a. Detail A.

Detail

(2) Pier Shape.— Unlike for spillway and outlet works intake with well-curved

(rounded) Pier shape

b. Plan.

Figure 1-72. -- Intake pier noses.

upstream pier noses, the shape of the power

intake noses is incised by the trashrack

slots (fig. 1- 72). To overcome the

disruption in the pier shape by the trashracks, the trashrack bars that lie within the

pier limits are located and slanted such that they are continuous with the pier face

through the trashracks.

To obtain the desired flow

distribution to the stay

ring, the downstream noses

of the intake piers are

asymmetrical (fig. 1-72).

The noses themselves are cylindrical with a minimum radius of 12 inches (30 cm) for

structural reasons to avoid stress concentrations.

On the side of the main spiral entrance (either left or right) the 12-inch radius in most

cases is tangent to a straight pier face. The other side of the pier curves towards the

pier nose to guide the flow into the spiral.

E. SURFACE POWERHOUSE SUBSTRUCTURE

Substructure The substructure of surface (type A) powerhouses (all six

subtypes illustrated in subsection B.1), basically comprises the part of the

structure that lies below the turbine floor and the lowest floors of the sub-bays, i.e.,

the structure below the top of the water conduit roof line

For powerhouses with vertical axis units, the substructure houses the draft tube

elbow, the draft tube structure and the semi-spiral or spiral case, whichever is used.

Figure 1-73.— Staning, Enns, Austria. Units: 3@ 11 MW; Hmax= 14 m (45.9 ft).

(Ennskraftwerke, 1946).

For horizontal axis units, part of the inlet and the draft tube are housed in the

substructure.

Figure 1-74. — Pierre-Benite, Rhone, France. Units: 4@ 20 MW; H= 7.95 m (26.1 ft).

(Compagnie Nationale Du Rhone, 1963).

Integral intakes for vertical axis units and intakes for horizontal axis units form, with

the substructure, the structure that retains the reservoir. The hydrostatic, plus seismic

loads if applicable, are transmitted through intake walls and piers, and draft tube

walls and piers, into the continuous intake and draft tube foundation slab and,

through it, into the foundation.

Thus the intake and draft tube walls and piers are the principal buttresses that resist

the lateral and vertical loads and carry them into the foundation.

1. Powerhouses on Soft Foundations

2. General. — The substructures on soft foundations, such as sand, gravel,

various moraine

a. Powerhouses on soft foundations deposits, or soft rock, must be designed for

low allowable foundation pressures to control differential settlements (figs.

1-4, 1-10, and 1-21). Consequently, the foundation slabs should be thick

enough to provide the necessary stiffness for acceptable load distribution. The

structure should be proportioned to obtain foundation pressures as uniform as

possible.

b. Intake and Draft Tube Foundation Slab. — The intake and draft tube foundation

slabs for the Sam Rayburn plant (fig. 1-4) were for instance, 13 and 9 feet (5.8 and

2.7 m) thick, respectively, and heavily reinforced. During conceptual studies, the

above aspects should be recognized to preclude substantial changes in excavation

and concrete quantities when final design is made.

(1) Preliminary Slab Foundation Thicknesses. — Preliminary slab thicknesses

can be quickly determined based on rough stability calculations. Approximate shear

and moment depth requirements should be assessed for the foundation slabs at

the wall and pier faces (based on conservative shear strength or design stress

assumptions).

(2) Reinforcement Layers. — For moment requirements it is not considered

advisable, in Reinforcement

interest of acceptable concrete placement, to use more than three layers of

reinforcement.

layers For the same reasons, bar spacing should not be less than 8 inches

(20 cm) o.c. To avoid surprises, these requirements should be assessed during

conceptual studies.

2. Powerhouses on Competent Rock Foundations

Powerhouses

When powerhouses are founded on competent rock, as most powerhouses are, the

loads are transmitted to the foundations through the rigid walls and piers of the

intake, where such are integral with the powerhouse, and the draft tube piers on

competent rock foundations

a. Intake Foundation Slabs. — The intake foundation slabs must be thick enough to

resist uplift pressures that vary between the reservoir and tailwater pressures, with

appropriate reductions when drains are provided as discussed elsewhere in the

Guidelines.

Depending on the head, the intake foundation slab thickness can vary from 4 to 20

feet (1.2 to 6 m), or even more. Therefore, the thickness requirements should be

quickly checked as briefly outlined for slabs on soft foundations.

Draft Tube Elbow Encasement. — The draft tube elbow encasement should be of

conservative concrete thicknesses to receive the thrust acting through the stay ring.

Draft tube elbow encasement

At least 3 feet (0.9 m), better, perhaps 4 feet (1.2 m) clearance is needed around

the draft tube steel liner for erection of same. The surrounding space is backfilled

after erection of the liner as second-stage concrete.

The first stage concrete should not be less than 3 feet thick at the narrowest

section to pro-vide sufficient space for reinforcement and ease in concrete

placement.

Thus, the total minimum elbow concrete thickness adds to about 5 feet (1.5 m)

for structures of medium size founded on competent rock.

The draft tube elbow foundation thickness is greatly affected by the

configuration and thickness of the intake foundation slab (figs. 1-4 and 1-8).

Unwatering

Foundation slab

c. Unwatering Pipe Embedment. — At the downstream end of the draft tube elbow,

the pipe embed -foundation slab is usually thickened to provide for embedment of the

draft tube unwatering pipes. These connect to each draft tube and run under

the draft tubes either to the unwater ing sump located, generally, in the erection

bay, or also between units in multi-unit powerhouses.

ment.

The size of the unwatering pipes, depending on the draft tube volume, may vary

between 12 to 18 inches. With a clearance of at least one pipe size diameter

provided at the top and bottom for embedment, the minimum slab thickness may vary

between 3 feet, for smaller powerhouses, to 6 feet or more, for medium and larger

powerhouses.

Structural requirements to resist uplift loads may require thicknesses larger than

indicated above, in the order of 10 to 12 feet (3.0 to 3.6 m).

The powerhouses of the La Grande complex of James Bay, Quebec, do not have

embedded drains for complete unwatering of the draft tubes. Instead, a drain

header below the draft tube access gallery dewaters the draft tubes below the

access opening. Portable pumps are used for complete unwatering [Ludwig and

Olive, 1980].

d. Draft Tube Foundation Slabs. — In the draft tube area, where the draft tube walls,

and intermediate piers for larger draft tubes, transfer the load to the foundation, the

draft tube floors can be rather thin and need to resist only uplift pressures from

tailwater.

Design for full uplift pressures is required only when the rock is very jointed and

highly permeable and cannot be improved with consolidation grouting.

Draft tube slabs on rock foundations that are reasonably watertight (no open joints

visible on the excavation surface or with joints that can be sealed) can be

provided with weep holes to relieve pressures.

Weep boles The weepholes are spaced approximately 8 to 10 feet on centers, and

are drilled 2 to 5 feet (0.6 to 1.5 m) or more into rock. Assuming that 50 percent of

the weepholes eventually clog, the draft tube foundation slabs can be designed for,

say, 50 percent of the actual uplift pressures.

On soft rocks with higher compressibility, it is assumed that foundation pressures

act also against the draft tube floor and the floor is designed accordingly.

For conceptual studies, it is recommended to check draft tube slab thicknesses for

the latter approach which will assure conservative excavation and concrete

quantities. Design refinements, that reduce quantities, can be attempted during the

final design stage if contractual provisions allow for such changes without price

increase.

For initial layouts, draft tube slab thickness can be assumed about 4 to 6 feet thick

and then quickly checked for shear and moment requirements.

Moment requirement check for a moment of 0.1w!2, will suffice for such purposes.

Heavier foundation slabs may be required if a powerhouse, especially a semi-indoor

type, is designed for high-flood tailwater levels. It should, however, be kept in

mind that deeper foundation slabs require deeper excavations and also result in

higher uplift pressures along the foundation contact area.

Heavier thicknesses may also be required if the draft tubes extend appreciably

downstream beyond the draft tube piers and resist foundation pressures as

cantilevers. Cantilever type designs should be avoided. Instead, draft tube piers

should be extended to brace the draft tube extensions.

3. Foundation Slab Outlines

It is recommended that the foundation slab contact area with the foundation be

developed as simple and with as few break points in foundation planes as possible.

The more complicated the excavation is, the more it costs. Also, reinforcement

becomes more complicated requiring additional lap lengths at each break point.

Any deepened "cutoff' keys (figs. 1-2, 1-9, 1-10, 1-13, and 1-15) at the upstream

ends of integral intakes foundations and draft tube outlets should, preferably, be

avoided. The following disadvantages result with provision of such keys:

Disturbance of good rock by blasting resulting in a more effective seepage path,

i.e. the opposite effect is achieved

Complication of excavation at premium cost

Complication of reinforcement resulting in higher cost

If seepage "cutoffs" are needed at the upstream and downstream ends of the

structure, other, more effective means can be utilized depending on the type of

foundation material:

Steel sheet piles where they can be applied

Consolidation grouting

Concrete aprons with sealed joints at the structure

Impermeable clay blankets

Grouting and a.

Grouting and Drainage Galleries.

— If cut-off grouting and drain curtains are needed then the upstream end of the

intake foundation slab

Drainage for an integral intake powerhouse, m a

galleries have to be deepened to include a grouting/drainage gallery (fig. 1-36).

The grouting gallery size and location with respect to the upstream face of the

foundation slab should be as indicated below:

Figure 1-75. — Grouting and drainage gallery location.

b = Gallery width, should at least 5 feet which has proved to be adequate for

numerous f ounda t i on g rou t i ng ga l l e r i es i n conc re te dams .

h = Gallery height, 8 feet minimum.

t = Gallery clearance with respect to surfaces exposed to reservoir. USBR

recommends that the distance be not less than 0.05H, where H is the reservoir head

at the gallery level, or not be less than 5 feet. These minimum requirements should

he confirmed by check for shear requirements at the gallery roof and floor.

d = Thickness of gallery floor slab when in contact with foundation. This thickness

can be somewhat less than the clearance, t, because there is no direct exposure to

the reservoir in case of cracking. A minimum thickness of 3 feet is recommended,

however, the actual requirements to resist actual uplift and grouting pressures shall

be determined.

Walls and4. Walls and Piers of Integral Intakes and Draft Tubes piers

Intermediate piers are provided for structural reasons when the intake and draft tube

width become too wide to:

Be closed off by a single gate •

Support the concrete gravity weights at and above the intake and draft tube roof

level

Intake walls and piers

Intermediate

peer

Resist the reservoir and tailwater loads and to deliver them to the foundation

a. Integral Intake Walls and Piers. — Except for horizontal axis unit intakes and

integral intakes for small power plants, usually integral intakes are designed with at

least two gate openings and one central intermediate picr. This provision is made for

two reasons:

To provide needed buttress effect against the reservoir pressures

To keep the service pier widths, and thus the water loads acting on the gates and

their hoists to a minimum so that a fast gate closure is ensured

The integral intake acts as the dam. It resists full reservoir pressures when the

gates are closed. The intake walls and intermediate piers must be designed

accordingly to resist all the imposed loads from the reservoir.

Because the main purpose for the intake is to convey water from the reservoir to

the turbines, the hydraulic requirements dictate the width of the water passages.

The thicknesses of the intake walls and piers are governed by structural

requirements imposed by the need to contain the flow and the resulting

hydrostatic effects within the water passages.

By necessity, the design of the intake walls and piers shall be conservative to limit

deformations and cracking. However, overly conservative designs will result in

excessive widths of the unit monoliths, with the resulting cost increases,

especially for multi-unit powerhouse.

For free-standing walls (not placed against concrete and anchored thereto) the

thicknesses may range between 4 feet and more than 10 feet depending on the

head. For most cases a thickness in the range between 5 and 8 feet will suffice.

The thickness of the intermediate piers will be somewhat less than that for the

walls, except when minimum pier thickness provisions, as discussed for draft tubes

in subsection D.2.e, govern to satisfy the depths requirements for gate slots and

minimum concrete thickness required between them.

Thus, minimum thickness for intermediate piers will be about 6 feet 8 inches

(see fig. 1-64) unless the depth of the gate slots can be less than 20 inches as

assumed in the referenced figure.

b. Intermediate Pier Downstream Nose Location. — The length of the intake

intermediate piers is dictated both by structural and hydraulic requirements.

Structurally, it is desirable to carry the intermediate pier downstream as far as

possible to provide all available support for the intake roof and the walls above.

With reference to figures 1-1, 1-3, 1-4, 1-5, 1-7, and 1-14, the downstream nose

preferably should extend far enough downstream to support the headwall of the

powerhouse that is exposed to the reservoir.

It is true that for supporting the gravity loads such watts can easily span between the

intake watts as deep beams, but it is also very desirable to obtain for the entire

structure as much rigidity as possible against the reservoir pressures.

The above structural considerations are, however, preceded somewhat by hydraulic

considerations in that the intermediate pier must be terminated at some distance

upstream from the stay ring.

The location of the downstream nose of the piers (if more than one) shall be

selected in such a way that the intermediate piers do not interfere with uniform

distribution of the flow around the stay ring. Moreover, the downstream noses of the

piers shall be arranged and shaped so that they serve as guides for uniform flow

distribution around the stay ring. Because of the foregoing requirements, if two

intermediate piers are provided, their noses may be slightly differently shaped and

may not be at the same location from unit center line. There are exceptions to this

simple arrangement in that some plants have very intricate intermediate pier

arrangements to obtain the desired flow distribution, e.g. Bonneville, Columbia River

[Mosonyi, 1957].

Since, generally, the turbine designers (manufacturers) are responsible for

determination of the spiral case inlet geometry, they will also determine the pier

nose location. For preliminary layouts, however, it can be assumed that the pier

Kaplan units

noses are located as indicated on figure 1-76 based on the expressions for the

dimensions given.

For the purpose of preliminary powerhouse layouts, the following information is

given from publications by de Siervo and de Leva [1976 and 1977] for the outlines of

spiral cases and draft tubes of Kaplan and Francis units, and by de Siervo and

Lugaresi [1978] for the casings and spiral cases of Pelton units. In addition to the

expressions given here, the referenced publications provide experience curves

plotted on the basis of statistical information obtained from a great number of plants

investigated.

Figure 1-76. -- Kaplan unit spiral cases.

[de Siervo and de Leva, 1977/1978].

Runner size:

Runner size

Dm= 84.5 ku(H„)°.5/n ku= 0.79 + 1.61 x 10-3n,

Dm/DM = 0.25 + 94.64/n5; Hm/Dm = 6.94ns-0.403

HilD„i= 0.38 + 5.17 x 10-5n,

Semi-spiral case dimensions:

Semi-spiral case

Inlet velocity: V2= 2.44 - 1.19 x 10-3ns

Dimensions (DM):

B2 = 1/(0.76 + 8.92 x 10-5ns) C2 = l/(0.55 + 1.48 x 10-5ns)

D2 = 1.58 - 9.05 x 10-5ns E2 = 1.48 - 2.11 x 10-5ns

F2 = 1.62 - 3.18 x 10-5n G2 = l.36 + 7.79/n,

H2= 1.19 + 4.69/n, /2 = 0.44 - 21, 47/ns

L2= 1.44 + 105.29/ns /M2 = 1.03 + 136.28/n,

Steel spiral case dimensions:

Steel spiral case

Inlet velocity: V1= 3.17 + 759.21/ns

Dimensions (DM):

A1 = 0.40n50.20 B1 = 1.26 + 3.79 x 10-4ns

C1= 1.46 + 3.24 x 10-4ns D1= 1.59 + 5.74 X 104ns

El = 1.21 + 2.71 x 10-4ns F1= 1.45 + 72.17/n,

G1 = 1.29 + 4.63/ns H1= l.13 + 31.86/n,

I1 = 0.45 - 31.80/n, L1=0.74 + 8.7 x 10-4ns

M1 = 1/(2.06 - 1.20 X 10-5ns)

Based on the above, concrete semi-spiral case width = 3.04 DM, and steel spiral

case width = 2.76 DM.

Spiral case width

To enable design of concrete spiral cases for heads between 30 and 40 meters (100

and 130 ft), their width should be reduced below the above value. This is possible

without sacrificing turbine efficiency.

Figure 1-77. — Kaplan unit wheel

dimensions.

[de Siervo and de Leva, 1977/1978].

elides centerlines

Figure 1-78. — Kaplan unit draft tube dimensions.

[de Siervo and de Leva, 1977/1978].

Draft tube dimensions:

Inlet velocity: V3 = 8.42+250.25/n,

Dimensions (Dm):

Ht= 0.24 + 7.82 x 10-5ns N = 2.00 – 2.14 x 10-6n,

O = 1.40 – 1.65 x 10-5n, I" = 1.26 – 16.35/n,

Q= 0.66– 18.40/n, R= 1.25 – 7.98 x 10-5n,

S =4.26 + 201.51/n, T= 1.20 + 5.12 x 10-4ns

Z = 2.58 + 102.66/n,

(2) Francis Units

Figure 1-79. — Francis units runner and spiral case outlines.

[de Siervo and de Leva, 1976].

Runner size:

D3 = 84.5ku(H n) " I n k = O 31 + 2 5 x 10-3n

u – • - s

D1/D3 = 0.4 + 94.5/n, D2/D- = 3 1/(0.96 + 0.00038n5)

o oc)I4

Hi/D3 =0:94 + 0.000025ns H2ID3= 0.05 + 42/ns for 50 <n, <110

H2/D3 = 1/(3.16 – 0.0013ns) for 110 <ns <350

Spiral case dimensions:

Inlet velocity: V1= 844 n5-144

Dimensions (D3):

A = 1.2 – 19.56/ns B = 1.1 + 54.8/ns

C = 1.32 + 49.25/ns D = 1.50 + 48.8/ns

E = 0.98 + 63.60/ns F = 1.0 + 131.4/ns

G = 0.89 + 96.5/ns H = 0.79 + 81.75In3

I = 0.1 + 0.00065ns L = 0.88 + 0.00049ns M = 0.60 +

0.000015ns

Draft tube dimensions:

Figure 1-80. — Francis units draft tube dimensions.

de Siervo and de Leva, 1976].

Inlet velocity: V2 = 574 + 248/n,

Dimensions (D3):

N = 1.54 + 203.5/n, O = 0.83 + 140.7/n,

p = 1.37 – 0.00056n, Q - 0.58+22.6/n,

R = 1.6 – 0.0013/ns S = ns/(-9.28 + 0.25ns)

T = 1.50 + 0.00019/n, U = 0.51 – 0.0007n,

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PROJECTED PLAN

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Figure 1-81. —Draft tube dimensions. [USBR, 1976: fig. 181.

Figure 1-82. — Pelton unit casing dimensions.

[de Siervo and Lugaresi, 1978].

Figure 1-83. — Pelton unit spiral case dimensions.

[de Siervo and Lugaresi, 1978].

Draft tube

c. Draft Tube Outline — Walls and Piers. — Preliminary draft tube outlines can be

deter-

walls and mined from the information given on figures 1-78 and 1-80 for Kaplan and

Francis units, piers respectively. The information presented is helpful to establish the

overall draft tube dimensions, which is all that is needed for conceptual studies.

Detailed dimensions that can be used for final designs for the draft tube geometry are

supplied by the turbine manufactured.

The draft tube floor can be sloped upward as much as 4 horizontal to 1 vertical for

economy in excavation.

The draft tube walls and piers perform somewhat similar functions to their intake

counterparts except to a lesser degree as far as hydrostatic thrust is concerned.

The draft tube walls and piers can be envisioned as extensions of the intake walls

and piers and, as such, they are involved to carry the reservoir pressure

overturning (lateral) effects into the foundation. They resist higher bearing

pressure intensities than the intake walls and piers because of the higher

foundation pressures under the draft tube.

Also, the draft tube piers and walls act as counterforts to resist lateral tailwater

pressures against the downstream wall and the uplift effects. The upstream areas

of the draft tube walls and piers receive substantial gravity loads from the

substructure and the superstructure above.

The need for intermediate piers will be governed by the overall width requirements

for the draft tubes, which is discussed under section D. Large, slow-speed units

require wider draft tubes with at least one or, in many cases, two intermediate piers.

The number of intermediate piers is governed mostly by structural requirements

discussed above and less for reasons to limit the gate width as is the case for

intakes. The draft tubes are not closed under operating conditions and their gates

serve only as closure bulkheads for inspection and maintenance.

For structural reasons, it is preferable to locate the upstream noses of the piers

as far upstream as possible, preferably, reaching under the downstream wall of

the generating bay. However, turbine design dictates that, for best flow conditions,

the pier noses be located approximately 1.35D (D = turbine discharge opening

diameter) downstream of the unit centerline, as indicated on figure 1-81. This should

be checked against the information on figure 1-78.

Semi-spiral case 5. Semi-Spiral Case

The term "semi-spiral" denotes an incomplete spiral when compared with a steel

spiral casing with a nose angle of approximately 320° to 340°. The angle for the nose

or baffle vane (the last stay vane in the semi-spiral) of the semi-spiral is

approximately + 180° depending on the design by the turbine manufacturer (fig. 1-

76).

The semi-spiral case is formed in concrete with varying cross-sections around the

stay ring (fig 1-76a). Preliminary information can be quickly obtained from [de Siervo

and de Leva, 1977/1978].

The walls of the semi-spiral cases are stressed in hoop tension in a horizontal plane

and as vertical wall sections spanning between the floor and roof. They must be

sufficiently rigid to control deformations and related cracking and must be designed

to be watertight. Low water content in the concrete mix and precooled aggregate for

concrete placement lead to good results. Reinforcement requirements should be

checked for acceptable crack widths which, preferably, should not exceed 0.008 inch

(0.2 mm).

Designs with concrete semi-spiral cases are feasible for heads up to about 130 feet

(40 m).

It is advisable to make the semi-spiral cases as narrow as feasible hydraulically to

reduce not only the overall length of the powerhouse but also, especially for

the upper head ranges, to reduce the hydrostatic pressures on the roof and the

corresponding reactions acting as tension forces on the walls. For this latter reason,

the upper and lower cones, which support the stay ring, should be made as wide as

hydraulically feasible at their contact with the roof.

According to Mosonyi [1957], the semi-spiral case width varies between 2.7 and

3.5D where D is turbine discharge opening. This variation in width is related to

the specific speed of the turbine as given by the preceding expressions by de Siervo

and de Leva. The lower values of the above coefficients should be used for the

higher head ranges for reasons explained above. An ongoing study for a plant

with 180-MW units to be operated under a 36-meter (118-ft) head and a semi-

spiral case width of 2, 750 feet has been confirmed by manufacturers.

According to the USBR, spiral case inlet ama should be checked so that the inlet

velocities do not exceed 0.14(2gh)0•5. More recent information is given in the

preceding expressions of de Siervo and de Leva.

a. Semi-Spiral Case Roof. — The roof of the semi-spiral case spans between the

stay ring cones and the outside walls. On the upstream side, at the hila, the roof

gains support also from the upstream head wall or upstream wall of the generating

bay (fig. 1-84).

The semi-spiral case roof protects the interior of the powerhouse against the

water in the spiral case acting on the underside of the roof under reservoir

pressures.

The top of the roof forms the turbine floor, which may contain blocked-out channels

for mechanical piping, electrical cables and conduits. There are also large

blockouts for the servo motors.

The blockouts reduce the effective structural depth of the otherwise massive roof

slab and, therefore, when the slab thickness is estimated to determine the turbine

floor level, an additional thickness of at least 12 inches should be added to the

structural slab depth that otherwise would be considered adequate.

Steelspiral case

Figure 1-84. — Semi-spiral case roof support.

Minimum semi-spiral case roof thicknesses are in the range of about 5 feet or even

somewhat less.

6. Steel Spiral Case

For heads over about 130 feet (40 m), when integral intakes and semi-spiral cases

become structurally infeasible, steel spiral cases are used.

Steel spiral cases may be connected to reinforced concrete conduits (fig. 1-25) in the

lower head ranges, or to steel penstocks when concrete conduits cannot be

designed economically for the reservoir pressures resulting from higher heads (fig.

1-24). For preliminary layouts, steel spiral case dimensions can be obtained from

figure 1-76.

To control cracking of the concrete contact surface with the steel spiral case under

operating pressures, spiral cases should preferably be embedded under

hydrostatically pressurized conditions. The pressures maintained during embedment

shall be equivalent to normal operating pressures. Spiral case embedment follows

hydrostatic pressure testing usually performed under pressures equivalent to 150

percent of the static head. For plants with high heads, spiral case embedment

under pressure, from a civil engineering point of view, is the preferred procedure

because less load is transferred to the concrete. Reduced internal pressures against

Concrete cover of spiral case

Draft tube roof

the embedment concrete greatly reduces the possibility for cracking and, thus, the

potential for seepage from tailwater.

Lately, some manufacturers have recommended that, even for high-head plants,

spiral case pressurizing is not required during embedment. In that case, appropriate

structural investigations should be performed to determine how the surrounding

concrete would be affected.

Because steel spiral cases are designed to resist full operating

pressures, including any dynamic effects, the encasement concrete

resists only a part of the latter.

a. Concrete Cover over the Spiral Case. — The concrete cover over the steel spiral

case is required to from the turbine floor and the foundation for the generator barrel.

AH the embedment concrete also provides a part of the weight needed for

powerhouse stability and the necessary mass to control vibrations. The larger the

units capacity-wise, the more important the presence of the mass is.

Minimum concrete cover over the spiral case can be less than the thickness of the

roofs for reinforced concrete semi-spiral cases. However, the thickness should he

sufficient for embedment of the generator barrel reinforcement. Based on the above:

Minimum spiral case cover should not be less than about 3 feet

The thickness of the cover will, for most cases, exceed 3 feet and be in the

range of 5 to 8 feet, and, quite often, more than that depending also on convenient floor

level selection.

7. Draft Tube Roof

The draft tube roof can be divided in two parts:

The downstream part of the draft tube inlet cone and elbow embedment

The part that forms the draft tube downstream of the elbow extending beyond the

generating bay (figs. 1-1 through 1-7 and others)

Downstream head wall

Because of the draft tube elbow geometry, the upstream part of the draft tube

roof (fig. 1-85) is a very deep massive feature that receives a substantial part

of the gravity loads of the structure, the weight of the turbines and generators, and

the hydraulic thrust. Its weight contributes to overcome the uplift forces.

The massive outline of the elbow roof offers a convenient mass for provision of a

service gallery along the downstream side of the units. Access ways to the

individual draft tube inlet eones for inspection and maintenance of the turbine

runners are provided as stubs from the service gallery (fig. 1-85a and b). Some

plants, for ease of installation of maintenance platforms through the draft tube cone,

also have an upstream service gallery.

The downstream pan of the roof usually tapers in downstream direction or the

downstream end of it can be of uniform thickness, depending on provisions made

above the draft tube.

If a downstream service bay is provided above the draft tube, the top of the draft tube

roof forms the lowest floor in that bay. Consequently, the draft tube roof must be

designed for the applicable uplift pressures. For major plants a minimum draft tube

roof thickness of 4 feet or 0.10H (H = depth of submergence), whichever governs,

is provided at the inside face of the downstream wall to ensure adequate

watertightness and reinforcement tie-in with the downstream service bay wall,

which acts as a headwall against the tailwater (figs. 1-85 and 1-86).

The minimum roof thickness should be checked against actual shear requirements

at the wall and pier faces assuming that only concrete, without any shear

reinforcement, resists the unbalanced shears.

8. Downstream Headwall

If a downstream service bay is provided, its enclosure against the tailwater is

provided by the draft tube roof and the downstream headwall (fig. 1-86).

The headwall, sometimes also called the bulkhead wall, spans vertically between

the draft tube roof and the draft tube deck. It may also be supported horizontally

by the draft tube piers. Interior floor slabs provide intermediate supports. For this

reason, such slabs shall be sufficiently thick to control slenderness. The minimum

thickness recommended is 18 to 24 inches, or even more, depending on the wall

submergence and the resulting hydrostatic pressures.

Figure 1-85. — Generating and downstream service bays, Sam Rayburn,

Angelina,

Texas. (Courtesy of Harza).

The draft tube piers, depending on how much they are extended downstream of the

head wall, may effectively act with the wall section as T-beam webs. The wall itself

then also spans horizontally between the piers.

For deep tailwater submergence, the length of the pier protrusion downstream of the

head wall should he determined to suit the structural design requirements. On the

other hand, if the piers support a transformer deck, they will probably provide more

width than required for the wall design.

During conceptual studies, it is advisable to be on the conservative side and to

assume that the wall is supported vertically in one-way action spanning between the

slabs without the assistance of the piers. The wall thickness, as before for other

principal structural components, shall be quickly assessed on basis of simplified

shear and moment calculations.

As in the case of the draft tube floor, for major plants the minimum thickness

provisions at the draft tube roof level should be 4 feet or 0.10H. This thickness will

be substantially more for deeper submergences and can be reduced gradually, by

tapering the outside face of the wall, for higher elevations. The thickness can also be

reduced in steps at the intermediate floor levels thus gaining more space, if needed,

for the upper floor levels. With this latter arrangement, the inside face vertical wall

reinforcement becomes offset and, thus, more complicated where the offsets occur

(fig. 1-86b).

Draft tube deck

Function

Thickness

Figure 1-86. — Downstream service bay wall.

9. Draft Tube Deck

a. Function. — The draft tube deck functions primarily as access to the draft

tube gate slots for handling of the draft tube gates. For indoor powerhouses (figs.

1-3 through 1-7, 1-9, and 1-10), an appropriate traveling gantry crane is located

on the draft tube deck to handle the gates. The main powerhouse gantry crane

for semi-indoor (figs. 1-11, and 1-13), or outdoor powerhouses (fig. 1-34) generally

reaches over the draft tube gate slots negating the need for a special draft tube

gantry crane.

When the draft tube deck is also used as transformer deck, the deck width increases

appreciably. The additional width required is for the transformer bank itself, with

adequate passage clearances with other transformers during installation and

possible removal for their maintenance.

Ultimately, the draft tube deck functions also as the roof over the space of the

Downstream Service Bay. As such, it must be reinforced for crack control to make

the deck watertight

b. Thickness. — If the deck does not carry any transformers, it can, depending

on the spans, be a relatively light slab, yet still about 8 to 16 inches thick, spanning

between the generating bay wall and the downstream headwall, if the latter exists.

Otherwise, a spandrel for the gantry crane rail or some wall framing below (figs. 1-1,

1-7, and 1-14) would provide the support. If there are no rooms below, the piers

provide supports in the longitudinal direction.

Figure 1-87.— Draft tube deck framing.

With transformers located on the deck, the framing requirements increase

substantially, depending on the size of the transformers and, because of the pressure

of the oil overflow sumps below the transformers.

Two choices are available for the deck design:

Slab and beam construction

One-way flat slab construction with local beam reinforcement or beams as

needed under the transformers only

The first choice, with the beams running in the upstream and downstream directions

and framing into a spandrel along the gate slots, or supported on a wall is a

conventional approach; however, it results in a rather complicated framing (fig. 1-87)

of the deck slab, beams, spandrel, and the oil sumps under the transformers.

Wherever possible, it may be advantageous to eliminate the beams supporting the

deck and make the latter thicker, say 24 to 36 inches, as needed to support the

loads. More concrete will be required but the construction will be simplified. The flat

soffit of the deck is also preferable for any exposed conduit or piping runs hung from

the deck if the headroom is limited.

Superstructure

Generator barrel outlines

Generator barrel

F. SUPERSTRUCTURE

1. General

Under "superstructure" it is understood that the part of the powerhouse that lies

above the turbine floor (figs. 1-73 and 1-74) and the draft tube roof, when a

service bay is provided, may comprise, depending on the concept of the

powerhouse, the following components:

Generator barrel (or pedestal) with or without generator floor, again depending on

the layout

All interior framing and walls above the turbine floor in the generating bay

Crane support columns and girders if such are provided

Exterior walls

Roof

All interior framing. walls and roof of any adjoining service bays

2. Generator Barrel

The generator barrel supports:

The weight of the turbines and generators

The hydraulic thrust from turbine operation

Dead and live loads from the generator floor if such is provided

Short-circuit torque effects from the generator

Hydrostatic pressures against downstream bulkhead wall or downstream wall of the

generating bay if such loads are transmitted into the generator floor

Similarly, the barrel may also support headwater pressures against the head walls

(or upstream generating bay wall) if generator floor is provided, or the barrel itself

may be in contact with the wall.

a. Generator Barrel Outlines. — The generator barrel outlines are mostly dictated by

the:

Outlines of the equipment it supports,

Access provisions

Erection and maintenance clearances and access provisions,

Structural requirements to obtain a rigid foundation for the equipment, concrete

dead loads and live loads supported,

And last, but not least, the designer's preference.

The inside outline is an extension of the cylindrical turbine pit outline which, by a

provision of a circular corbel, necks down to a circular opening, slightly larger than

the turbine head cover. The corbel serves as the bearing bracket support (fig. 1-88).

Also the opening above the bearing bracket corbel is cylindrical.

The exterior outline of the barrel can be square, rectangular, octagonal, or round.

The shape used is mostly the designer’s prerogative, based on considerations for

obtaining adequate space for other equipment on the turbine floor and for passage

for personnel and for equipment during installation and maintenance.

Square and rectangular shapes are the simplest to form, but some valuable

turbine floor space may be sacrificed. The reinforcement is of simple configuration,

but more concrete is required.

If floor beams support the generator floor, the flat wall faces in both directions offer

simple framing for the beam supports.

Octagonal shapes are still simple to form, however, reinforcement becomes

somewhat more complicated, but concrete volume is decreased and more floor

space is gained. The forming costs are, of course, increased. The generator floor

girders, if provided, may, however, have to be framed in barrel faces skewed under

45°, i.e. the bearing length on the barrel varies along the two sides of the girder. This

usually affects only the reinforcement.

Cylindrical generator outlines are quite common, they occupy the least space on the

turbine floor and, when exposed to the view without the presence of the

generator floor, have a pleasing appearance by matching the generator housing

outline (mostly cylindrical, too).

The forming of the cylindrical barrel is more expensive than for the other types; also

all horizontal reinforcement consists of curved bars, which are more expensive

than straight bars.

If the generator floor is supported with beams, their framing into cylindrical

generator barrels presents some complications for the beam-barrel tie-in area.

For that reason, sometimes rectangular support pilasters are incorporated into the

barrel to offer a less complicated beam framing into the barrel (fig. 1-88).

The generator barrel (pedestal) wall thickness may vary between 1.5 feet for small

units and, perhaps, 8 feet or more for large units. In most cases, a thickness of 4

to 6 feet will suffice.

b. Access Provisions in Generator Barrel.

Blockouts are provided in the barrel walls to gain access to the turbine pit, located

below the turbine floor level, and for access to the thrust bearings above.

Blockouts are needed for the passage of the low-voltage leads.

The turbine pit access should be at least 4 feet wide and with headroom of at

least 6.5 feet. This clearance requirement dictates where the top of the bearing

bracket support can be set. Assuming the bearing bracket sole place anchor bolts

to be 3 feet long and a cover of 6 inches over the access opening, the bearing

bracket is set approximately 8 to 10 feet above the turbine floor. This assumes

that the roof of the turbine pit access slopes. The 10 feet would apply if the roof

would be horizontal.

Other openings are needed for access, to the air coolers between the stator and the

generator pit wall, or housing. Their location and sizes will be determined by the

generator manufacturer.

Generator floor

Framing without columns or beams

Figure 1-88.— Generator barrel arrangement. 3. Generator Floor

The generator floor, if provided, frames into the top of the generator barrel

and the upstream and downstream walls of the generating bay.

For most designs, the generator floor does not support any permanent equipment

loads but may be used extensively during equipment installation as laydown area or,

quite frequently, as a temporary erection floor in extension of the erection bay floor or

where space is available between the generators.

If the generator housing protrudes above the generator floor level, access to the top

of the generator housing and the exciters is provided from the generator floor.

It is preferable to design generator floor slabs without columns and beams. The

columns are undesirable because they take up space on the turbine floor and may

interfere with the equipment.

The beams take up valuable headroom below and may interfere with the routing

of conduits, cable trays, and piping.

It is recommended to span the generator floor slab between the barrel and walls in

direction parallel to the flow, and to cantilever them from the barrel in the direction

perpendicular to the flow. With this arrangement, the slabs of the adjoining units

should be keyed along the contraction joints to control, and prevent differential

vertical off-sets and also, possibly, to control vibrations of the cantilevers.

If, because of wide unit spacing and, therefore, long slab cantilevers, beams are

needed to support the slab along the contraction joints, columns will be needed along

the contraction joints.

Without beams, the minimum slab thickness should be about 12 to 24 inches

depending on the spans. With beams, the thickness should not be less than 12

inches to provide mass around the generator barrel. To control vibrations of the

larger units (say, 100 MW or more) in major plants, a 24-inch slab thickness is

appropriate.

If generator floor slabs are used as lay-down or erection area for heavy equipment,

the slab thickness may have to be increased above the minimum recommended.

Slab thickness

4. Other Interior Floors

Other floors

The floors of the service bays usually can be of slab construction if the bay width is

limited to approximately 30 feet. Beams may be needed for heavy equipment

loads, or when the bay width is increased much beyond 30 feet.

The minimum slab thickness should be 12 inches. Appreciably greater thickness

will be needed if the slabs are used as struts against hydrostatic pressures. In that

case, slab thickness of 24 to 36 inches are not unusual.

The above minimum thicknesses may be excessive to support the floor loads, but

it is prudent to provide more rigidity in the framing systcm and also mass to control

vibrations.

5. Generating Bay Walls

6. Generating bay walls

7. The generating bay walls perform several functions:

Enclose the generating bay along its sides and support its roof.

Provide support for the generator floor, if such is provided (figs. 1-3, 1-4, 1-7, 1-11,

1-26, 1-34, 1-41, and 1-47).

Support the powerhouse crane (same figs. as above and others).

The upstream wall may act as a headwall to retain the reservoir if the powerhouse

is integral with the intake (figs. 1-3, 1-5, 1-7, 1-9, 1-10, 1-11, 1-14, and others).

The downstream wall may also act as head wall to retain tailwater if there is no

downstream service bay head wall (figs. 1-11, 1-13, 1-27, 1-31, and 1-41).

a. Upstream Wall of Generating Bay. — ( I ) Upstream Wall Integral With Intake. —

If the

Upstream wall support

Upstream wall powerhouse is built integral with the intake, the upstream wall of the

powerhouse is

exposed to the reservoir and resists the applicable hydrostatic pressures applied

against it.

The headwall is tied-in with the intake piers and walls and spans between them.

This wall, in conjunction with the intake forms the dam and, when the water

passages are not closed, the head wall represents the watertight barrier between the

reservoir and the interior of the powerhouse, i.e. it functions as a fixed bulkhead.

Consequently, the thickness of the headwall must be determined on the basis of

conservative shear resistance values and considerations illustrated on figure 1-89.

Figure 1-89. — Headwall support.

As for the downstream bulkhead wall, discussed in section E.8, the minimum

thickness for major plants again should be 4 feet or 0.10H determined at the base

of the wall. This criteria is considered as a minimum requirement to control

seepage (watertightness), but usually structural requirements govern. For

most major plants, the actual thickness will be appreciably more (fig. 1-5).

For minor plants a lesser wall thickness in the range of 2 to 3 feet will suffice.

With respect to the reservoir pressures (fig. 1-89), the wall "hangs" from the intake

walls and piers. Consequently, a potential crack could develop as indicated on

figure 1-89 and the headwall could lose support.

Thus, adequate thickness to resist shear, induced by the reservoir pressures, is not

the only design provision that is needed. The wall should be tied back into the intake

walls and piers by adequate reinforcement assuming that water pressure acts also

over the full width of the potential cracks.

In the vertical plane, the headwall spans between the intake end walls with additional

supports on the intermediate piers (if such are provided) of the intake.

The headwall itself supports the spiral case roof with equipment loads acting

downward, and uplift pressures, against the spiral case roof. Unless cast integrally

with the spiral case roof, keys should be provided if construction joint is introduced

at the downstream face of the wall (fig. 1-90).

PROVIDE KEYS IF CONSTR. JOINT IS PROVIDED AT WALL FACE

Figure 1-90. — Headwall–semi-spiral case roof joint.

(2) Upstream Wall Separated from the Intake or for Detached Powerhouses. — If the

generating bay is separated from the intake by a service bay (figs. 1-4, 1-6, 1-8,

and 1-13), or if the powerhouse is located at the toe of the dam or for detached

powerhouses (figs. 1-36 through 1-47g), the upstream wall of the generating bay is,

in most cases, a free-standing wall. The wall is supported on the substructure and

has lateral support by the generator floor (if provided) and by the roof.

The upstream wall may have to support:

End walls

Type of construction

Walls for indoor houses

Reinforced concrete walls

Live and dead loads from floors, decks and roofs framing into the wall, including

the equipment loads they carry.

High-voltage line pull-off loads (fig. 1-24, 1-35, 1-41, and 1-45).

Vertical and lateral crane loads (fig. 1-2, 1-3, 1-4, 1-5, 1-7, 1-11, 1-12, etc.).

Tailwater pressures transmitted from downstream wall through floor slabs, decks

or roof (figs. 1-11, 1-13, 1-30, and 1-41).

Conservative wall thickness should be provided (see subsection d (3) in this section)

to control deformations from combination of all load effects.

b. Downstream Wall of Generating Bay. — The downstream wall of the generating

bay is Downstream similar to the upstream wall of the type described in paragraph (2)

above and supports similar loads wall of generating bay

For high tailwaters, the downstream wall may be exposed to hydrostatic pressures in

addition to the other load effects.

The downstream wall usually contains the equipment and personnel access

doors. In case of high flood levels, they may have to be bulk-headed unless all

access is from the roof level (semi-indoor type powerhouses).

It is recommended that the access level for indoor type powerhouses is set above

maximum powerhouse design flood level, say with 200-year return frequency. If

the PMF level is not substantially higher, the access level should be set at or slightly

above the PMF level.

For substantially higher PMF levels semi-indoor powerhouses with walls designed

for the high water levels should be selected.

c. End Walls. — The end walls close off both ends of the generating bay — at the

erection bay and at the last unit. They may be extended to serve as end walls

also for the service bays, if any provided. Endwalls, depending on their construction

type, may have to support the following loads:

Loads transmitted by longitudinal walls connected to them, normal and

parallel to the end wall plane

Loads from interior floor slabs

Longitudinal crane thrust

Roof loads

Exterior fill loads

Tailwater or groundwater pressures

The end walls receive and transmit any exterior fill and hydrostatic loads to the

interior slabs and the longitudinal walls framing into them. If they retain fills or water

loads, their outside faces are usually battered to account for the varying pressures.

The thickness of the wall at the top should be 24 inches minimum to allow for

concrete placement.

d. Wall Type Construction. — The construction type of the superstructure walls

can be dictated by:

The type of the powerhouse, indoor or semi-indoor

By considerations of economic aspects, availability of material

Preferential treatment by owner

(1) Walls for Indoor Powerhouses. — Superstructures of indoor powerhouses

can be designed for a great variety of construction types and materials:

Solid cast-in-place reinforced concrete walls

Cast-in-place reinforced concrete columns with brick, concrete block or precast

concrete panel infills between the columns

Structural steel framing faced with material as for concrete framing plus various

types of insulated metal wall panels

Figures 1-l, 1-2, 1-3, 1-4, etc. illustrate typical integral-intake powerhouses and

figures 1-24, 1-30, 1-39, 1-41, etc. illustrate powerhouse designs with cast-in-place

concrete wall construction. The arrangement may vary somewhat from project to

project depending on unit setting, generator size and interior floor arrangement.

The upstream wall is thicker than the downstream wall because the former resists

hydrostatic pressures or higher pressures than the latter.

Minimum thickness requirements for the walls are discussed in subsections 5.a, 5.b,

and 5.c.(3). If the concrete walls are not exposed to water or fill pressures, their

minimum thickness will still be in the range of 2 to 3 feet, perhaps, even more

depending on the crane size they support and structural support provided.

The wall thickness is reduced above the crane rail corbels. This thickness, however

still must be approximately 18 inches thick to provide at least 12 inches for the roof

girder bearing and approximately 6 inches minimum, better 8 inches, at the girder

blockouts.

The crane corbels protrude the inside face of the walls to provide for the

required crane Crane support. The crane rail should be set to provide minimum

clearances between the crane and clearances the wall. For conceptual studies, the

information in table 1-1 can be used, subject to confirmation by crane designers.

Cast-in-place pilaster-type superstructure walls for a detached powerhouse are

illustrated on figure 1-91. The walls appear recessed at the pilasters for

architectural treatment to break up an undesirably plain, bunker type of effect of the

wall type discussed above.

A very pronounced architectural treatment was obtained for the Grand Coulee Third

Power Plant with a cast-in-place, folded-wall design, as shown on figure 1-92.

Table 1-1. — Crane clearance data.

Span (ft)8 A B C D (in)

• 200 T Crane 13-6 12-4 8-0 13

90 13-9 12-6 8-6 13

300 T Crane

70 16-6 15-0 8-6 1580 16-9 15-3 8-6 1590 17-3 15-6 9-0 15

400 T Crane

70 17-3 16-9 11-6 1780 17-6 17-0 11-6 1790 18-0 17-0 11-6 17

500 T Crane

70 18-9 18-0 19-3 2080 19-0 13-0 19-6 2090 19-3 18-6 19-6 20

600 T Crane

70 20-0 19-3 20-6 23

80 20-3 19-6 20-9 2390 20-6 19--6 21-0 23

700 T Crane

70 21-6 20-6 20-6 2680 21-9 20-9 20-6 2690 21-9 21-0 21-0 26

800 T Crane

70 22-0 21-0 21-0 2780 22-3 21-0 21-6 2790 22-6 21-6 22-0 27

Figure 1-92. — Grand Coulee Third Powerhouse. (Courtesy of USBR).

Another attractive example of superstructure wall and roof treatment is shown on

figures 1-93a and 1-93b, the auxiliary powerhouse of the Angat Plant in the

Philippines. A folded place roof is supported on tapered columns. The crane girder,

cast integrally with the wall, spans over doubly tapered columns that are also cast

with the wall.

Structural steel framing

Walls for semi-indoor powerhouses

Figure 1-94 shows a design with reinforced concrete framing, a thin cast-in-place

upstream wall and glazing for the downstream wall, the cast-in-place reinforced

concrete crane girders tie the columns (pilasters) and provide support for

the walls above.

Superstructures with structural steel framing offer the distinct advantage that they

can be erected faster than cast-in-place concrete structures, but they have to be

painted and maintained during the plant's life.

Steel framing also saves one column at each contraction joint, i.e., no double column

arrangement, as for concrete structures, is required because any movements

resulting from temperature differential are taken up by sliding in the boli holes.

Similar to the concrete design, also for structural steel designs appropriate

corbels or brackets are needed for the crane girder support. Simple rolled column

shapes, with welded-on brackets offer the simplest, most economical solution (fig. 1-

96). However, a more pleasing effect can be achieved with tapered columns, as

shown on figure 1-95. Such a design costs slightly more, but so does the

concrete design shown on figures 1-92 and 1-93. Considering the lifespan of the

power plants of at least 50 years and more, a small investment for a more pleasing

appearance would appear justified.

Such decisions, however, ultimately may be made by the owner who may prefer to

the simplest, most economical solution, unless convinced by the designer otherwise.

For, it should be remembered that powerplants are not only energy producing

facilities but also places of work for the operating and maintenance personnel and

facilities frequently exposed to visitors and the surrounding communities.

Figure 1-95 illustrates a powerhouse superstructure consisting of steel bents, faced

with 12-inch-thick cast-in-place reinforced concrete walls for the lower half, and

porcelain aluminum panels for the upper half. A row of glass block panels is also

included in the upper wall portion to expose the interior of the powerhouse to

daylight, and also as part of the exterior architectural treatment.

Figure 1-96 illustrates application of precast panel wall along the transformer bank,

and insulated metal wall panels for the other wall, both types supported on

structural steel framing.

(2) Walls for Semi-Indoor Powerhouses. — Semi-indoor type powerhouses are

normally used under conditions when there is insufficient headroom for access at

the erection floor level. This usually is the case for low-head installations when the

units are set relatively high with respect to tailwater and when the maximum flood

tailwater may be close to the roof level. Figure 1-12 illustrates an example of the

former case. In the given example the maximum tailwater is below the service

walkway allowing fenestration in the downstream wall. Because of the relatively

heavy gantry crane and the loads it carries, the wall design is in cast-in-place

reinforced concrete, braced by the roof deck which controls the lateral load effects.

Superstructure wallthicknesses

Figure 1-96. — Kinzua pumped storage powerhouse (conventional unit), Allegheny,

Pennsylvania. Unit: 25 MW; H = 873 ft (266.2 m).

(Courtesy of Harza).

With higher tailwaters, the downstream wall must be in solid concrete as illustrated

on figure 1-11. In this case, the roof was not constructed integrally with the walls and,

therefore, the downstream wall had to be designed accordingly.

(3) Superstructure Wall Thicknesses. — During conceptual studies of powerhouse

layouts, realistic wall thicknesses must be indicated to preclude surprises during

final design. Undersizing of walls initially may require changes in the overall

superstructure width later, causing a chain reaction of changes in other areas.

Smaller wall thicknesses will be required with units of smaller physical size because

of the smaller span for the cranes, indoor or outdoor. The headwall thickness will

be affected by the width of the intake openings and the hydrostatic head on the

wall. Tailwater pressure may control the thickness of the downstream wall and the

end walls.

Minimum wall thicknesses as given below in table 1-2 may be considered for

inicial layouts which, however, should be confirmed by rough calculations (M =

0.1WL; V = 0.5W at support; W = total load) for the uniformly distributed loads, such

as hydrostatic pressures, or for lateral crane loads, tension due to floor slab or roof

restraint, backfill pressures with surcharges and, possibly, others.

With higher tailwaters, the downstream wall must be in solid concrete as illustrated

on figure 1-11. In this case, the roof was not constructed integrally with the walls

and, therefore, the downstream wall had to be designed accordingly.

Superstructure

(3) Superstructure Wall Thicknesses.

During conceptual studies of powerhouse layouts, realistic wall thicknesses must be

indicated to preclude surprises during final design.

Wall thicknesses

Superstructure roofs

Undersizing of walls initially may require changes in the overall superstructure width

later, causing a chain reaction of changes in other areas.

Smaller wall thicknesses will be required with units of smaller physical size because of

the smaller span for the cranes, indoor or outdoor. The headwall thickness will be

affected by the width of the intake openings and the hydrostatic head on the wall.

Tailwater pressure may control the thickness of the downstream wall and the end

walls.

Minimum wall thicknesses as given below in table 1-2 may be considered for initial

layouts which, however, should be confirmed by rough calculations (M =

0.1WL; V = 0.5W at support; W = total load) for the uniformly distributed

loads, such as hydrostatic pressures, or for lateral crane loads, tension due to floor

slab or roof restraint, backfill pressures with surcharges and, possibly others.

Table 1-2. Minimum wall thicknesses for major plants .

Type of wall Minimum wall thickness t, inches

Headwall at integral intake or downstream

bulkhead wall resisting water pressure

48 or 0.1 H

(H = head on wall)Cast-in-place walls without columns, supporting cranes

24

Cast-in-place walls not supporting cranes 12

Cast-in-place walls between columns (pilasters) 12

Insulated precast wall panels supported on framing 5

Brick or concrete block8 (maximum unbraced length or

height 18t)

Concrete columns supporting main powerhouse crane 30 deep

Steel columns supporting main powerhouse crane 24 deep

The information in table 1-2 should be checked against similar existing designs, if

available, and the latter information, if different from information given above,

should be used for preliminary layouts. If information of existing projects is not

Structural steel roofs

Prestressed concrete girder framing

available, the above minimum dimensions can be checked against the principal

governing loads.

6. Superstructure Roofs

a. General. — The superstructure roof is provided for indoor and semi-indoor type

powerhouses and the roof design may, by necessity, be substantially different for

each type of powerhouse.

For outdoor powerhouses, the roof is similar to that of semi-indoor powerhouses,

except is penetrated by the generator housing (fig. l-34).

The primary function of the roof is to protect the interior of the powerhouses

against weather. However, it may also be used to support switchyard structures

(figs. 1-1, 1-35 1-44, and 1-96) and as laydown space for light equipment during

equipment installation for semi-indoor type powerhouses.

b. Roofs for Indoor Powerhouses. — For indoor type powerhouses (figs. 1-91

through 1-95), the roof structure can be of relatively light design, supporting only

snow loads (in climates where such are applicable) and loads expected during

construction and maintenance of the roof. Such roofs are preferably framed, for

expediency in construction, either with structural steel or prestressed concrete

girders. The type of framing may depend on what material is available within an

economical hauling distance to the plant and on the type of framing used for the

crane support.

Cast-in-place concrete beams or girders are not recommended because of the

high cost of shoring and forming.

(1) Roofs with Structural Steel Framing. — Structural steel girders can be used with

cast-in-place concrete walls or, of course, as part of a structural steel frame.

The girders can be seated in blocked-out pockets in the wall, or on corbels protruding

from the wall which reduces the span length. On the other hand, wall pockets

complicate the wall reinforcement and may be less accessible than corbels for

maintenance.

Adequate number of transverse beams shall be provided as lateral bracing in

addition of the deck.

The decks may be of following construction:

a. Steel roof deck with built-up roofing

b. Cast-in-place concrete, 4 to 6 inches thick, placed in steel deck used as forms,

0.75 percent reinforcement for water tightness; 0.5 percent reinforcement if

nonshrink cement used (preferable in cold climates)

c. Cast-in-place concrete deck formed conventionally, forms supported from girders

and beams, otherwise same as item b

d. Precast, post-tensioned concrete flat panels, or channel-shaped elements,

fastened to the girders and topped with reinforced 2-inch-thick cast-in-place

surfacing (this design is used mostly with precast concrete girders)

Figure 1-96, shows a cast-in-place, reinforced concrete deck slab with concrete

pads to support switchyard structures.

In all designs, adequate number at transverse beams shall be provided to obtain

economic spans for the deck type used and also to brace the girders.

(2) Roofs with Prestressed Concrete Girder Framing. — Prestressed concrete girders,

compared with structural steel girders, cost less to maintain because no corrosion-

preventive painting is needed. This aspect should especially be evaluated in humid

areas.

The prestressed concrete girders can be supported in the same manner as the steel

girders, except that seating of girder ends on individual corbels may not be advisable

for safety reasons — a continuous ledge, with shear pockets, is recommended

instead. Usually, a row of two or three cast-in-place cross-beams (diaphragms),

doweled into the webs of the girders, are needed.

The deck types that can be used with prestressed, precast girders are the same as

described under items b, c, and d for the steel girders.

Interior space requirements

Figures 1-97 and 1-98 show prestressed concrete and cast-in-place slab construction

(Bath County Powerhouse) with steel deck as forms — details at a contraction joint

and a girder seat at the wall. The slab was designed for 500 lb/ft2 load to allow for

equipment laydown and vehicle traffic. The slab contained 0.75 percent

reinforcement for watertightness. This amount could have been reduced to, say, 0.5

percent because it was decided shortly before construction to use shrinkage

reducing cement. The approximately 65-foot-wide slab contains no construction

joints without any signs of cracking.

Similar other alternatives, most likely, have been used in other designs. Either

structural steel or precast—prestressed concrete girder construction has given

satisfactory service. Which type to use should be determined on the basis of

availability of the prefabricated material and appropriate cost comparison studies.

1. General

In most cases, once the unit bay width is determined for the water passage

requirements as outlined in section D, the resulting overall length of the

powerhouse usually suffices to obtain the necessary space for station and unit

services.

With minimum provisions for station services, such as offices, personnel and public

service rooms and arcas, it is possible to locate all station and unit service equipment

within the generating/erection bay area as has been done in numerous plants even

with fairly large units.

Figure 1-97. — Roof design with prestressed concrete girders and cast-in-place

slab,

Bath County. (Courtesy of Harza).

With expanded station service requirements, combined with transformer location

either on the draft tube deck or upstream of the generating bay, the need for a

downstream or upstream service bay, respectively, or both, arises.

The service bay length, in most cases, matches the length of the generating bay,

including the erection bay. Such an arrangement simplifies the design (no re-entrant

jogs in the structure), construction (simplified forming with continuous end walls),

and also simplifies access at both ends of the service bay.

A service bay width of about 30 feet enables simple, one-way slab construction

without any beams. Such a design simplifies cable, conduit, and piping routing

and may save in floor heights.

Unit service equipment

Station service

equipme

Figure 1-98. — Roof girder wall seat detail.

The available space may also depend on the powerhouse location with respect to the

dam. Powerhouses located at the toe of dams (figs. 1-24, 1-30, and 1-37) or

immediately downstream of an integral intake with long water passages (fig. 1-4) may

afford provision of relatively inexpensive additional space for service

bays because the foundation (substructure) is already there.

For detached powerhouses (fig. 1-41) the substructure of the generating bay is

extended under the service bay and, therefore, requires additional excavation and

concrete, which may be combined with penstock encasement concrete (figs. 1-42

and 1-46).

2. Unit Service Equipment and Provisions

The unit service provisions include the mechanical and electrical portions for the

control of each unit installed and access to the equipment for installation and

maintenance.

Unit service equipment shall be placed adjacent to the unit and consists of:

a. Mechanical equipment

Servo motors to control wicket gates

Raw water piping with valves, pumps, strainers, etc

Oil, air, and cooling water systems

Fire protection system

b. Electrical provisions/equipment

Common leads and gallery

Low-voltage switches, breakers

Instrument transformers

Lightning arresters, reactors

Station service connections

c. Access and erection provisions:

Passages to equipment, turbine pit

Stairs between different levels

Access to generator housing and exciter

There should be a convenient, free passage between the upstream and

downstream sides at the turbine and generator floor (where such is provided) levels.

3. Station Service Equipment and Provisions

Station Service equipment and provisions are those features that serve the entire

plant and may consist of:

a. Equipment access, unloading and erection bay, usually located at one end of the

powerhouse in a separate monolith, next to the first unit monolith, or other

arrangements

b. Maintenance shops with machine and electrical shop and testing equipment

c. Material storage spaces

d. Draft tube and station unwatering sumps

e. Powerhouse and draft tube cranes

f. Control room, cable spreading room

g. Auxiliary mechanical equipment

Lubricating and insulating oil pumps, purifiers, storage tanks

Raw water pumps and strainers

Air compressors and receiving tanks

Water treatment equipment

Sanitary facilities, septic tank

Fire protection for oil system

Tailwater gauge well

Emergency diesel generator usually provided outside the powerhouse

Heating, ventilating and air conditioning equipment

h. Auxiliary Electrical Equipment:

Main auxiliary power switchboards

Station service transformers

Battery room

Motor generator sets

Station service switchboards for lighting, heating

Communications room

i. Personnel and Public Service Facilities

Offices

Wash rooms, lockers

First aid room, laboratory, dark room

Visitors reception and display room, rest rooms

Elevators, stairs, corridors

The provision of the various facilities will vary from project to project depending on

the plant owner's needs.

4. Sizes of Individual Equipment Rooms

There is no standard for sizing of the individual space provisions. The room size

will depend on the size and number of equipment installed and the space needed

for the necessary erection and maintenance clearances.

The space requirements will be governed by the plant size and owner's preference. It

also depends whether a plant is built by public or private sector. The private sector

generally invests less for provisions other than the generating plant equipment

that is necessary to make the plant operable.

The public sector, on the other hand, will generally make provisions to receive the

public, and other space provisions will tend to be more generous.

Table 1-3 gives approximate space provisions for three plants, the 52-MW (2 units)

Sam Rayburn plant in Texas, 200-MW Angat (4-unit pit-type powerhouse) in the

Philippines, and the 1000-MW Karun (4 units) plant in Iran.

Table 1-3. - Room sizes for three power plants. Width/length in meters.

Room or space Sam Rayburn Angat Karun

1. Air compressors,

receivers

8.2/33.5 7/10

2. Air conditioning, ventil. 4.6/29.6 5.5/55 11/145.5/31/5

3. Battery 4/6.1 0 -4. Carrier current 3.8/7 5.5/14 11/115. Cable gallery 8.9/16.8 5.5/62 -6. Control 9.2/18.3 5.5/30 11/197. Diesel generator 4/7 outside 7/238. Electrical shop 3.7/9.8 4.5/9 5.5/189. Erection bay 12.8/14.6 15/16 19.5/2510. First aid, toilet 4/4.6 5.5/8 6.5/8.511. Machine shop, tools 9.2/12.2 9.9 11/1912. Mech. station eqpt. turb. floor 5/47 11/5313. MG room 4/5.5 in open in open14. Offices 4/15.2 5.5/53 8/2015. Oil storage 5.5/30.5 10.15 11/1916. Sewage treatment 3.7/6.4 5/5.7 6/717. Storage 6.1/13.1 6/9 5/17

6/5018. Switchgear, electrical eqpt. 8.2/30.5 5.5/49 11/82

3.5/919. Toilets, janitor 6.l/10.7 5.5/10 11/1520. Unloading bay - 10/16 7/19.5

7/1221. Unwatering pumps 3.1/4.6 4.3/7 6.5/1022. Visitors facilities 9.2/10.l - -23. Water treatment 6.l/10.7 8/10.5 -

The vertical distance between individual floor levels below the access (draft tube

deck) level will be dictated first by the level of the turbine floor and, second, by the

level of the lowest floor provided (usually in the downstream service bay).

The generator barrel height may range between 15 and 20 feet which represents

the floor height between turbine and generator floors.

The number of floors provided below the entrance level draft tube deck for indoor

powerhouses, and the roof for semi-indoor powerhouses, will generally depend on

the available distance between the access levels and the lowest floor level selected.

For rooms housing station or unit service equipment, the floor height should not

be less than, say, approximately 15 feet to allow sufficient clearance for cable

trays, ventilating ducts and piping runs. Recommended height is in the range of 17

to 20 feet.

Floors supporting office and general non-equipment spaces may be limited to a

height between 13 to 15 feet. The clear height to the suspended ceilings should

be about 9 to 10 feet.

The clear structural height for all floors will be the above recommended heights less

the structural floor thickness — to the bottom of beams, or slabs of floors without

beams.