4.2.2 Design Considerations

Sleeves will not affect the design of headed anchor bolts subjected to tensile

loads, because the tension in the bolt is transferred to the concrete via the anchor bolthead and not by the bond between the bolt and the concrete.

In the design of an anchor bolt with a partial sleeve, the embedment depth of thebolt should be determined as recommended in Chapter 3. However, the distancebetween the bottom of the sleeve and the anchor-bearing surface should be sufficientto ensure that the concrete below the sleeve will not fail in shear from tensile loads

causing the bolt head to snap through the sleeve. The minimum distance betweenthe bottom of the sleeve and the anchor bearing surface should not be less than 6­bolt diameters or 6 inches (150 mm), whichever is greater.

The applied shear force may be resisted by anchor bolts only if the sleeves arefilled with grout. If the sleeves are not filled with grout, the anchor bolts will not beeffective in resisting the applied shear force. The sleeve, combined with isolation ofthe bolt from the grout, is desirable to prevent short radius flexing of the anchor boltdue to a horizontal component of the vibration or as a result of thermal growth of theequipment and is an effective way to avoid the most common failure mode ofcompressor anchor bolts.

4.3 PRETENSIONING

4.3.2 Development

Pre tensioned anchor bolts should be designed for an embedment development of

at least 80% of their ultimate capacity (0.8 fut).

4.3.3 Methods

Methods that may be utilized to apply the required pre-load are as follows:

a. Hydraulic jacking

Hydraulic jacking is the most accurate method and is recommended if thehydraulic equipment is available and if the physical clearances that exist aroundeach bolt permit its use.

b. Turn-of-nut

Turn-of-nut is the easiest to perform by a construction crew and gives a

reasonably accurate result provided that:

* Conditions of grout and base plate can give a consistent "snug-tight" resu It

* Stretching (spring) length of bolt can be accurately determined.

Certain conditions make it desirable to pretension anchor bolts to enhance the

performance of the bolt or the performance of the system.

The recommended pretension load is one-third the tensile strength of the boltunless otherwise required.

Nut Rotation _ 360 I AfT_ t t Ie

E Ad

where:

(4.1)

Anchor bolts may need to be retightened one week after initial pretensioning tocompensate for pre-load losses from strain relaxation within the system.

4.3.1 Pretension Applications

Pretensioning of anchor bolts should be used for the following situations:

bolt stretch length, in (mm)tensile stress area of bolt, in2 (mm2)

desired tensile stress, ksi (kPa)

bolt threads per unit length, thds/in (thds/mm)

elastic modulus of bolt, ksi (kPa)nominal bolt area, in2 (mm2)

a. Tall process towers sensitive to wind (as a rule of thumb these are towersover 100 feet (30 meters) tall or with a height-to-diameter ratio of 15 ormore).

b. Reciprocating compressors or other pulsating or vibrating equipment.

c. High-strength anchor bolts (to prevent load reversals on bolts susceptible tofatigue weakening).

4-3

If the bolt is to be retightened to compensate for any loss of pre-load, thismethod requires that nuts be loosened, brought to a "snug tight" condition, and thenturned the number of degrees originally specified.

c. Torque wrench

Torque wrench pretensioning provides only a rough measure of actual pretel1sionload but can be the method of choice if equipment for item a. is not available and

4-4

stretch length of anchor cannot be fixed as required by the "turn of nut" method.

The Industrial Fastener Institute recommends the following formula for determiningthe proper tightening torque.

Pretensioning should only be implemented when the stretching (spring) length ofthe anchor bolt extends down to near the anchor head of the bolt. On a typicalanchor bolt embedment, as a pre-load is placed upon the bolt, the bolt starts to shedits load to the concrete through its grip (bond) on the bolt. At that time, there existsa high bond stress at the first few inches of embedment. This bond will relieve itself

over time and thereby reduce the pre-load on the bolt. Therefore, it is important thatthe bond be prevented on anchor bolts to be pretensioned. Bond on the bolt shaft

can be prevented by wrapping the shaft with plastic tape or by heavily coating thebolt with grease immediately before placing concrete. Grout must not be allowed to

bond to the anchor bolt. Tape the portion of the anchor bolt through the grout zoneand to within one inch (25 mm) of the bolt head, below the sleeve. (See Figure 4.3)

TAPE

"

~I::C

UI­1-"~zI-~cn...J

T.O. ROUGHCONCRETE

TAPE _

Figure 4.3: Anchor Bolt Stretch Length

NOTE; STRETCHING LENGTH = THAT PORTION OF ANCHOR BOLT

ALLOWED TO FREELY STRETCH.

-~

~----

b. Second stage should apply full pre-tension load to all bolts.

a. First stage should apply 50% of full pre-tension load to all bolts.

FDN.

A. BOLT

GROUT --

BASE PL. -

Anchor bolts should be tightened in two stages:

Bolts should be tightened in a criss-cross pattern. (See Figure 4.4 for circularbolt pattern sequence.)

4.3.5 Tightening Sequence

(4.2)

tightening torque, kip-in (kN-mm)torque coefficient, dimensionlessfastener diameter, nominal, in (mm)

bolt tensile load, kips (kN)

"K" varies from 0.06 to 0.35 (use 0.20 for typical anchor bolt)

where:

TKDP

T = KDP

4.3.4 Stretching Lengths

Tape or grease should not be applied closer than one inch (25 mm) to the anchorbolt head or anchor plate. Anchor bolt sleeves should not be positioned closer than6D to the bolt head to preclude failure by the head of the bolt pulling through thesleeve.

Sleeved anchor bolts to be pretensioned should have that portion of the boltbeneath the sleeve taped or greased.

The stretching length of the bolt which is pre-loaded within the elastic range actsas a spring in clamping the base plate down against the foundation.

:112 ..--.. 5--- ......••..

g/ '\!f \

4£ : •. C EQUIPMENT\ : /J

10\ . /~

\'/"11'-2 TIGHTENING SEQUENCE

Figure 4.4: Anchor Bolt Tightening Sequence

4-5 4-6

Figure 4.5: Detail for Anchor Bolt for Vibrating Equipment

4.4 CONSIDERATIONS FOR VIBRATORY LOADS

4.4.1 Sleeves

Provide sleeves on all anchor bolts installed on vibratory equipment and isolatebolt from any grout (see Figure 4.5).

4.4.2 Pretensioning

Pretension all anchor bolts installed on vibratory equipment, unless specificallyprohibited by the manufacturer.

This pretensioning (stretching) of the anchor bolts creates a spring effect thatwill absorb the vertical amplitude of the vibration without fatiguing. This springeffect also serves well in clamping the equipment base against the grout without thenut loosening if the amount of anchor bolt stretch exceeds the vertical amplitude ofthe vibration.

4.5 CONSIDERATIONS FOR SEISMIC LOADS (ZONES 3 AND 4)

Anchorage capacity, including capacity ofreinforcement, must exceed minimumspecified tensile strength (based on fut) of the bolt to ensure that any reserve capacityof the bolt can be utilized and that the failure mode will be ductile and in the bolts.

Friction capacity from gravity loads shall not be considered effective in carryingany seismic lateral loads.

Friction capacity may be considered if anchor bolts are pretensioned to twice thecalculated seismic uplift force. Friction may then be considered, except frictionshall not exceed 50% of that provided by pretension loads.

4-7

ANCHOR BOLTSLEEVE

PRE-TENSIONED ANCHOR BOLTSIN ACCORDANCE WITH VENDOR'S

INSTRUCTIONS OR DESIGN DWG,

ASE PLA TE

FILL ANCHOR BOLTSLEEVE WITHELASTOMERICMATERIAL.

4-8

Ad

Aefr

A"

A"AI

NOMENCLA TURE

nominal bolt area

effective anchor bolt area for resisting tension

the area of vertical pier reinforcement per bolt

area of cross-section of one leg of tieanchor bolt tensile stress area

Jl

r/J

r/J.

r/J2

friction coefficient

strength reduction factor

strength reduction factor for tension load

strength reduction factor for shear load

BC = bolt circle diameter

Bn = nominal bearing capacity

D = fastener diameter

E = elastic modulus of bolt

f I = actual tensile stress

FI = allowable tensile stress

f v = actual shear stress

F v = allowable shear stress

f y = anchor bolt yield stress

F y = minimum specified yield strength of reinforcement steel

K = torque coefficient

bolt stretch length

M = maximum moment on vessel

n = number of legs in the top 2 sets of ties resisting the shear force (V u)N = number of anchor bolts

P = bolt tensile load

P n = nominal tensile capacity of bolt

T = tightening torque

Tie = bolt threads per unit length

T u = factored tensile load per bolt

V u = factored shear force resisted by anchor bolt(s) located in the pierV UJ = factored shear force per bolt

W = minimum weight of vessel

A-I A-2