Eric Sobel 2941 Fairview Park - Penn State Engineering ... · the architecture almost identical,...

Eric Sobel 2941 Fairview Park Structural

4/20/2003 Fairfax, VA Prof. Geschwindner

1:33

Executive Summary: 2941 Fairview Park is a 16-story speculative office tower located in Northern Virginia. The

steel superstructure is clad in a metal panel / strip window curtain wall on the East and West sides,

and a curvilinear precast / strip window curtain wall on the North and South faces. An architectural

canopy adorns the building and casts a distinctly modern shadow over the adjacent Capital Beltway.

The main tower has a slender footprint, with an even more slender central core running down

its longitudinal axis (North-South). This core is home to not only architectural amenities such as

elevators, bathrooms, and stair towers, but also to braced frames that serve as the main lateral force

resisting system. In the East-West direction, these braced frames are augmented by moment frames

to reduce drift. The bottom three floors have additional area on the Eastern side of the building that

is defined within the curvilinear context of the North face of the building.

The goal of my redesign was to determine how a precast concrete system compares in terms

of cost and serviceability to the existing composite steel structure. Design constraints were to keep

the architecture almost identical, meet or exceed all strength requirements set forth by code, limit

drift to a tolerance defined by the building envelope.

Two very different forms of structural concrete were used in the redesign of 2941 Fairview

Park. The gravity system was composed of a precast concrete frame of hollowcore planks, inverted-

tee girders, and 4-story columns. The lateral system consisted of site-cast concrete shearwalls

located within the building core. Spread foundations were used beneath columns, while a matt

foundation was used beneath the core of the building.

Structural design was performed to get typical sizes and identify general behavioral

response. A new building façade was chosen and designed to better facilitate both the structural

system and the construction schedule. Finally, a construction plan was devised upon which

scheduling and estimates were based.

Overall, the redesign proved to be a success. All designed members met or exceeded

minimum design strengths. When compared to the existing structure, inter-story drift was reduced

significantly. The construction schedule was shortened by four weeks, and rough estimates showed

that there was not a significant difference in cost after the schedule reduction was considered.



2:33

Background: Fairview Park is one of the premier office parks in Northern Virginia. Located on a 200 acre

site in Fairfax, Virginia, Fairview Park lies just inside the Capital Beltway and straddles US Route

50. Fairview Park Marriott, located in South Fairview Park, provides lodging and conference

facilities. Zoned as “dense commercial”, development rights for the complex include over four

million square feet of office and miscellaneous space. As part of the development, interchanges

from both US Route 50 and the Beltway were created, thus providing outstanding vehicular access.

Fairview Park Office Complex 2941 Fairview Park represents the first large office tower in the Fairview Park complex. The

building consists of two primary structures. First, an office tower of 361,101 gross square feet rises

sixteen stories above grade. Second, a five level parking structure with 406,766 gross square feet

adjoins the building.

2941 Fairview Park is a modern looking office tower with a slender profile and architectural

ornamentation atop its penthouse. The building towers above the trees, and presents a modern look to

onlookers from the Beltway. The building envelope is sheathed in a metal panel / strip window

curtain wall on its East and West sides, while a curvilinear precast concrete / strip window curtain

wall adorns its North and South faces.



3:33

Views from the North, East, South, and West In plan, the building has two main shapes. The lowest three floors have an almost circular

appearance. There is a café on the ground floor, and an upscale restaurant in the basement. A three

story atrium in the Southwestern corner greets visitors as they enter the building. The remainder of

the space is occupied by offices. There is a connector bridge on the ground level between the

building and the parking garage to the West.

Above the third floor, the office tower rises as a slender rectangle, with curved elements on

its North and South ends. Small exterior decks on the 14th floor in the Northwest and Southeast

corners of the building add interest to the building shape. The core of the building, measuring only

20’-4” in the east-west direction, includes the elevator shafts, MEP rooms, restrooms, and stair

towers. Exterior bays extend 39’-10” from each side of the core. Floor-to-floor height within the

tower is 13’-8”. Overall footprint dimensions in the tower are 232 feet in the North-South direction,

and 102 feet in the East-West direction.

Typical Tower Floor Plan



4:33

On the west side of the building, a large plaza paved with granite welcomes guests into the

atrium. A small, one-level parking structure exists below the plaza. An ornamental steel canply

adorns the building to accent the main entrance.

Plaza Atrium Great effort was spent on landscaping the 34 acre site. Existing mature hardwood trees were

both preserved and transplanted to create a lush landscape. These trees are complimented by natural

stone walls, rock gardens, paved walking paths, and several small waterfalls. Walking trails allow

tenants as well as nearby residents to enjoy the Fairview Park North community. Fairview Park Lake

is accented by a water fountain, and boasts a large duck population.

Waterfall near the Restaurant Landscaping at the Atrium Fountain & Ducks



5:33

Players: Owner: 2941 Fairview L.L.P.

Developer: The David Orr Company

Architect: Boggs & Partners

Contractor: Clark Construction

Structural Engineer: Cagley & Associates

Geotechnical Engineer: ECS

MEP / FP Engineer: Allen & Shariff

Civil Engineer: VIKA

Curtain Wall: PCC Consultants

Landscaper: EDAW

Site: Tree preservation was a major goal in the construction of this project. Clearing limits were

clearly marked on site. Root pruning and installation of protective fences was performed under the

supervision of the Project Arborist before any clearing and grading could be done. Some trees were

transplanted to the preservation area.

The geotechnical report showed that in most locations the soil could support the weight of the

proposed building. In some cases, foundations would have to extend further down to reach

undisturbed soil. The soil had good subsurface drainage qualities and showed adequate strength and

cohesiveness, but was susceptible to erosion.

Since the building is located outside the District of Columbia, it is not governed by the 1899

Building Heights Act.

MEP / Telecommunication Systems:

Mechanical, electrical, and plumbing design was performed by Allen & Shariff. The electrical

system consists of circuit breaker panelboards (typically 480/270 and 208/120 volt) on each floor in an

electrical room with K-4 rated, dry type transformers for the lower voltage. Mechanical equipment and



6:33

large motor loads operate at 480 volts. Lighting operates at 277 volts, and convenience receptacles are

at 120 volts.

A chilled water VAV air conditioning system is used to cool the building. 65 ton units are

provided for each floor to meet individual cooling demands of the occupants. Additional cooling units

are located on the penthouse level.

Series fan powered VAV boxes are provided for perimeter office areas with electric heating coils

to meet the heating needs of occupants. A penthouse mechanical room houses additional equipment. A

computer regulated emergency Management System monitors and controls all the mechanical

equipment.

Telecommunications equipment is centered about a telecommunications room located in the core

on each floor. By providing a dedicated room for each floor, tenants are able to have precise control

over their equipment, while minimizing security risks.

Building Envelope: The building envelope is a combination of detailed

glass curtain wall strip windows, metal panel, and precast

concrete. Clement Enyeji of PCC Construction

Components, Inc. was the consulting engineer for the

building envelope. He oversaw the design and

construction of the curtain wall, including 76,000 square

feet of insulated glass, 60,000 square feet of Reynobond

Panels System, 10,000 square feet of insulated metal

panels, as well as entrance doors. Precast concrete is used

on the North and South faces, while metal panel is used on

the East and West sides.

Curtain Wall

In section 01190-2 of the building’s specifications, inter-story drift is limited to 0.0063 for

seismic events, and 0.0025 for wind gusts. These numbers are most likely borrowed from section

1633.2.4 of the 1997 Uniform Building Code.



7:33

Roof: The roof structure is comprised of a modified bituminous membrane. 1½” 20 gage steel roof

deck spans between structural members. The deck supports a cementitious fill that is sloped ¼” per foot

for drainage. The roof membrane, 2” insulation board, a filter fabric are all sandwiched between the

cementitious fill and concrete pavers. Adjustable pedestals allow for leveling of the concrete pavers.

The roof structure above the lower portion of the building is somewhat different that that of the

penthouse roof. Here, rigid insulation is placed above the metal deck. The insulation is sloped to allow

for proper drainage. The rigid insulation is protected by a layer of ballast.

Transportation:

2941 Fairview Park employs both elevators and stair towers to facilitate vertical transportation.

Five elevators, including a freight elevator, service floors B-1 through the penthouse. Four additional

elevators services floors B-1 through floor 9. The 10th floor houses an elevator machine room for the 4

elevators that service only the lower portion of the building. The elevator contractor was Lerch Bates.

Two stair towers provide emergency exit from the building. One stair tower is continuous from

the ground to the roof, while the other is disjointed at level 3. Structural design was originally

performed so as to allow additional stairs to be added at the tenant’s discretion.

Fire Protection:

Spray-On Fireproofing

The entire office building is protected by an

automatic sprinkler system. An additional security

system is also provided that will be fully supervised,

addressable, non-coded, voice alarm system conforming

to the requirements of both the BOCA High Rise Code

and the Americans with Disabilities Act. The fire control

room is located next to the atrium on the lobby level.

All structural steel members (not including deck)

are protected by spray-on fireproofing. Firestops are used

between the slab and the metal panels.



8:33

Costs: The total cost for the project, including land development, design, construction, and financing

approached $73 million. $11.9 million was spent to acquire and develop the 34 acre site. Construction

costs, including the adjacent garage and tenant allowance, comprised nearly two-thirds of this budget,

with a total of almost $47 million. A further breakdown in this area showed the office tower cost $28.4

million, the parking garage $8 million, and $10 million was carried for tenant allowance. Construction

costs came out to be $128.39 per square foot of rentable space.

Design costs for this project totaled $2.27 million dollars, and represented approximately 3% of

the total costs. Administration fees, including contingency, insurance, and development fees, totaled

over $8 million. A contingency of $2 million was carried throughout this project. Financing of $3.65

million was provided for this job, including $3 million in construction loan interest.

Codes Used: Original Design:

BOCA 1996

Virginia Uniform Statewide Building Code

AISC Manual for Steel Construction, Allowable Stress Design, 9th Edition

ACI 318-95

ASCE 7-95

My Analysis:

AISC Manual for Steel Construction, Load and Resistance Factor Design, 3rd Edition

ACI 318-02

ASCE 7-98

Wind Loading: Wind loading was calculated in accordance with Chapter 6 of ASCE 7-98. Values obtained

through this analysis differed from those on the plans, which were calculated with ASCE 7-95, by as

much as 2 psf. Differences were most acute above 100 feet in height. Further differences were

observed when procedure 6.5 of ASCE 7-98 was used to determine wind loading. This procedure



9:33

accounts for building dimensions. Results obtained from using this procedure were greater in magnitude

in the North-South direct, and diminished in the East-West direction.

East-West Wind North-South Wind Seismic

Seismic Loading: A seismic analysis was performed using the Equivalent Lateral Load Method of ASCE 7-98,

Chapter 9. The main lateral force resisting system was taken to be braced frames without special

seismic connections. Calculations performed showed failure occurred in the connection before the

member, thus confirming the use of an R value of 3. Originally, a seismic response coefficient of 8 was

used, which accounts for variations between calculated and existing values for seismic loading.

Upper level mechanical rooms, including the penthouse and intermediate roof level, house heavy

pieces of mechanical equipment. The mass associated with this equipment greatly increased the seismic

loading.



10:33

Existing Gravity System:

A typical tower floor plan is shown below. Composite steel girders span from the central core to

the exterior supports and form 20’-0” bays running North and South. Composite beams, typically three

in number, span between the girders and support the deck. All steel members are grade ASTM A572

steel. Steel deck is 19 gage with 2” flutes. A 3 ¼” lightweight concrete slab covers the steel deck (5 ¼”

total depth). ¾” A108 shear studs are placed along the beam to resist longitudinal shearing forces, thus

enabling composite action between the beam and slab.


Existing Lateral System: The lateral system in the East-West direction is a combination of braced frames and moment

frames. Five braced frames of nearly identical size and member constitution exist in the building. All

five braced frames adjoin either stair or elevator towers through the 10th floor, at which point one frame

is left without an accompanying opening. Braced frames are shown in red above.

Five major moment frames also exist in the East-West direction. The three moment frames are

approximately forty feet in depth. The moment frame along grid line 2 runs the full width of the

building (about 100 feet), and the moment frame along grid line 10 runs approximately fifty feet. The



11:33

moment frames all rise to the top of the penthouse. Additional moment frames exist within the building,

but are not of significant size. Moment frames are shown in blue above.

The lateral system in the North-South direction is comprised of two braced frames in the tower,

and a smaller braced frame in the lower portion of the building (East side). The two braced frames lie

on opposite sides of the core (grid lines C and D), and are offset from one another so that each one is

adjacent to a different elevator tower. Each of these braced frames rises the full height of the building,

from the foundations through the high roof. Another braced frame lies along Grid line H, and terminates

at the roof of the lower portion (4th floor). This braced frame increases stiffness to the lower portion of

the building, which is an outstanding leg.

Load Path: A load path was found to exist between the load collecting elements and the main lateral force

resisting systems. The curtain wall was designed and detailed to transmit lateral forces into the floors.

The slab acted as a diaphragm to transmit the load to the main lateral force resisting system. Detailing

near the braced and moment frame columns revealed no weak points such as soft joints or gaps.

Existing Foundations: The basement floor is comprised of a 5” normal weight concrete slab on grade. Strip footings

were used beneath basement walls. Spread foundations were used beneath most columns in the existing

design. These foundations varied in size from 4’x4’x18” to 13’x13’x48”. The top of interior footings

was at 1’-0” below grade, and 2’-0 for exterior footings.

In addition to these typical foundations, buried mats were poured beneath the East-West braced

frame foundations. Two of these mats, with thickness of 5’-0”, were buried so that the top of the mat

was 9’-0” below grade. Another 5’-0” thick mat, slightly smaller and located in-between the other two,

was buried 12’-0” below grade. The braced frames were extended into the mat foundation by way of

concrete encasement. This system was employed, with the approval of the geotechnical engineer, to

resist uplift forces that could occur during heavy wind storms.



12:33

Plaza & Parking Garage: The entrance plaza is a concrete structure comprised of site cast one-way slabs spanning between

reinforced concrete beams. The lateral system consists of moment frames in each direction. A stairwell

in the North-West corner of the plaza provides additional lateral support.

The parking garage is a site cast concrete structure with one-way slabs spanning between post-

tensioned beams. Moment frame provide lateral resistance. Stair towers add to the lateral stiffness of

the parking garage.

Pros / Cons of Composite Steel: Composite steel was chosen as the structural system for several reasons, including both

economic and serviceability concerns. Structural steel is quick to erect, which cuts down dramatically

on construction costs. The dead weight of a steel structure is relatively small, minimizing foundation

costs. Composite steel is also one of the easiest systems to retrofit to meet individual tenants’ needs.

With all the advantages of structural steel, there are a few drawbacks. The fabrication time

necessary to acquire structural steel requires designers to choose shapes and quantities early in the

design process, when not all loads (such as mechanical equipment) are known precisely. Changes to the

design, such as member size changes and architectural dimension changes are fairly difficult to

implement because of the fabrication process. The low dead weight can cause vibration to be an issue

when designing either long spans or structures without a lot of mass. Low dead weight may also cause

wind uplift may be a problem. This requires a more complicated (and expensive) foundation system.



13:33

Redesign Overview:

Although concrete was originally determined to be a significantly more expensive structural

system, only site-cast concrete options were evaluated. I therefore chose to undertake a redesign using

precast in addition to site cast concrete.

The redesign of 2941 Fairview Park utilized two very different forms of the same basic

material: structural concrete. Portions of the building, including the core and the foundations used the

most common type of concrete, site-cast concrete. Concrete structures cast in situ are typically

thought of as being both stable and monolithic; two very important properties. The remainder of the

building, including the bulk of the gravity system and the building envelope, employed a more

industrial form of concrete: precast concrete. Precast concrete members are fabricated in an off-site

production facility, then transported to site for quick erection. Higher quality members and reduced

erection time offset the costs associated with off-site production and transportation.


Precast Hollowcore Planks:

Precast hollowcore planks comprised the bulk of the floor system. The planks were sized

through use of design tables in the PCI handbook. 4’-0” x 8” lightweight plank was chosen for typical

spans, which were approximately 20 feet. Wider plank could be selected so as to reduce erection costs,



14:33

however many manufacturers are not able to produce anything wider than 4’-0”. Some 2 foot wide

planks were also used for more complete floor coverage.

28-day concrete strength for the planks was 5,000 psi, and initial strength was 3500 psi. Strand

pattern 66-S was chosen for the superimposed service loads. Initial camber was 0.5”, and long-time

amber was .6”, as read from the PCI design handbook design tables.

Hollowcore planks were grouted and reinforced so as to transmit lateral loads and achieve

diaphragm action. The plank ends rested upon bearing pads that were designed to transmit vertical

loads, but minimize displacement restrictions that could result in induced stress due to shrinkage. Weld

plates were used to connect the tops of the planks to the girders. Shrinkage occurs in the bottom half of

the members because this is where the tendons are, and is therefore not a problem.

Detail at Hollowcore Plank / Inverted-Tee Interface A thin slab was poured over the precast floor to level the surface. Estimated depth of this slab

was 1” at the ends of the planks (less at mid-span due to camber), just enough to cover the tops of the

cambered members, but not enough to require reinforcing or add considerable weight to the structure.

The weight of this slab was assumed to be 7 psf, as recommended by Mr. Mark Taylor. The topping

slab was not reinforced and is therefore allowed to crack. Cracking in this slab is not important because

it is non-structural and will be covered by a finished floor.



15:33

Precast Inverted-Tee Beams: The hollowcore planks spanned between inverted-tee beams spaced at 20’-0” on center. These

beams spanned between the building core and the exterior of the building in the tower (and between

columns in the lower portion). The inverted-tee beams were prestressed with straight tendons so as to

meet the structural requirements set forth by ACI 318-02 for uncracked sections.

A higher strength concrete was chosen

instead of draping tendons to simplify

production. Initial concrete strength was also

minimized to reduce the expense of admixtures

required to achieve high strength before transfer.

In practice, coordination between the architect,

structural engineer, and precaster would allow

for the use of fewer strands and a lower concrete

release strength, which would reduce the pre-

production cost of the precast girders.

Typical Inverted-Tee Beam

Inverted-tee beams rest upon bearing pads on corbels in the columns. The tops of the inverted-

tees were welded to angles which were in turn attached to the columns. This allowed transmission of

any lateral loads required to keep the column stable (as required to resist moment induced through

eccentric loading). The angle/bearing pad connection did not restrict lateral movement of the bottom of

the tee, thus no induced loading was caused by shrinkage.

Detail at Exterior Column Beam End Detail



16:33

Girders that framed into site cast walls used a

proprietary connection so as to avoid complicated

corbel formwork. The BSF “Invisible Corbel”

connector, manufactured and distributed by JVI-Inc.

was chosen due to simplicity of construction. This

connector employs two metal pieces; a bearing box and

a knife-plate. Site welding is not necessary, speeding

up erection dramatically. The box is cast into the

girder, and the knife-plate is attached to the supporting

member. All BSF connectors were 200/30.

BSF Connector

The “Invisible Corbel” connector was also used when girders were brought into circular

precast columns. This connection is most easily seen on the exterior of the building on the lower

levels where the columns reveal themselves by protruding from the skin of the building.

One major drawback of the BSF connector is that it is a gravity only connection, it can not

handle torsion forces resulting from unequal loading patterns. Temporary shoring, which can be

expensive, must be provided during construction unless it can be guaranteed that no torsional loading

will occur. I therefore chose to use haunches on the opposing end of all beams that used the BSF

connector to add some torsional support. Planks were loaded onto the beams from the interior support

(BSF), alternating from each side every few planks, to minimize torsional loading. Analysis showed

that more that seven planks could be erected on one side prior to instability.

Dapped Beam End

In three locations per floor (occurring

in the North and South regions near the

center of the building), one beam end must

bear upon another beam. Beams running into

the North-South girders were dapped to

simplify the connection. Both members were

check for strength requirements, and rebar

was sized appropriately.



17:33



18:33

Precast Columns: Column locations were determined based on existing column locations to maintain the

architect’s original plan. Column sizes were chosen based on the least possible dimensions without

required excessively high material strength, reinforcing, or prestressing. It is the engineer’s intention

to avoid the use of column wraps so as to eliminate material and labor costs. Column dimensions were

kept constant throughout the building to present a more uniform architectural element. Once designs

were submitted to the architect for review, it would be up to his discretion if he wanted to increase the

column sizes to match existing column wraps.

Architecturally exposed columns are only possible with precast due to the high-quality of their

formwork. Precasters typically use steel forms that impart only minor blemishes in the concrete

surfaces. These flaws, as well as those that may occur during transportation, can be eliminated with

proper finishing techniques.

All columns are 20”x20”, with concrete strength, reinforcing patterns, and material weight

varying by location and elevation. Lightweight concrete was used in the upper floors to reduce the

dead weight of the structure. Lightweight concrete was not used in the lower columns (which carry

greater axial compression loads) to minimize column shortening. Reinforcing patterns were kept

constant, with only the size of the bar changing.

To minimize the number of pieces that need to be erected, precast columns are typically

brought to site in sections that span several stories. I chose to use 4-story columns because that

allowed for the least number of pieces without overly complicating the transportation process.

Corbels were cast into the columns to allow the inverted-tees to bear on them. Because the

corbels produce an eccentric loading from the beams, additional analysis had to be done to insure

rotational and lateral stability of the columns. PCI design handbook tables were used to construct

loading spreadsheets for all columns designed. Moments were obtained at eight different locations

throughout the column, and served as the applied moment in the column design process. The

maximum lateral force required at each level was also determined. Beam to column connections were

designed based on these lateral forces.

The loadings for each column (4-story element) were then taken to ADOSS PCA Column for

analysis. Unfactored loads were used so as not to create a problem between ACI 318-02 and 318-05



19:33

(which PCI Column is based upon). To be conservative, the largest moment and the largest axial force

were each used to design the column. Columns were designed so that all moments were less than the

balanced moment, therefore insuring no chance of a failure due to insufficient axial compression.

Corbel dimensions were

determined based on PCI Design Tables.

Reinforcing for the corbels, as well as

the beam ends and the dapped beam

ends was determined through the use of

PCHELP, a piece of software developed

by LeapSoft that replicates formulas

used in Chapter 6 of the PCI Design

Manual.

Corbel Detail

Bearing Pads: Neoprene bearing pads were chosen to serve as the main bearing element throughout the building

for several reasons. Unlike metal bearing plates, neoprene will not rust. Rusted plates can exert enough

horizontal force to restrict movement due to shrinkage, thereby inducing large forces into the structure.

Neoprene pads permit ample rotation so as to behave similar to the analytic model of simple support.

They also help to distribute bearing forces more evenly over a large area.

Building Core: The building core is composed of concrete cast in situ. This was done to increase stability

during erection, as well as ‘tie’ the building together. The walls in the building core, shown in red

above, were considered as shear walls and form the main lateral force resisting system in both

principle directions. Site-cast concrete floors were used in this section of the building to better

facilitate the large number of openings for mechanical, electrical, and plumbing equipment. Deep

beams were cast in-between walls and columns along grid lines C and D to add continuity and

stiffness to the building core (shown in light blue above).



20:33

Lower Portion: The lower portion of the building is home to both precast and site-cast concrete members.

Columns and beams running North-South along Grid Line H were cast in situ so as to create a moment

frame. Girders running East-West from this moment frame, as well as the planks were precast. This

system was chosen to add stiffness in the North-South direction to this portion of the building.

Lower Portion of Building Currently, a braced frame is used along Grid line H between two columns. This significantly

hinders rentable area, especially on the first floor, where a cafe is located. A moment frame was

favored over a shearwall for these reasons. At this location, the building is only three stories tall, so

large moments are not present and can be accommodate by economic members. The moment frame

runs in the same direction as the planks, so little gravity load is imparted on these members.

A control joint was placed along Grid Line G to separate the two portions of the building and

keep load from the tower from flowing into the moment frame through the floor diaphragm.

Movement in the North-South direction was made independent between the two sections. Movement

in the East-West direction was kept dependant so as to alleviate the need for a lateral system in this

direction in the lower portion as well as eliminate the possibility of pounding.

This joint was accomplished through the use of slotted connectors. Teflon bearing pads, which

have a very low coefficient of static friction, were specified for these bearing connections so as to

minimize friction forces.



21:33

Foundations: Relatively high-quality soil allowed the use of spread footings as the main foundation system.

Due to the increase weight of the concrete structure, there was no longer a net uplift force, and

therefore buried foundations were not necessary. Foundation sizes increase from roughly 9’ x 9’ to

12’ x 12’ for typical columns. A large 40’ x 130’ x 5’ mat was cast beneath the core of the building.

This foundation system was chosen to simplify formwork costs and create a foundation for the

shearwalls that would behave rigidly.

Spandrel Beams: The spandrel beams were designed to perform the functions of both spandrels and exterior wall

panels. This reduces the number of members and simplifies curtain wall connection details. The

spandrels bear on column corbels, and are connected via connector plates to transfer tensile loads

generated by the floor diaphragm. Since the bulk of the mass is in the beam portion, this member is

inherently stable. Please refer to the Building Envelope section of this report for more information.

Building Ends: The curvilinear portions of the building, just inside the North and South faces were not

conducive for precast plank members due to their tapering spans. The spandrel and interior beams

both had ledges which were able to support formwork. Metal deck was cut to shape and placed so it

was bearing on the ledges. Rebar was laid down, and the small slabs were cast. The thickness of the

slab led me to believe that they would not be a concern for either fire-rating or acoustics.

Building Plan Section



22:33

Fire Performance: Concrete construction, be it precast or site-cast, is inherently more fire-resistant than steel

construction. This is especially true for members that are restrained laterally at their ends by others

bays of the structure. Heat from a fire below heats the underside faster due to thermal lag. This

causes a thrust force to be exerted on the bottom half of the member by the restraining bays, and

results in a bowing up of the member. Bowing is a positive effect because this increases positive

moment capacity, thereby increasing its load capacity. If allowed to continue, the fire will further heat

the underside of the concrete member until the concrete becomes less stiff; this further increases

bowing. This effect can add many hours of additional fire-protection to a hollowcore slab.

The performance of simply supported members due to a fire does not experience the dramatic

gains that restrained members do. Since no bowing action is developed, the imposed loading is

allowed able to continue to act without an increase in strength capacity. As the concrete begins to

become less stiff and the reinforcing begins to lose its strength, the moment capacity decreases. It is

this fire-rating that is presented in the Manual for the Design of Hollowcore Slabs.

Fire performance is further defined by member thickness and aggregate type. Hollowcore

planks are typically converted from their real thickness to what is known as equivalent thickness,

which is the average thickness of concrete once the cores are considered. 8” Plank has an equivalent

thickness of a little over 4”, depending on the manufacturer. In general, lighter aggregates increase

performance because there are a larger number of small air pockets which resist the flow of heat. The

addition of gypsum wallboard ceilings typically have little affect on fire-rating because to be effective,

they would have to last longer than the planks.

In the redesign of 2941 Fairview Park, the hollowcore slabs are detailed in such as way as to

achieve simple support and ample room exists at both ends of the slab to allow for expansion.

Thrusting forces are not encountered, and thus the values obtained from the Manual for the Design of

Hollowcore Slabs should be used. As seen in Figure 6.2 of the Manual for the Design of Hollowcore

Slabs, sand-lightweight 8” planks have a fire-rating of in excess of 2 hours.



23:33

Pros / Cons of Precast: Precast can be constructed in near optimal conditions with more precise and accurate formwork

and rebar placing. Scaffolding requirements are virtually eliminated, and formwork is used with

greater repetition. Precast members are often steam cured in ideal conditions. Finally, because of

there greater control employed over concrete mixtures, concrete strengths of 8000 psi are common,

and do not come at such a premium as they do with site-cast concrete mixes.

Precast hollowcore planks offer superior acoustical isolation and fire rating. 8” hollowcore

plank made with lightweight concrete has an inherent fire-rating of 2.5 hours. This inherent fire-rating

eliminates the need to consider special assemblies, and also affects insurance rates.

Hollowcore slabs offer superior acoustical performance when compared to many other

construction assemblies. An STC value of 50 dB is conservative for all assemblies using an 8”

hollow-core plank. Impact noise is reduced by 30 dB. This is a 10 dB improvement for each criterion

over the existing composite slab.

Precast concrete allows the building to be enclosed quicker than many other structure systems,

thereby allowing other trades to begin work, and therefore reducing the overall construction schedule.

Erection speed is comparable to steel structures, but the extra step of making final connections is

eliminated.

Despite its advantages, precast

concrete does have a few drawbacks. Its

highly prefabricated nature does not allow

for easy retrofit to meet tenants’ needs.

Hollowcore plank floor units make

attachment of non-structural items, such as

pipes and ducts, slightly more complicated.

Preliminary estimates show that heating

units might require several hangers.

Fan Powered VAV Unit w/ Heating Coils



24:33

Building Envelope:

Often on the critical path, enclosing the building is a vital part of the construction process

because it helps define when trades such as MEP tradesmen and finishers may begin to work.

Currently, a proprietary metal panel curtain wall system is in place, and minimal information is

available. Detailed schedule information is also not available. I therefore chose to redesign the

building envelope to insure the following requirements:

1) Identical appearance to existing system

2) Fast erection time

3) Minimal crane use

The building enclosure system is

based around the precast concrete spandrels

than form the perimeter of the building.

These members perform two major duties.

First, they play a major role in the lateral

diaphragm, providing significant flexural

resistance. Second, they provide the

structural support for the metal panel

curtain wall.

Precast Spandrel

Existing Connections

The existing system is connected to the

structure by a variety of connecting elements. It

was my original intention to derive a system that

did not need such connections. The spandrel

section is stable due to its large dead weight

acting directly over the column corbels. No

additional pieces of metal need to be attached

(except for diaphragm action), and no fireproofing

is necessary.

The redesigned curtain wall utilizes a Wausau window system. This proprietary product,

pictured below, allows for easy installation from the inside of the building. When the spandrel beams



25:33

arrive on site, the head and sill pieces are installed along with the appropriate flashing. The beams are

then lifted into place, ready for window installation. Window installation can proceed once the

spandrel beam on the floor above has been erected.

Head & Jams Sill Installation of the windows from the inside of the building proved to be critical because it

minimized crane use. The cranes only need to drop off a few pallets of windows per floor. These

windows can then be handled by smaller pieces equipment prior to installation. Additionally, all

sealing can be done from the inside as the windows are being secured, thus further reducing the

amount of work that needs to be done on an unsupported area.



26:33

Construction Management: When designing a precast concrete structure, construction must be at the front of the designer’s

mind. Large, heavy members must be brought to site and erected in a safe, stable, and timely manner.

Well orchestrated precast structures can be erected quickly, saving money. I therefore chose to take a

much closer look into the aspects governing the construction of my structure.

Construction: Construction throughout the building

proceeded in complete harmony between many

different trades. A site-cast concrete crew was

first on site. After completing the foundations,

they began to on the core of the building at a pace

of one floor per week. Precast erection began two

weeks after completion of the foundations.

Precast concrete was erected at a pace of one floor

per week so as to keep up two floors behind the

building core.

Construction Rendering Precast erection of each floor took part in three distinct stages. First, the columns on the East

side were erected and stabilized with cables. The second half of that day, beams and girders were placed

so as to further stabilize the structure. The following day, the hollowcore plank was erected on that half

of the building. Plank was laid starting at the inside of the building and proceeded outward, alternating

bays every few planks for torsional stability reasons (see above). The third and fourth days were

similar, but on the West side of the building. The final day of the week was spent placing all remaining

planks and lifting materials such as metal deck, windows, and materials for the core. Grouting and

reinforcing of the keys was performed by a separate crew on the side of the building opposite from

where members were being lifted. It was at this point that the end bays were cast. Finally, a leveling

slab of high water content concrete was poured.

The precast crane was used to lift window pallets up onto the building. When half a floor was

done, windows would be lifted up onto the other half (a floor below), and moved beneath the completed



27:33

half to minimize exposure to weather. Window installation could occur once the spandrel beam on the

floor above was set. I chose to schedule the window installers two weeks behind the precast erector to

give the precast erector some leeway. This procedure minimized crane use for windows to only a few

lifts per week while allowing other trades to move into an enclosed building quickly. This had the

overall effect of reducing the schedule by two weeks.

Two possible crane locations were considered for this job. First, a single crawler crane could

work from one side of the building. The length of land that needed to be prepared for the crane was

minimized, but at the expense of some long lifts. The second option was to run the crawler crane down

each side of the building, depending on where pieces were being placed. While this allowed for quicker

lifts and a smaller crane, additional site preparation needed to be done. Further research was not

conducted because this is highly dependant on what cranes the precast erector owns.

Schedule: A rough schedule was determined for the existing system based on values obtained from R.S.

Means, D4 Cost Estimator, and the general rule of thumb of “30 pieces of steel per day”. Bid is AwardedProcure SteelProcure MEPSite PreparationGroundwork, UtilitiesExcavate BuildingCIP FoundationsSteel SuperstructureComposite FloorErect Curtain WallRoofingFireproofingElevatorPlumbingDuctworkElectrical & TelecomPlaza StructureFinish ItemsLandscapingSig. Completion

Composite Steel Schedule Using numbers given to me by Mark Taylor for precast erection speeds, and values taken from

RS Means, I was able to put together an approximate construction schedule for the precast system. This

schedule was then available for use with my 4D CAD model, as well as for direct comparison against

my schedule for the existing building system.



28:33

Construction of the precast portion of the building was estimated based on a 20 pieces (girders or

columns) or 40 planks per day figure given out by Mark Taylor during his precast presentation to thesis

class. This allows erection to proceed at a rate of approximately one floor week.

Bid is AwardedProcure PrecastProcure MEPSite PreparationGroundwork, UtilitiesExcavate BuildingCIP FoundationsCIP CorePrecast FrameErect Curtain WallRoofingCIP Grid HErect Lower PrecastElevatorPlumbingDuctworkElectrical & TelecomPlaza StructureFinish ItemsLandscapingSig. Completion

Precast Schedule As can be seen by a quick comparison between the two schedules, the precast frame allows for a

slight decrease in construction duration. Specifically, four weeks were removed from the schedule. By

pushing up the move occupancy date, the return on capital can begin sooner, thereby increasing

profitability of the building.

This increase in erection speed occurred in three major locations. First, the core of the building

was begun two weeks prior to the first precast member even arriving on site. Two floors of building

core were constructed during this time, and allowed precast erection to start off running. Second, the

precast frame can be erected faster than the steel frame. This is partially due to the reduced number of

beams, and partially due to the ability to move on from one floor to another without casting a structural

slab. Finally, the façade I chose reduced erection time significantly and allowed other trades to begin

work in an enclosed building.



29:33

Transportation: Construction of the precast members was taken into consideration during design so as not to

specify any members that would be prohibitively expensive to create in the precasting plant. Column

corbels were placed on three sides and not four, which is the demark point for being very difficult to

cast.

Transportation is perhaps one of the most difficult exercises in scheduling. Precast trucking

limits are typically dictated by the physical properties of the trucks as well the local department of

transportation. Given truck capacity and member weights, only two columns or girders could be hauled

per truck. Additionally, since there were haunches on three sides of each column, arrangement on the

truck’s bed was limited to a side-by-side configuration, with filler pieces between the two columns so as

not to damage any of the haunches. Girders did not present this additional dilemma since they did not

have any thin projecting elements. Six hollowcore plank units or two spandrel beams could also be

taken per truck. Special transportation methods were required for the spandrels so as to not damage the

metal panel finish.

Materials were brought to site early in the morning, and trucks were unloaded and ready to leave

the site by early afternoon so as to avoid DC traffic. The Fairview Park development has its own exit

off the Capital Beltway, so use of local roads with heightened weight restrictions was not an issue.

There were no sharp turns so truck size was not limited by turning radii.

Transportation Route



30:33

Estimate: A rough estimate was performed on those parts of the project that were affected by the redesign.

This included the superstructure, foundations, as well as the effect of the reduced schedule.

Cost data for precast members was obtained from Mark Taylor. Costs for other systems were

obtained via an assemblies estimate carried out through R.S. Means. The costs per assembly were then

adjusted to account for time differences and the two total values were compared. Once a location factor

was added, the structure was taken as a percent of the total budget and compared to a R.S. Means square

foot estimate to determine if the numbers sounded reasonable. Overall, the precast system was about

$120,000 more expensive.

The different significant completion date affected the length of the construction loan. Rough

calculations showed a reduction of almost $200,000 in construction interest (from $3 million). Overall,

this produced a savings of about $80,000. This difference represents approximately 0.2% of the overall

budget, and just over 1% of the structural budget. Clearly, this is insignificant due to the limited accuracy

of the estimates I performed. I therefore conclude that the two systems are roughly equal in cost when

both construction costs and construction interest are considered.



31:33

Conclusions: As shown in the previous sections of this report, a precast concrete gravity frame, coupled

with a site cast core, can be a very effective structural system. Building serviceability, including

story drift, sound transmission, and thermal transmission were all improved. Both sound

transmission and impact noise between floors were improved by approximately 10 dB (p205, Egan).

Thermal transmission between the inside and outside environments was controlled further by an

increase in thermal resistance of approximately R=5.

Façade improvements can be based on two main changes. First, the insulated concrete panels

have greater connectivity with the rest of the structure. This reduced air leakage, a major source of

energy loss in buildings subjected to the hot, humid DC environment. Second, the system proposed

allows building enclose to occur much sooner in the project, thereby reducing the total schedule

duration by thee weeks.

The effect of this different structure on the other trades is somewhat minimal. Typical

structural depth was increased by approximately 5”; however it was reduced by nearly 1” where

there were originally moment frames. As can be seen in photos taken inside 2941 Fairview Park,

this should have minimal impact upon the ductwork and other systems that reside within the ceiling

plenum. Attachment of non-structural items to hollowcore slabs, however, presents some difficulties

that are somewhat more complicated than with a composite metal deck.

Recommendations: Clearly, there are many benefits brought about through the use of a precast concrete

structural system. There are numerous ways to construct precast concrete systems, and just one of

them was attempted in this project. Variations that would merit consideration include the use of the

PRESS system, and the use of a structurally composite slab. At this point, I am prepared to

recommend further evaluation of a system similar to what is proposed above, but with the addition

of a structurally composite slab. This would reduce precast member size, material strength, and

prestressing force. Additionally, floor diaphragm reinforcing would be simplified significantly. I

believe the savings in construction costs would outweigh the increase the overall schedule length of

approximately one week.



32:33

Acknowledgements: I would first like to thank Joe Ajello and Jimmy Lakey of Cagley & Associates. I worked

both of them for two summers. As the original designers of the building, they were able to answer

many of my questions and were able to provide a lot of information for me. I would also like to

thank everyone else I worked with at Cagley & Associates for making my two summers enjoyable

and educational.

Next, I would like to thank Mr. Mark Taylor of Nitterhouse Concrete. In addition to the

presentation he gave our thesis class on March 6th, he answered many of my questions about precast

via email. He was also instrumental in putting together a rough estimate.

I would also like to thank Professors Parffit, Geschwindner, Boothby, Burnette, and all other

faculty for helping me along with the senior thesis and answering my numerous questions.

Mike Patton of Boggs & Partners was very helpful in answering some of my project oriented

questions in the beginning of the year. He was also able to provide data that helped immensely.

Clement Enyeji of PCC Consultants was also helpful to the extent governed by proprietary rights.

Finally, I would like to thank my family and Karen for their continuous support,

especially these last two semesters.



33:33

Bibliography:

Bljuger. F. Design of Precast Concrete Structures, Ellis Horwood Limited, London, 1988 Egan, D.M., Architectural Acoustics, McGraw-Hill, New York, 1988 Gerwick, Ben C., Construction of Prestressed Concrete Structures: 2nd Edition, John Wiley & Sons, Inc. New York, 1993 Haas, A. M., Precast Concrete: Design and Applications, Applied Science Publishers, London, 1983 Nawy, Edward G., Fundamentals of High Strength High Performance Concrete, Longmadn Group Limited, Bath, 1996 Nilson, Arthur H., Design of Prestressed Concrete 2nd Edition, John Wiley & Sons, New York, 1987 PCI, Manual for the Design of Hollow Core Slabs 2nd Edition. Chicago, 1998 PCI, PCI Design Handbook: Precast and Prestressed Concrete 4th Edition

Date post:	26-Mar-2021
Category:	Documents
Upload:	others
View:	2 times
Download:	0 times

Eric Sobel 2941 Fairview Park - Penn State Engineering ... · the architecture almost identical,...

Documents