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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.
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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.
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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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
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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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.
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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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.
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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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.
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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.
Typical Tower Floor Plan
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
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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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.
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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.
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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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.
Typical Tower Floor Plan
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,
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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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.
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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.
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
17:33
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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).
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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.
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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.
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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.
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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.
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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.
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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.
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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.
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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.
Eric Sobel 2941 Fairview Park Structural
4/20/2003 Fairfax, VA Prof. Geschwindner
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