philadelphia community college - center for business & industry

Philadelphia Community College - Center for Business & Industry 18th & Callowhill Streets - Philadelphia, PA Bernie Dougherty Penn State AE Senior Thesis Structural Option

AE 482A - Spring 2004 Consultant – Geschwinder

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Table of Contents

Executive Summary - Comparing Alternate Structural Systems 3

Introduction 5

Primary Project Team 8

Loads 8

Alternative Structural Systems: A General Overview 12

Live Load Reduction 17

Composite Steel Joist System

Composite Steel Bar Joist System Design 18

Composite Steel Bar Joist System Overview 19

Steel Layout 22

Gravity System Design 26

Mechanical System Changes 27

Lateral System Design 29

Foundation System Design 31

Base Plate Design 32



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Vibration and Deflection 33

Fire Protection 34

Acoustics 35

Bridging Requirements 36

Architectural Modifications 37

Advantages of Steel Joist Design 39

Disadvantages of Steel Joist Design 41

Conclusions

General Conclusions 44

Conclusions: Overall Comparison of Structural Systems 46

Acknowledgements 47

Bibliography 48



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Executive Summary

Comparing Alternate Structural Systems

The Philadelphia Community College’s Center for Business & Industry is a five

level, mixed use facility located in downtown Philadelphia, PA. The building is nearly

rectangular in shape and consists of three stories of classrooms and administrative space

over a two story parking garage. The decision of choosing the optimal building structural

system was a large consideration in the development of this construction project.

While some buildings may only have few practical options for their structural

systems, the relatively simple layout of this particular building along with its low height

suggest many different framing options. This thesis compares the original composite steel

framing system to the proposed composite steel joist system. The advantages and

disadvantages of each system will be thoroughly examined in later sections. By looking in

depth at the strength parameters of the proposed system design as well as serviceability,

economic, scheduling and architectural concerns, it will be possible to develop a more

insightful decision for the project’s optimal structural system. The following details the

research, design, and analysis of the proposed structural system and its implications on cost

and scheduling concerns.



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As noted earlier, the Center for Business & Industry was built using composite steel

construction as its framing system. After evaluation, it was determined that the next most

practical alternate framing system for this particular building was a steel joist system,

preferably taking advantage of composite action when necessary. The depth of this

research involves the design and evaluation of this alternate system and the breadth of this

study examines the scheduling and cost estimation of this alternate system. Fire ratings,

acoustical effects, vibration, and overall constructability were also studied in some detail

and incorporated into this report. This comparison between these two framing systems

shows that the original composite steel system has several desirable features that, in

comparison to the alternate system, outweigh the benefits of the proposed composite joist

system.



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Introduction



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The Philadelphia Community College’s Center for Business and Industry is a five

level, mixed use facility. There are two sub grade parking levels and three above grade

levels designated for educational and administrative space. The footprint of the building

is in the shape of a four sided polygon with one side on an angle relative to its adjacent

side

Typical Floor Plan for the Center for Business & Industry



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The dimensions of the building footprint are approximately 180 ft by 240 ft. The

building houses a new cyber café, a two story atrium, classroom space and office space

for administrative personnel. The project is located in downtown Philadelphia, PA and

will cater to the needs of the community in an expanding college campus downtown. The

approximate cost for the building was 19 million dollars including land acquisition,

development and construction.

The project was constructed using a very efficient framing type. The building

features a composite steel system with composite metal deck. Lateral reinforcement is

provided by some moment frames in the short direction established by the full depth

connections welded at all girder to column connections. The original depth of the slab is

6 inches with three of these six inches of concrete being used in compression for the

design of the beams. Concrete strength was specified to be 4000 psi. Floor to floor

heights are 12 ft below grade and 14ft above grade. With the exception of the sloping

wall, the column grid is fairly regular. Typical beam and girder spans range from 30 to 40

feet and initial site work began in August 2001 and concluded in January, 2002.



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Primary Project Team

• Owner – Binswanger, CBB

• Architect – Cope Linder Architects

• Structural Consultant – O’Donnell & Naccarato

• MEP Consultant – McHugh Engineering

• Civil Consultant – Schnabel Engineering

Loads

General design loads are as follows:

DEAD LIVE

Slab on Grade 50 psf 100 psf

Upper & Lower Parking 85 psf 50 psf

Floors 1 through 3 60 psf 100 psf

Storage Areas 60 psf 125 psf

Loading Dock Area 110 psf 64 psf

Roof 25 psf 30 psf



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• Ground snow load: 30 psf

• Basic wind speed: 70 mph

• Basic wind Pressure (P):12.5 psf

• Peak velocity related acceleration (Av) : <0.05

• Peak acceleration (Aa) : <0.05

• Seismic performance category: B

All loads are taken from Chapter 16 of BOCA 99

Seismic Loads were evaluated using the equivalent lateral force procedure



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Cross Section Showing Governing Wind Loads on Critical N-S Face



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Cross Section Showing Governing Seismic Loads on Critical N-S Face



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Alternative Structural Systems: A General Overview

With much more time, a more thorough analysis of every different available structural

framing systems could prove to be valuable in the selection of the ideal structural system

for the Center for Business & Industry. There were many different preliminary options for

the structural frame of the building; however, only a couple of them were worth further

consideration. In practice, it would be important to consider, in more depth, every

available system. Only the most practical system was chosen as the subject of this thesis.

Some other typical framing systems that could have been considered more seriously and

investigated in more detail are:

• A non composite steel system

• A non composite steel joist system

• A one way concrete system

• A two way concrete system

• A precast concrete plank system

• A wood framed system



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There are many advantages and disadvantages of each system listed below. Due to the

nature of this report, they will not be investigated further in the remainder of this report.

There were several reasons for selecting the current composite steel joist structural system

for the Center for Business & Industry.

Due to the nature of the parking garage, a grid column system as opposed to a bearing

wall system should be used to transfer loads to the foundation and consequently maximize

parking area in the plan below grade. This eliminates some framing options very early in

the selection of the structural system. For example, any masonry or wood framed bearing

wall system could potentially cause a load transfer problem at the garage level. This transfer

could result in thicker slab depths, and therefore, an increased floor to floor height. These

line loads could also dictate some architectural features, such as overall building height.

This would then result in more scheduling time and budget contributions for the project in

general.

Foundation types were also a driving force in the selection of the framing type. It was

essential for cost reasons to use concrete or steel piers to transfer the gravity loads to a

spread footing system. Soil conditions were good and spread footings seem to make sense

here. This would prove to be a very cost efficient decision in the construction sequence.

This is due, in part to the fact that loads seen by the foundation are fairly low

Some other structural alternatives were simply eliminated due to their lack of efficiency

when considering strength. A non-composite steel system with a flat, one way slab would



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not be as nearly as effective as the existing composite steel system. On the other hand, a

composite steel system would prove to be less efficient than a prestressed, concrete plank

system.

A preliminary design check between the non-composite steel system and a one way

flat slab showed that ceiling thickness would need to be increased by up to 4” per floor in

locations where the spans were longest. This intrusion into the floor heights was considered

unacceptable for the purposes of the alternate design.

Due to the nature of the project, and the atrium especially, the need for long spans

was a major concern when choosing the optimum system. The architectural intent of the

design was to keep the plan as open as possible, especially in the atrium, for locating non

load bearing interior walls. Flat slab concrete systems along with non composite steel

construction systems were originally eliminated due to this desire for maximizing spanning

capacities while minimizing the ceiling thickness. The original design of the composite steel

system seems to be a good fit here. The first floor plan looks fairly unobstructed by

columns. Column sizes are small as well as plenum depth compared to any concrete

system. The supporting column’s faces are coincidental with the interior walls that separate

the administrative space from the atrium space on the first floor.

Since the openness of the atrium and the stairs on the first floor are one of the

main architectural features of the building, efficient systems were very desirable. This

openness creates a feel that the space is bigger than it actually is, which, was the architect’s



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original intent. If a non composite concrete on metal deck system was used, construction

time would be faster for the floor system due to exclusion of activities such as shear stud

welding, but, costs would eventually increase due to the fact that the composite strength

wasn’t being fully utilized. This would then result in larger beam sizes as well. It makes

sense here to use composite action if you are choosing any metal decking system with steel

supports.

Time was a critical factor in decision making on the project. This factor basically

drove any structural concrete system out of the running for the selection of the optimal

structural floor system. The time required for formwork construction, stripping and

reshoring, and also the time needed for tying and placing reinforcement cages would be

very intensive. This is very undesirable in a metropolitan area when trying to minimize

traffic congestion. The need for long spans was also another driving force for the selection

of a steel system. These are all reasons why concrete systems were excluded early in this

process.

After some initial investigation, a steel bar joist system seemed to be the best option

for several reasons. First of all, the availability of mechanical space seemed to play a key

role in the decision. This need for long spans could still be accommodated by the joists.

Also, in the event that extra strength was needed, composite action could be utilized by the

joist system.



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Lateral force resistance will become a more critical issue in the use of bar joist due

to their decreased rigidity. This issue will be further examined in the lateral force resistance

section of this thesis. The existing moment frames can produce thinner beams due to

moment distribution, but the cost of these moment connections proves bigger than the

savings here. On the other hand, all girder to column connections were identical. These

connections avoid construction confusion and also ensure a certain level of quality.

Normally these types of connections would be reserved for taller structures with a relatively

small base. The use of some moment frames and connections here seems to be work well

for this project.

Wood trusses could also provide a great range of application through this type of

construction; but due to the size of the necessary mechanical equipment, these web

spaces would prove to be unutilized anyway. If any truss system would be used, it would

need to be steel.

The final option was a precast plank system. Using literature published by

Nitterhouse Concrete Products, it was determined that a 10” plank with 2”

topping was necessary for the case involved. In addition to the plank depth a 16”- 18”

girder was necessary. The entire assembly would be at least 28” deep. This is close to the

required depths of some other systems, but material costs are increased dramatically with

the use of a precast system. Also, materials staging could also prove to be a critical issue in

a metropolitan job such as this.



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While some of these systems are definitely applicable, others offer some favorable

advantages. After analysis of a variety of systems, it was determined that a steel joist system

would be the most practical alternative. The wood truss construction mentioned could

provide a great cost savings as well as a plenum for MEP equipment in this type of

application; however, there was no credible merit for that system here. If web space was

going to be utilized, it would need to be a steel joist system. Therefore, a steel bar joist

system would prove to be the best alternative worth further, in depth investigation.

Reduction of the Live Load

Throughout the course of this thesis, it was noted that the building’s structure

seemed to be somewhat over designed. The live load used for the design of these floors

was 100 psf. This seems extremely high for a space that falls into just educational and

administrative use. A live load reduction was carried out for all the interior beams and

columns. With the use of this live load reduction, and my own personal judgment, the

design live load was reduced from this previously mentioned 100 psf to 70 psf. This is

done in confidence because of the large tributary areas that are often unloaded in the

framing system. These often unloaded areas were previously designed as if they were all

simultaneously loaded to the prescribed loading. This change affects every aspect of the

framing system including member sizes, cost analysis and scheduling.



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Steel Joist System



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Steel Joist System: General Overview

Composite steel bar joist construction has much in common with the existing

composite steel construction type. The load and resistance factor design theory was used in

the design of both types of systems. Both the existing system and the proposed systems use

steel wide flange columns to support the floor system. Typical spans for the existing steel

shapes range from 30 to 40 feet. For the most part however, spans are fairly constant at 30

feet. On this basis, a transition from w shaped beams to steel bar joists should not be a very

difficult task. Basically, the existing column grid will be used here to retain the architect’s

original intent. In any instances where the column grid would need to be changed, the

appropriate reasoning will be given. Some such reasons would include satisfying deflection

criteria or facilitating mechanical and electrical layouts. These structural steel framing

systems can use a variety of lateral systems for shear support such as the use of a moment

frames or braced frames, or the inclusion of some type of concrete or masonry shear wall.

This lateral force resisting system will be investigated in further detail in sections to follow.

As a logical alternative to the original composite steel construction type, the

Philadelphia Community College’s Center for Business and Industry was redesigned using

the existing concept of composite steel construction, however, with the use of steel bar

joists to replace the existing wide flange beams. This option was chosen over others for



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several reasons. Primarily, maintaining the efficiency of a composite steel system was

heavily emphasized. This was due mainly to the need to maximize the span distances and

maintain the architect’s floor plan as much as possible.

The design of the floor system was verified by hand using the LRFD 3rd edition,

the USD deck manual, and the Vulcraft steel joists and girders manual. The model code

used in the proposed design was the BOCA 1999 code, which was the code originally used

in the design. This was done in order to make system comparisons as easy as possible by

keeping these code requirements the same. This decision will make comparison between

the original design and the redesign much easier. The original composite steel design used

the same fully restrained connections in the shorter east-west direction as the lateral force

resisting system. This lateral system was used again here in order to aid in global system

comparisons and also because it seems to fit with the designers desire to ensure quality

construction. This redundant connection detail will minimize the amount of questions as to

where fully restrained connections should be located. Joist to girder connections will also

be investigated in more detail in sections to follow.

The non-load bearing interior walls are comprised of light gage steel studs. This is

consistent with the original design and is fairly consistent with any project of this size, type

and use.



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In the case of the Center for Business & Industry, the structural system was

designed at the same time as the architect began floor plan development. This allowed for

much communication during this early, influential phase. The project engineer here could

facilitate the needs of the architect and vice versa early in the design development phase.

The repetition of structural members, especially in the floor system, will aid in the alternate

framing system cost and scheduling comparisons to be addressed later in the research. The

use of repletion here will also ensure deflection requirements and strength requirements

without fully investigating every framing member in the floor system. Some factors that

should be included in the overall system comparisons are cost implications, scheduling

feasibility, constructability, fire protection and floor vibrations. These factors will be

investigated in some detail in sections to follow.



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Steel Layout

The original column grid remained virtually unchanged with the introduction of the

steel joist system. In the new system, the joist spacing was changed to every 15 feet. Since

the spacing was changed, the joists carried one and a half times more live load per foot.

The spans were kept the same, so all the column loads remained the same. The typical bay

was basically changed from two intermediate supports to one intermediate supports.

Plan View of a Typical Existing Bay



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There is one area along the sloping exterior wall where the existing spans are low. This is

due to an extra line of supporting columns. These columns were eliminated because the

joist system could accommodate these spans. The MEP equipment will be then run

through these joists.

Since the volumes of air to be conditioned are smaller here, and thus area for air

transmission are smaller, these cross sectional areas of the ductwork are lower here anyway.

The MEP equipment can be run through the joists here rather easily.

As noted earlier, the support spacing was changed to fifteen feet in the proposed

system.

Plan View of the Typical Proposed Bay



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This changing of the beam spacing has some direct effects worth mention here.

Primarily, the deck spans go up, therefore the minimum thickness of concrete will need to

be increased from 6 to 7.25 inches. This has an effect then on the budgeting for pouring

concrete. This will all be later examined and analyzed in the cost analysis sections to follow.

In the proposed system, the columns were oriented so that their strong axis was positioned

in the east-west direction of the building. The girders would span in this direction and pick

up the load from the joists in the perpendicular, north-south direction. This is consistent

with the original design.

As noted earlier, the beam spacing was changed to 15 feet. This spacing was

changed to aid in minimizing the material costs and also to increase joist depths to allow for

mechanical intersections. It was intended for this slab to stay as thin as possible, while still

satisfying all the necessary strength and deflection requirements Using the USD deck

manual, it was decided that a 3” lok floor system with a 4.25” concrete composite topping

could span just over 15’ based on the loads for this project. It would have been unfeasible

to increase this joist spacing, and therefore, deck spans if the live load were not reduced

somewhat. Using hand calculations and the Vulcraft joists manual, it was determined that

the original spacing used wouldn’t produce the necessary depths for MEP space.



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In the original concept, joists were to be spaced at 10 feet on center. The analysis

here produced 18 LH 6 joists keeping the spans at 30’. This depth was not big enough to

fit supply air ducts through the web space. Therefore, the joists were to be spaced at 15 feet

on center. They would then pick up more load and need to be deeper. The new joists were

sized to be 28 LH 05 joists.

The spacing change means that material costs for joists were reduced by one third

over the initial joist system with the ten foot support spacing. Also, decking was going to be

more expensive now. The second run showed deeper joists, which was the goal of the

system originally.

The floor layout is basically identical for the first through third floors. The lower

levels were not redesigned. It was decided here that since the space was unconditioned, the

smallest plenum depths would be found using the existing composite steel system. This is

true because the w shapes working in composite action prove to be smaller than the joist

depths. The roof however is slightly different because the live load is lower here. Here, the

spans of these angling members are bigger because the intermediate supports are removed

also. Again, this works well with trying to facilitate the needs of MEP space.



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Steel Bar JoistHVAC Ducts

Concrete Topping

W Shaped Column

Shear StudsMetal Decking

Column TopPlate

Gravity System Design

After readjusting the layout, the gravity system was designed using the Vulcraft

manual and hand calculations. The design yielded desirable joist depths. The girders

ranged in size from 20 to 30 inches deep and most shapes were around 28 inches deep.

The joists ranged in size from 24 to 32 inches deep. The difference here is typically an

increase of 8 inches, which would originally be considered undesirable. This depth

however actually facilitates the MEP equipment crossing the joists.



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It is not necessary for these joists to be fully composite in all cases, so shear studs

for composite action are not provided for all joists. In the event of needing studs however,

the shear stud placement is limited to the spacing of the ribs. For the girders however, the

deck is running parallel, so it is necessary that the ribs line up with the girder’s top flange.

For the joists, 2 shear studs will be used to ensure a full connection by making sure that the

studs don’t fall in between the angles that comprise the top flanges of the joists.

The column design for the gravity system showed fairly small required sizes. W12s

were more than adequate in most locations. Here the columns still worked because they

still take the same amount of tributary gravity load. These smaller sizes are due to the

relatively small axial load on the columns. The building, after all, has only three above

grade levels.

Mechanical System Changes

In the redesigned system, the mechanical ductwork will be able to be sandwiched

inside the joists. The increase of the support spacing facilitates this decision by increasing

the required depth of the joists. This was actually done in order to try to reduce the overall



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depth of the plenum as well as to increase overall constructability and minimize material

costs.

This joist system will fit the MEP runs through the webs of the joists. In the original

design, the typical tributary return air ducts were typically 12” by 18”. In the new system,

the main return line will require a cross sectional area of 1.5 square feet as in the previous

system. In this case however, the rectangular returns will be resized as circular to fit through

the joists, just as the supply lines do. The new areas, as said, will be 1.5 square feet. This

results in a duct diameter of almost 9 inches. This value is well below the maximum

recommended diameter of a circular duct crossing through the webs of the specified joists.

This decision to use circular return lines will increase material costs for the HVAC

equipment. These circular lines are typically reserved for supply lines because of the level

of the head loss between the AHU and the space to be conditioned. Using circular lines for

returns will be wasteful, but it will enable the HVAC lines to fit in to the plenum space

without having to drop them below the joists. This will also make construction more

difficult for MEP trades trying to weave to lines through the joists. This will add labor time.



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Lateral System Design

The layout of the floor plan doesn’t really offer a very easy solution to the issue of

lateral force restraint. On the good side however, the building is rather short which means

that these loads are fairly low. The interior elevator shafts aren’t located where they could

pick up lateral load without creating a major torsion issue. Moving these shafts was not

allowed due to the architectural restraints. Some moment frames are being used in the

shorter, 180 foot direction. The rigidity of the existing composite joist system was

determined to be adequate using a STAAD model. Since the building only has three

above grade levels, wind loads aren’t very high and gravity design sizes prove to also work

in lateral design.

The original wide flanged girders were kept in the new framing system. This is

because these girders which were originally sized for the 100 psf live load are still smaller

than the required depths of the joists girders designed with a live load of 70 psf. These w

shapes also allow for increased rigidity for this lateral force restraint. This decision will also

not increase floor to floor heights for the redesign. These steel girders were modeled as the

primary element that carries lateral load from column to column in the partially restrained

frames in the short direction.

The use of the moment frames are carefully placed to ensure that torsion is

minimized. This decision would then ensure that the centers of rigidity and mass for the



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structure are located very near each other. The eccentricity is 5’ in the x-direction and 7’ in

the y-direction. This does not provide for a very large loading when considering torsion.

The gravity system also proves adequate here. This moment arm was also increased an

additional eccentricity of 0.5 percent of the building dimension as required by BOCA.

Also, due to the low seismic performance category, it was not necessary to account

for any further torsional amplification. There are no torsional irregularities as defined by

BOCA. As previously mentioned, wind forces governed over seismic forces by a significant

amount due to the nature of the light mass of the structure. This is typical of a composite

steel system in general. This load was even reduced further because of the decisions to use

a joist system as well as changing the spacing of the joists and also by the use of the

lightweight concrete

The building was analyzed in both the North-South and East-West direction. The

resultant force was much greater in the east-west direction because of the greater area for

resultant load. Direct and torsional forces were distributed according to stiffness. It was

determined that total building drift was considerably less than h/400. Overturning was also

not a problem due these low lateral loads. Although the semi restrained connections are

the only element designed to take lateral load, additional resistance can come from the

building’s composite floor system as well as the metal decking itself. Since the deck

thickness was increased; this provides even more resistance to lateral forces and drift. Also,

the exterior glass fiber reinforced panels could carry some load in the long direction.



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The steel frame, although designed and modeled mostly as a series of simply

supported beams, will also provide some lateral stability to the structure by the nature of

the actual, partially restrained connections. Modeling the steel frame as moment resisting

could further attain some lateral support but would not be recommended. Here, some

braced frames or the introduction of a shear wall could alternatively have been added at

this time in the design development to provide greater lateral force resistance.

Foundation System Design

The building’s foundation was designed as spread footings at the base of the

columns and strip footings at the base of the elevator and stair tower walls and along the

exterior walls. Hand calculations were used to verify the following footing sizes.

In the original design, footing sizes ranged from 6 feet by 6 feet to 12 feet by 12

feet. Typically, the common footing size was 8 feet by 8 feet. In the redesign however,

footing sizes ranged from 4 feet by 4 feet to 10 feet by 10 feet and the typical footing size

was 6 feet by 6 feet. The footing depth ranges from 12” to 24” and typical footing depths

were 18 inches. These new sizes turn out to be significantly smaller than the original

design sizes.

The sizes were found using an allowable soil bearing capacity of 4500 psf and a

concrete strength of 3000 psi for footings. Also, a safety factor for general failure was set



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at 3.0. The spread footings are relatively lightly reinforced, with #6 bars in both

directions every 8 inches. The continuous footings are 4 feet wide with similar depths and

reinforcement as the spread footings.

There are some issues that are worth mention here because they contribute to

these differences in footing sizes. First of all, the load was lowered for the steel

contribution to the dead load due to the spacing change as well as the decision to use

joists over beams.

Secondly, the dead load was increased due to the increased contribution from the

concrete. Because the new system needs to increase the spans for the decking, concrete

thickness, and therefore its weight per square foot, must also be increased. The main

contributor to the smaller footing sizes here was the decision to reduce the live load from

100 psf to 70 psf. This task proves to be one where costs are hard to cut down on, but the

material savings here are still worth mention.

Base Plate Design

The column base plates were designed using RAM and verified by hand. Base

plates were designed using 36 ksi steel, and required sizes ranged from 8” x 8” x 0.5”

base plates to 12” x 12” x 1” thick base plates. The typical size was a 10” x 10” x 0.75”.

These sizes are not very large in any case. The moderate sizes are due to the low axial load



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on each of the columns. This is primarily due to the fact that the building is only three

above grade levels. The reduction of the live load has a noticeable effect on sizing the

plates here. This is worth mention here because, using repetition, costs can be saved in the

purchase of these plates.

Vibration and Deflection

Due to the loss of rigidity in the supporting beams, vibration issues and deflection

requirements must be reassessed. The original composite steel system, though it has the

ability to span much farther than most other materials, lacks the natural damping abilities

of some other system alternatives such as wood truss design or structural concrete.

The longest span of a beam in this framing system is 30 feet. Composite

construction can fairly easily span this distance; the problem lies in its serviceability, mainly

live and construction load deflection and also vibration.

Deflection issues were addressed using live load deflection criteria of L/360. This

overall deflection limitation was included to prevent damage to non-structural elements

such as ceiling and floor finish materials. The joists were cambered typically 1inch to

accommodate the live load deflection.

The vast majority of structures do not exhibit floor vibrations that are large enough

for concern. For the purposes of this thesis however, vibration issues were addressed. The



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typical joist was checked for vibration concerns here. Some type of floor vibration will

occur in every building construction. Unlike any type of steady state vibration with constant

amplitude or frequency, human excitations are much more difficult to isolate and therefore

control. Since human sensitivity to vibration varies, some criteria were established to study

the floor system. Due to the occupancy type of this project, harmonic excitations may

occur due to walking. Any excitations caused by dancing or aerobic activity were neglected

since there is no gym or ballroom in the building. Studies show that vibration problems

arise when spans are near 30 feet and floor thicknesses are in the range of 2.25 inches.

Some measures taken by the redesign were fully consistent with minimizing any

vibration. For example, spacing was increased from 10 to 15 feet and the slab was

thickened. These two decisions would prove to be very beneficial in minimizing these

vibrations. If it were necessary to reduce the frequency of the floor further, this could be

accomplished by adding more concrete to the topping slab or adding additional bridging.

These decisions could prove to have undesirable cost and scheduling implications. This

would also slightly increase the dead weight of the floor system.

Fire Protection

BOCA 1999 code was also used for fire proofing requirements. In most

occurrences, the requirements for fire protection range from 1 to 2 hours. The code states



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that a 2 hour wall separation is required for fire walls, ceiling assemblies and also for the

enclosure of exits. The building does has a sprinkler system, but fireproofing will become a

much more difficult task in the proposed system. Fireproofing the webs of the joists would

add schedule time and other associated costs. Also, considerations must be made to keep

enough clearance for the passage of MEP equipment.

To avoid the fireproofing of the joists themselves, a double layer of 5/8” staggered

seam gypsum board will be used to fireproof the entire ceiling assembly. This addition

brings the fire rating of the ceiling assembly over the necessary 2 hours. The new, increased

deck size does, however increase overall fire rating characteristics of the floor system in

general. Assuming that the system originally worked with a 6 inch slab, it will definitely

work for a 7.25 inch slab thickness. This difference will save costs and time because the

ceiling layer was existing but the activity of spray on fireproofing is now eliminated.

Acoustics

The acoustics of this building were of particular interest because of the intended

occupancy of the building. Since the Center for Business and Industry is comprised of

various uses, including quiet classroom spaces along with social spaces, occupants would

expect high degree of privacy from fellow students, especially in spaces designated as study

spaces. Sound transmission class values are recommended by the Federal Housing



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Administration between these types of uses. The STC values for unit separation assemblies

were kept above a rating of 50. The STC values for floor ceiling assemblies were kept

above the value of 52. Again, with the addition of the slab thickness, acoustical properties

were only bettered. This is typically the case for a thick slab like this, but the slab plays into

the broader goal of reducing the overall building height. This also works very well here.

Bridging Requirements

The issue of bridging is one of the bigger drawbacks of the joist system. Since the

joists are fairly slender, and more slender than the original beam system, some type of

lateral torsion resistance is recommended here. This bridging will increase both the

overall budget and also the projects schedule. There are many activities that can’t be

undertaken until this bridging is installed. The Vulcraft manual specifies that for this type

of loading and joist depth, joist spacing should be kept less than 10 feet. In this case

however, the composite nature of the floor slab will provide bracing for the top

compression flanges of the joists. After some inquiry here, it is recommended that

bridging be installed at midspan of the joists. This decision will complement the

compression flange bracing provided by the composite decking above the joists.



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Architectural Modifications

The original design for the Philadelphia Community College’s Center for Business

and Industry consisted of a composite steel framing system with light gage steel studs used

for interior, non load bearing partitions. It would have been very difficult to change the

framing system to another material, such as concrete, without affecting the floor plan in

some way. Due to the desire the maintain as much open space as possible, by maximizing

spans and minimizing column sizes, the proposed joist system seemed to fit here. These

changes due to the use of another material would prove to be inadequate here.

One of the largest changes to the architecture of the building is the change in floor

depth. The original design of the building used a 6” concrete slab on metal deck. The

proposed joist system uses a 7.25” slab with typical joists measuring 24” in depth. The

entire joist assembly brings the floor depth to about 32”. This depth is significantly larger

than the original depth of the structural system; however, there is no need to drop the MEP

equipment here, so this number is the total depth of the plenum space. The structure will

remain true to the architect’s intent, but now the floor to floor heights can actually be

lowered by 6 to 8 inches per floor. This has influences all over the board. It decreases the

unbraced length of the columns, thus requiring smaller sizes. It also lowers the overall

amount of building height that needs both interior and exterior finishes. This may seem to

be insignificant here, but this could be a source of great savings in a building with more



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stories by saving 6” of finish materials per floor. This could amount to saving 5 ft in the

total building height the building were say 10 stories instead of three.

Another advantage of reducing the total building height is the influence of wind

loads. Exterior areas are now smaller; therefore, wind loads are also smaller.

Due to the nature of the composite joist design, a deep area is available between the

bars in the webs. Since the beams are spaced at 15’ on center, there is sufficient room to

run MEP through the joists.

Compared to the original composite steel design, the steel columns are smaller in

cross section. This is due to the combination of lowering the live load, reducing dead loads

somewhat by using the joists, and decreasing effective column lengths for buckling. The

original steel columns ranged in size from 12”x16” to 14”x28”. The new steel joist design

requires only W8 or in some cases W10 columns. This change in column size may be

somewhat beneficial in the cost of materials for the proposed framing system. This may be

seen as a strong benefit of the redesign. This reduction in column size makes this option

more viable in terms of architecture because there will be slightly more available floor area

for occupancy. These differences will also lower the cost per occupy able floor area

per square foot. On the other hand, overall costs could be kept constant while some finish

materials can be upgraded. This would be quite favorable for the architect.



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Advantages of Steel Joist Design

As mentioned earlier, the beam system and the joist system have a great deal in

common. Both systems share basically identical spans, and basically the same column grid

is used to transfer vertical loads. The original beam design used PR connections as does

this composite steel design. Because of their similarities, it is important to look at the

benefits of this design over the original system. The choice of framing systems in the

development of a project is typically dependent on these advantages or disadvantages.

The most apparent advantage of the proposed system over the existing system is the

change in the beam spacing. This change reduces the amount of members needed for the

construction sequence. This will have its effects during the erection phase of the project.

The composite beam system can easily span large distances. This is another feature

of the joist system. The longest span in the redesigned steel framing system is 40’. With the

same spans, but bigger joist loads due to the change in spacing, these joists must be fairly

deep. This actually helps to fit all the MEP equipment in the open webs of the joists and, in

result, lower floor to floor heights. Again, increasing the spacing also reduces labor costs,

material cost, and also saves time.

By reducing almost every size slightly by reducing the live load, changing the

spacing and using the lighter weight joists, seismic loads will be globally reduced. This



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could be seen as more of an advantage in a higher seismic zone which is worth mention

here.

It should be noted here also that an advantage of the joist system is the reduction of

the wind load by reducing the building size. There should also be some further

investigation here because of the wind load being lowered. The seismic loads may control

the design here. And again, had this project been a in a zone of high seismic activity, the

lightweight nature of this design would be a great benefit.

One of the biggest advantages of this alternative system is that the MEP equipment

doesn’t need to be dropped under the joists. With this addition, drops are unnecessary

eliminating the problem of intruding into the occupy able volume below.

The fact that footing sizes are now also smaller contributes to saving money on both

field labor and material costs. As noted earlier, the typical footing was reduced from 8 feet

by 8 feet to 6 feet by 6 feet. This greatly helps to get the structural frame topped out with

the spacing change factored into it. Along with these major advantages, vibrations have

been decreased by increasing the spacing and the floor thickness. Acoustical properties

have also been bettered by increasing the slab thickness. It seems that the fireproofing

activities will be faster and less expensive here as well



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Disadvantages of Steel Joist Design

While there are many advantages of this system over the original, it is necessary to

examine the drawbacks of using joists over w shapes. Since the project was, in fact, built

using beams, there was probably better fit for the owner, architect and the designers.

The first criticism of the composite joist construction is the fact that the thickness of

the deck needs to be increased. This was necessary once the support spacing was changed.

This means that more time and money needs to be spent in purchasing, setting and storing

the deck as well as pouring the floor.

Another disadvantage of this system is that weaving MEP equipment through the

joists could prove to be tedious relative to setting it below the beams. This decision may

create tricky trade coordination as well. The joists will be a constant interference for these

trades. This will slightly slow the installation of all service systems in general. On the good

side however, these aren’t tasks that are critical in the schedule. Most MEP tasks have an

associated float with their schedule. The need for additional man hours will, however,

definitely increase costs for field labor.

Another criticism of this construction method is deflection due to the construction

loading. The construction loads actually dictate the size of the decking. This limits the

strength capacity of the cured concrete because the spans could accommodate more loads

if not for the water weight in the curing process.



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Many other considerations have to be taken into account in these system

comparisons. Fireproofing is one of these. In the original system, fireproofing consisted of

spraying the underside of the deck while spraying the beams as well. In the joist system, it

becomes a much different approach because the joists themselves aren’t even sprayed.

This could be good, however, now fire blocking must now be provided in any holes in the

ceiling enclosure. This could be very expensive and time consuming. On the good side, the

fire rating of the floor itself will be increased because the slab thickness will

be increased. Vibration must also be considered in further detail in order to avoid any

discomfort or distraction for the occupants. Lateral bridging also becomes a task that wasn’t

originally planned for. This adds additional costs and time to the project.

Lastly, and very importantly, in the new system, the joists are much more expensive

than the w shapes in the original system. Even with the increased spacing, material costs are

still increased. These joists also have less lateral stability for force resistance from lateral

loads. Connections can also become tedious in both field labor and also design time

allocation.

General Conclusions

The differences between composite steel design and the composite joist design are

many and widespread due to the fact that loads were also lowered.



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Steel construction has a typical amount of advantages as well as drawbacks

associated with any construction method. Overall cost is not much of a factor, according to

an R.S. Means square foot estimate of the building; the cost difference is almost negligible

when comparing the two systems. The joists may have the advantage when it comes to

weight, and scheduling, but it has many disadvantages when compared to constructability

and serviceability. Overall, it does not appear that for this particular building, composite

joist system would have any major advantages over the steel beam system.

Conclusions: Overall Comparison of Structural Systems

After looking at a variety of structural framing systems, it becomes apparent that this

building was designed architecturally with the framing systems in mind. The use of long

spans to minimize column intrusions was both an architectural and structural decision.

If the designers of this building had originally opted to use another system such as

A two way flat plate or masonry bearing walls with precast planks, the architectural layout

would have reflected this. According to this analysis, the original structural framing system,

composite steel construction seems to have the least drawbacks of all the systems. While

being the optimal system, this construction type does have its drawbacks as well. All things

considered, this construction type is still the best fit for the project.



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The steel joist design showed few solid advantages over the original steel beam

design. There was no major cost or time advantage. Steel erection time may have been

slightly shorter, but cost savings would have been counteracted by serviceability factors. As

far as serviceability, the design didn’t perform as well as the original system and fireproofing

and deflection criteria could be problems. This construction may have other applications in

other sectors such as lower overall building heights, but it is not the best system to use in

this particular case.

In conclusion, although composite steel beam construction is the optimal

construction type on the Center for Business & Industry, the composite joist system does

offer many advantages. Due to the architectural layout, few other construction types,

excluding some type of composite steel system, could be used optimally. Had the

architectural design been based on the assumption of another structural system such as a

two way flat plate system, the results of this analysis as well, as the constraints faced, would

have been very different.

This thesis notes that this proposed alternate joist system could be used very

successfully if there was a driving need to minimize floor to floor heights. If an addition was

being built on an older structure that didn’t have the vast amount of mechanical equipment

that is involved in today’s building construction, this joist system could be used to meet

those requirements to match the finish floor levels. Here the addition on the existing

structure would need very small floor to floor heights.



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Acknowledgements

Vivian Palmer Binswanger, CBB Tom Dreyer Structural Consulting Louis F. Geschwinder Professor of Architectural Engineering Linda M. Hanagan Assistant Professor of Architectural Engineering Moses Ling Assistant Professor of Architectural Engineering Jonathan Dougherty Assistant Professor of Architectural Engineering Schnabel Engineering Cope Linder Architects McHugh Engineering



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Bibliography

The BOCA National Building Code (1999) ANSI / AF&PA NDS – 2000 (revised standard) “National Design Specification for Wood Construction” Manual of Steel Construction, AISC Load and Resistance Factor Design (3rd ed.) United Steel Deck Design manual and catalog of Products United Steel Deck Inc. Steel Joists and Joist Girders The New Colombia Joist Co. Load and Resistance Factor Design Louis F. Geschwinder Steel Design Guide Series 11 Floor Vibrations due to Human Activity Building Code requirements for Structural Concrete ACI 318-02 And Commentary ACI 318R-02 Design of Concrete Structures 12th edition Arthur H. Nilson

philadelphia community college - center for business & industry

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