pro-con structural study of alternate floor systems...and having an allowable bearing capacity of 20...

Michael Gray Structural Option Faculty Consultant – Dr. Linda Hanagan Arlington Gateway Hotel Arlington, VA

Rendering courtesy of WDG Architecture

Arlington Gateway Hotel - Technical Assignment 2 - 1 -

Pro-Con Structural Study of Alternate Floor Systems October 27, 2004

Executive Summary

For this assignment four different flooring systems were designed and

evaluated for a common bay of the Arlington Gateway Hotel. The four

systems along with the current system are evaluated in a pro-con format.

These systems are:

♦ Standard one-way reinforced concrete joist system

♦ Two-way waffle slab

♦ Open web steel joists on non-composite steal beams

♦ Non-composite steel beams and composite deck

♦ Two-way flat plate – Current system

Through the evaluation, the floor systems were designed for a minimum total

height of the flooring system to compare to the shallowness of the two-way

flat plate design.

The designs have a strict tolerance to reduce vibrations, deflections, and to

meet a 2-hour fire rating. At this time, there is a need to eliminate the

concrete joist system because in keeping with minimum system thickness,

layout and deflections are not desirable. All other systems have

advantages and should be further investigated.




Design Parameters

The hotel was designed using a live load of 40 psf for typical floors and 70 psf

for corridors. In my design I only designed for gravity loads, no lateral loads

were calculated. I chose to design the whole interior bay using the 70 psf

live load for corridors, as they pass through the center of the bay. Exterior

bays shall be designed using 40 psf live loads. These loads were taken from

ASCE 7 code book. Dead loads on the structure are made up of 15 psf for

partitions, 10 psf for MEP, and material self weight. At this time, fire rating of

the structure is not available. I designed the systems using a 2-hour fire

rating. I used a deflection of l/360 for all systems.

Design is of the common, center

bay of floors 4 through 14 as

displayed in the figure. The

cross-hatched area is the total

floor area of floors 10 through 14,

with an outline of the building

footprint shown. The bay is 26’-

11” for east to west and 19’-9” from north to south.




Two-way Flat Plate – Existing Condition

The current system is a two-way flat plate using a concrete compressive

strength of 4000 psi and 60 ksi steel reinforcing bars. A slab thickness of 8” is

used throughout the entire floor with #4 bottom bars spaced 10” each way.

The column strips have top reinforcement of

#5 bars spaced as per structural drawings.

Columns dimensions are 16” x 32” and use

5000 psi concrete on the fourth through

eighth floor and 4000 psi concrete on the

remaining upper floors. An existing layout of

the bay being designed is provided on the

left.

A major advantage to a two-way flat plate is the ability to have a relatively

small floor to floor height. This reduces the amount of exterior cladding

needed to enclose the building; thus, reducing the overall cost of the

structure. Also, it is a good system due to the flatness of the bottom slab

which allows for fast application of formwork. Concrete structures require

very little lead time, therefore can be designed and built in less time than

steel structures.

One-way Concrete Joists – Alternate System 1

The first alternate system I evaluated was a one-way concrete joist system. I

used the CRSI Handbook 2002 to aid in the design of this system. I used 4000

psi concrete with 60 ksi steel. In trying to design the thinnest system I

designed the system with a total floor thickness of 12.5” including a 4.5” slab




that provides a 2-hour fire rating. The layout

uses 30” forms + 5” ribs at a spacing of 35”

center to center. Each joist is reinforced

with #3 + #4 bottom bars and having top

bars of #4 at 12”. The beams were

designed as band beams spanning

between columns. Due to the long span

and shallowness of beam, a 12.5” x 48”

beam is required with 6-#7 bottom bars and

6 - #9 top bars for reinforcing.

There are several advantages to using a

concrete joist system. It is lighter than a

two-way flat plate and can also span

longer distances.

Waffle Slab – Alternate System 2

The use of a two-way waffle slab is

another alternative to the existing

flat plate. On the right is a

diagram of the proposed waffle

slab column strip and middle strip.

The system was designed using a

concrete compressive strength of

4000 psi and steel strength of 60




ksi. The total floor thickness is 12.5”, comprised of 8” drop panels and a 4.5”

slab. Use of 30” x 30” dome forms will allow for 3’-0” on-center spacing of

the 6” ribs. A 13” x 13” minimum size column is required for shear stresses in

the floor. At this time sizing of shear panels around column have not been

calculated. Reinforcement in the column strip is 2 - #6 bottom bars and 21 -

#5 top bars and includes 4 ribs. The reinforcement in the middle strip is a #4

for the long bar and a #5 short bar for bottom bars, while the top bars are to

be 8 - #5.

Advantages of a waffle slab design are the ability of it to span long spans

with minimal deflection and self weight. A disadvantage to it is the amount

of time involved in placing formwork and rebar. The extra labor required will

add to the building cost and lengthen the time it will take to erect the

structure.

Steel Joists – Alternate System 3

The third system I chose was to use a total slab

thickness of 4.5” reinforced concrete with 6x6

W2.1xW2.1 welded wire fabric or a fiber mesh

on non-composite metal decking. The

concrete compressive strength is 3000 psi. The

1.5 C24 galvanized decking is on 18LH4 open

web steel joists spaced 4’-0” on center with 2

rows of bridging. The joists span from east to west and sit on W18x55 non-

composite beams. The beams spanning east to west between columns

were designed as non-composite W16x31 beams as illustrated in the




diagram is provided. All steel has a tensile strength of 50 ksi. The 4.5” slab

was required for a 2-hour fire rating and beams will need to have

fireproofing applied to them. The total thickness of the floor system is 22”.

The advantage of a steel structure is the quickness of erection of the

structure and the lighter weight of the total system over concrete.

Disadvantages are the requirement of fireproofing, which increases cost of

the structure and the lead time involved in ordering and fabricated of the

steel members. Also, floor to floor heights are greater than that of concrete

requiring more exterior cladding and also more surface area and higher

lateral loads throughout the structure.

Non-Composite Beams and Composite Decking – Alternate System 4

My fourth and final alternative is the use of non-composite beams with

composite metal decking. The system includes a 4.5” reinforced concrete

slab with 6x6 W1.4xW1.4 welded wire fabric or

the use of an acceptable fiber mesh. The

decking is 1.5VL18 galvanized composite

metal decking on W14x90 beams with tensile

strength of 50 ksi. The total thickness of the

system is roughly 20”.

The disadvantages and advantages are similar to that of the steel joist

system. The possibility of designing this system with composite beams exists

and will reduce beam sizes by weight and depth, which may provide a

more viable option.




Conclusion

The only system I can eliminate at this time is the one-way concrete joists. To

keep the total slab thickness the same, a 48” wide beam is needed

between columns. The current beam is at its limits for my desired deflection

of l/360. The three other systems require further investigation along with the

possibility of making the steel systems use composite steel beams to reduce

beam sizing.

The overall advantages and disadvantages of each system are as follows.

The concrete systems require no additional fireproofing, which steel requires;

thus, limiting its expense. With the foundation sitting on decomposed rock

and having an allowable bearing capacity of 20 to 40 tons per square foot,

weight of the building structural system is not a major concern. It is not a

factor in selection of a flooring system. All systems can be designed as

moment frames to resist lateral loads. As mentioned above, steel floor

systems require a larger floor-to-floor height creating a need for more

exterior cladding. Also, the building will be taller and have to resist more

lateral loads from wind due to the increase surface area of the façade.




Structural Floor Systems Comparison Chart

System Overall Thickness

Overall Weight Advantages Disadvantages Further

Investigation

Two-way Flat Plate 8" 100 psf

Thin floor system Ease of placing formwork Fireproofing not required

Heavy system Labor intensive Existing

One-Way Concrete Joist

12.5" 72 psf Fireproofing not required Low floor-to-floor heights Desirable finish

Labor intensive Takes longer to construct No

Waffle Slab 12.5" 91 psf Fireproofing not required Low floor-to-floor heights Desirable finish

Labor intensive Takes longer to construct Yes

Steel Beams and Joists 22" 60 psf

Lighter structure Quick construction Ease of construction

Requires fire proofing Long material lead times Taller structure

Yes

Steel Beams and Composite Metal Deck

20" 51 psf Lighter structure Quick construction Ease of construction

Requires fire proofing Long material lead times Taller structure

Yes




Appendix Title Pages 1. One-Way Joist System 10-11

2. Two-Way Waffle Slab 12

3. Steel Joists 13

4. Steel Beams 14

pro-con structural study of alternate floor systems...and having an allowable bearing capacity of 20...

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