Jonathan Hill Structural AE Faculty Consultant – Dr. Hanagan Lynde and Harry Bradley School of Technology & Trade Milwaukee, Wisconsin ____________________________________________________________________________________

Pro-Con Structural Study of Alternate Floor Systems

October 27, 2004 ____________________________________________________________________________________

Executive Summary

This report demonstrates the advantages and disadvantages of the existing structural system for the Lynde and Harry Bradley School of Technology & Trade. Alternate systems are designed and compared to the existing system. When analyzing the alternate systems, general assumptions were made and design criteria were simplified. These systems are not complete designs; however, for the purpose of this pro-con analysis typical designs are effective. The specific systems discussed in this report are as follows:

Existing System: One-Way Pan & Joist - Good fire resistance and durability - Fast construction (no lead time)

Alternate #1: One-Way Slab with Concrete Beams

- Good fire resistance and durability - Heavy system with a larger overall depth

Alternate #2: Two-Way Flat Slab – Square Panels with Drops - Good fire resistance and durability - Heavy system and difficult construction (complicated reinforcement)

Alternate #3: Two-Way Pan & Joist

- Good fire resistance and durability - Lightest concrete system - Difficult formwork needed for construction

Alternate #4: Composite Steel Beams with Composite Slab on Metal Deck

- Lightest overall system - Additional fireproofing needed

Alternate #5: One-Way Slab with Concrete Encased Steel Beams

- Good fire resistance and durability - Extremely heavy, inefficient, and costly

The existing system is the most effective system for Bradley Tech. Alternate #1, #3, and #4 are viable options based on their typical design details and their ability to work within the building’s changing design conditions.

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System Analysis Existing System: One-Way Pan & Joist

Design CriteriaDead load: 105 psf

(includes self wt.) Live load: 150 psf Wu = 1.2D+1.6L: 366 psf Typical bay: 32’-0” x 30’-0” f’c = 4 ksi, fy = 60ksi, NWC

Design Results

Total Depth: 25” Slab: 5” thick Pan & Joist:

20” pan + 5” slab Joist width: 8” Pan width: 53” or 66” typical Reinforcement:

Top: 3 - #10 Bot: 3 - #9 Stirrups: 16 @ 10”

Beams: Size: 28” x 25” or 24” x 25” Reinforcement:

Top: 7 - #11 Bot: 5 - #9 Stirrups: as needed

Advantages

A pan and joist system is more economical than steel systems or other concrete systems. In this one-way system, the concrete that is in tension (which is usually ignored in design) is eliminated with the use of pan forms thus reducing cost.

Due to concretes weight and stiffness, vibrations through the system are severely limited. Through design the deflection was limited to well less than L/360.

The main reason this system was chosen instead of a standard steel system is the advantage concrete has when considering fire resistance. The simple 5” slab supported by the concrete joists does not require additional fireproofing, whereas steel would need to be sprayed with a fireproofing material.

The ability of this system to adapt by changing spacing, depths, and widths allows the system to be used under changing loads and bay sizes.

Disadvantages

This system, like most concrete systems is still quite heavy, which could negatively affect the seismic response of the building.

Constructing this system is more time consuming than a simple steel erection. Pan forms must be used and reinforcement placed correctly, each of which requires additional man hours.

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Alternate #1: One-Way Slab with Concrete Beams Design Criteria

CRSI Handbook Interior continuous span Wu = 250 psf (slab), Wu = 2.6 k/ft (beams) f’c = 3 ksi (slab) = 4 ksi (beams)

fy = 60ksi, NWC Typical bay: 32’-0” x 30’-0”

Design Results Total Depth: 28” Slab:

Span: 8’-0” Thickness: 4” Reinforcing:

Top: #4 @ 12” Bot: #3 @ 12” T-S: #3 @ 15”

Weight: 50 psf Beams:

Span: 30’-0” Size: h = 20”, b = 12” Reinforcing:

Top: 2 - #14 Bot: 2 - #10

Girders: Span: 32’-0” Size: h = 28”, b = 20” Reinforcing:

Top: 5 - #14 Bot: 2 - #14

Advantages

A system with concrete beams and girders framed into concrete columns is very adaptable under various load conditions. The framing can be adjusted to accommodate changes in loads and spans which vary throughout this building.

With the use of concrete beams, deflection is kept to within acceptable limits and floor vibration is minimal.

Additional fireproofing is not necessary because the concrete cover is thick enough to provide at least a 2hr rating.

Disadvantages

Like all concrete systems, this current system tends to weigh more than the typical steel system. The extra weight is not that much more than the originally designed building, so it will have a minor effect on the seismic and foundation systems of the building.

Erecting and reinforcing the forms prove to be the biggest drawback. If beams and girders are not kept to a typical size and spacing, significant time will be needed to cast the system.

The large girders cause an unwanted large total system depth. 3

Alternate #2: Two-Way Flat Slab – Square Panels with Drops Design Criteria

CRSI Handbook Interior continuous span Wu = 250 psf – use 300 psf f’c = 4 ksi, fy = 60 ksi, NWC Typical bay: 32’-0” x 32’-0”

Design Results Total Depth: 23” Column Size: minimum 23” Slab:

Span: 32’-0” Thickness:

12” between drop panels Drop Panels:

10’-9” wide / 11” depth Reinforcing (e.w.):

Column Strip: Top: 22 - #6 Bot: 15 - #7

Middle Strip: Top: 12 - #7 Bot: 13 - #6

Advantages

This two-way system is a very durable design. There is little formwork required and placement of the system involves casting concrete over a large area. This saves time and labor costs.

The designed thickness of the slab provides an acceptable fire rating of 2hrs. Vibration and deflection are deemed acceptable for the typical span according to

CRSI Manual calculated values based on the stiffness and reinforcement of the concrete.

Disadvantages

The main drawback of two-way slabs with drop panels is the complicated reinforcement strategy that is needed. Each area throughout the slab has its own detailed reinforcement schedule that must be accurately followed. Extra time must be taken in placing these bars.

When looking at use of this system in Bradley Tech it seems to be too complicated. With so many varying loads, each section of the building will have to be carefully designed and detailed. Construction of such a detailed system would require extra time and attention. Both increase the cost.

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Alternate #3: Two-Way Pan & Joist Design Criteria

CRSI Handbook Interior continuous span Wu = 250 psf – use 300 psf f’c = 4 ksi, fy = 60 ksi, NWC Typical bay: 32’-0” x 32’-0”

Design Results for Typical Bay Total Depth: 15” Span: 32’-0” Total depth:

3”(slab) + 12”(rib) = 15” Pan & Joist:

30” x 30” voids 6” ribs @ 36”

Column: 19” x 19” with shear reinf. Stirrups: 4S62

Reinforcing (e.w.): Column Strip:

Top: 35 - #6 Bot: 5 ribs – 2 - #8 per rib

Middle Strip: Top: 11 - #6 Bot: 6 ribs – #6 long bars, #6 short bars

Advantages

Of the alternative concrete systems analyzed for use within this building the two-way pan & joist tends to be the lightest. Its thin slab reduced the overall weight.

The most efficient use of this system is with square bays. The majority of Bradley Tech has near perfect square bays; however in the areas where the bay sizes vary an adjustment to the void or rib dimensions would be needed. If done too often, adjustments could actually act as a disadvantage due to the increase cost.

Because no weight was added to the system, the foundations would not have to be redesigned, unless cost emerged as an issue. In that case the foundations could be decreased to reflect the new self weight of the system.

Disadvantages

Because of the thin slab used in this system, additional fireproofing measures may need to be taken in areas that require a 3hr rating. A simple 1” increase in the slab thickness could solve this problem.

Casting this system is very labor intensive. A complex arrangement of forms is needed, but if done properly large areas could be cast at one time - effectively off-setting additional labor costs.

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Alternate #4: Composite Steel Beams with Composite Slab on Metal Deck Design Criteria

Ram Model Interior continuous span f’c = 3 ksi, NWC fy = 36 ksi Typical bay: 32’-0” x 30’-0”

Design Results Total Depth: 26” Slab:

Span: 8’-0” System:

3” metal deck w/ 2 ½” slab 5” long - ¾” dia. studs

Reinforcing: 6x6xW2.9xW2.9 wwf

Beams: Span: 30’-0” Size: W16x26

Girders: Span: 32’-0” Size: W21x44

Advantages

This steel system is comparably lighter than the concrete systems previously used. The composite actions of the slab and deck help to reduce the size of the members.

Erection of steel is fast and easy. While a crane will be needed to place the steel members, the time-efficiency of placing the steel more than makes up for the extra cost.

Since this is a typical steel framing system it can be used in most building scenarios. Changing bay sizes or loading patterns only mean a simple change in beam size. Connection details remain standard throughout the building

Disadvantages

Steel is a high quality design material because of its strength. Steel suffers from vibration effects much more than a concrete system.

Deflections can become a problem when dealing with heavy loads and large spans, which are present in this building. A simple camber of the beam could rectify the problem; however, in some heavily loaded areas lateral bracing or mid-span reinforcement may be necessary.

The lateral system for this building is minimal because of the extensive use of concrete columns and floor framing members. If steel were to be used, the lateral system will have to be redone. Moment connections, braced frames, or other lateral supports will be needed to support the wind and seismic loads acting on the building.

Unlike its concrete counterpart, this steel system requires fire proofing. The large amount of steel needed only means an even larger amount of spray-on fire proofing, an obvious cost drawback.

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Alternate #5: One-Way Slab with Concrete Encased Steel Beams Design Criteria

Ram Model Interior continuous span f’c = 4 ksi, NWC, fy = 36 ksi Typical bay: 32’-0” x 30’-0”

Design Results Total Depth: 27 ½” Slab:

Span: 8’-0” System:

5” Slab 4” long - ¾” dia. studs

Reinforcing: 6x6xW2.9xW2.9 wwf

Beams: Span: 30’-0” Size: W16x26

Girders: Span: 32’-0” Size: W21x44

System Analysis

The main drawback to an all steel structural system is the expense of fireproofing the members. This was the principal reason the engineers elected to use a concrete system. To incorporate the fireproofing advantages and the accessibility of a monolithic concrete system with the overall strength and versatility of a steel system, a system using steel members encased in concrete meeting a one-way slab was conceived and analyzed. This system is, in effect, a combination of alternate #1 and #4 from above. Cover thickness were determined as follows:

Slab: ¾” cover (5” thickness) Column: 1 ½” cover Beams/Girders: 1 ½” cover

After analyzing the resulting beam and slab system according to AISC specifications, the moment capacity was compared with that of the standard steel system described in alternate #4. The results showed that the composite beams increased in strength 4-10%. This could enable the use of smaller steel members inside the concrete. However, the increase in weight of the system caused by the additional concrete severely affects the remaining building systems. The increase in load would cause the need to redesign the foundation and adjust for higher seismic forces. Floor-t- floor height is compromised for fire protection and a small increase in strength. Construction of this system is also a drawback. Forms for the members and placement of shear studs help to confirm the notion that this would not be an acceptable replacement system for the specific building.

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System Comparison

The comparison chart presented below was developed to summarize the different aspects of each structural system and how they respond to design criteria. Each system is compared using a series of factors that reflect the most important characteristics when looking at a structural system and its place in this specific building. The individual systems are ranked in order from 1 to 6 (1 being the best and 6 being the worst) according to how they respond to the individual factors. An average score is calculated and the systems are ranked.

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Factors Structural SystemsExisting One-

Way Pan & JoistOne-Way Slab w/ Concrete Beams

Two-Way Flat Slab w/ Drop Panels

Two-Way Pan & Joist

Composite Steel Beam & Metal Deck

One-Way Slab w/ Concrete Encased Beams

Weight 3 4 6 2 1 5Depth 3 6 2 1 4 5

Fire Protection 1 3 4 2 6 5System Cost 1 3 5 2 4 6

Floor Vibration 1 2 4 3 6 5Durability 3 1 2 3 6 5

Constructablility 2 4 3 5 1 6Construction Time 2 4 3 5 1 6

Score 2.0 3.4 3.6 2.9 3.6 5.4

Rank 1 3 5 2 4 6

Based on the chart above, I concluded: 1. The existing pan & joist system bets fits the structural system required for Bradley Tech. 2. A two-way pan & joist system as well as a one-way slab with concrete beams system also have acceptable scores. 3. An all steel system is viable option; however, fire protection for the steel will increase the cost. 4. A two-way slab and concrete encased beams were ranked very high and should most likely be avoided in this specific building.

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Conclusions After analyzing each building system a condensed list of advantages and disadvantages was created to determine the systems overall potential to be used in Bradley Tech. The list follows:

Systems Advantages Disadvantages Potential Existing One-Way Pan & Joist

- Fireproof - Adaptable to building

- Heavier than steel - Longer Construction than steel

Yes

One-Way Slab w/ Concrete Beams

- Fireproof - Adaptable to building

- Heavy system - Deep System

Yes

Two-Way Flat Slab w/ Drop Panels

- Fireproof - Simple formwork

- Heavy system - Labor intensive - Complicated reinforcement

No

Two-Way Pan & Joist - Fireproof - Adaptable - Reasonably light system

- Labor intensive - Complicated formwork

Yes

Composite Steel Beam & Metal Deck

- Light system - Easy to construct - High strength

- Additional fireproofing required - Total system redesign - Long lead time

Yes

One-Way Slab w/ Concrete Encased Beams

- Fireproof - High strength

- Heavy system - Footing redesign - Deep system

No

After looking at the system comparison chart and the advantages and disadvantages for each system, decisions were made regarding the effectiveness of each structural system based on the needs of Bradley Tech. The one-way pan & joist system initially used in the building seems to be the most effective and economical solution for structural demand. A waffle slab is a viable option as is a one-way slab supported on concrete beams. Both of the concrete options utilize the existing columns and would not require any redesign of the foundation systems. The final option that has potential is the steel system using composite steel beams and metal deck. This system however, would need column designs, a redesign of the foundation, and an elaborate lateral system that was not needed with the previously mentioned monolithic concrete systems. Calculations and Assumptions made in this analysis can be found in the Appendix.

Appendix Contents: A - 1: Existing: System Details B - 1: Alternate #1: Slab and Joist Design B - 2: Alternate #1: Girder Design C - 1: Alternate #2: System Design

D - 1: Alternate #3: System Design E - 1: Alternate #4: Ram Design Output E - 2: Alternate #4: Ram Beam Details E - 3: Alternate #4: Ram Beam Details E - 4: Alternate #4: Ram Girder Details E - 5: Alternate #4: Ram Girder Details F- 1: Alternate #5: System Design