emily whitbeck structural option faculty consultant ... · pdf filefaculty consultant –...

Emily Whitbeck Structural Option

Faculty Consultant – Professor Parfitt

RiverView Condominiums (Phase II) Chicago, IL

Technical Report #2 Pro/Con Structural Study of Alternate Floor Systems

Executive Summary This report contains: - A description of the existing floor system at RiverView II - An investigation of 8 alternate floor systems including: (2) composite steel systems Concrete Waffle Slab system Flat Slab w/ Drop Panels One-Way Pan Joist system Double-Tee Beam system Hollow Core Plank system Solid Flat Slab system

- A comparison and contrast of the investigated systems including their advantages and disadvantages

The existing floor system at RiverView II is primarily a flat plate system. There are small places where one-way slabs or other systems are used, but the two-way flat plate is the primary system. The tower is the focus of this report and in that area the slab is 8.5” thick and supports a superimposed dead load of 5 psf and a live load of 40 psf. None of the bays in the layout of the tower are of a regular shape and size. Therefore the largest overall bay was used so as not to under design any system.

Since a two-way system is the existing design even though the bays are not square, other two way systems are included in this report. Additionally, since the bays are not square, but are instead rectangular one-way systems and spanning systems were tried. The existing column layout was not changed because this is dictated by the architectural design and the condominium layout.

The option of reducing the weight of the two-way system was investigated in the waffle slab. This resulted in almost a 23% reduction in weight, but because of the complexity of construction involved which increases construction time and budget the flat plate system is preferred over the waffle slab system.

Steel construction results in lighter buildings with easy and speedy erection. Composite systems result in the highest overall reduction in weight by combining the strength of the concrete and steel. However these systems do pose problems when locating mechanical/electrical equipment. Because of the inherent pros, steel should be investigated further, although castellated beams or open web steel joists might offer more benefits and should therefore, be studied.

The one way pan joist system was chosen to try to reduce the weight of the flooring system. It was quite effective at that, reducing it by 40%. The only problem is that to support the slab on a girder increases the floor section depth at that beam. This may be explored further.

The two-way slab system was considered as an option to reduce the weight of the system. Instead it increased it and increased the floor section depth. This option will not be investigated further as the flat plate system is superior for this design.

The precast/prestressed systems were looked into because of some of the longer spans. The Double Tee and Hollow Core systems effectively reduced the overall weight of the system without adding too much complexity to the construction. More highly skilled construction personnel would be necessary when dealing with prestressing however, unless it is pretensioned at the fabricators in which case the material cost increases slightly. The solid flat slab system was ineffective at either reducing weight or depth and is therefore not going to be looked into further.

Introduction Using several different sources, the existing floor system as well as alternative systems will be investigated and compared/contrasted based on several different criteria. The loads used to design the systems are a superimposed dead load of 5 psf and a superimposed live load of 40 psf. This results in a superimposed service load of 45 psf and a superimposed factored load of 70 psf. To compare the systems a typical bay is chosen from the 13th floor that has the longest dimensions so that the systems won’t be under designed for the overall floor. The bay is taken from the northwest corner of the building and is shaded in Fig. 1. Criteria General criteria will be used to evaluate and compare and contrast the various systems including the existing one. These are not exhaustive ideas, but are meant to be a starting point for finding the ideal floor system. The criteria used in this report to govern the design of the systems are:

1. Column layout is to remain the same. 2. Ceiling-to-floor section depth should be

minimized to minimize overall height and maintain the same floor to ceiling heights for occupation. Fig. 1

3. Weight of the system should be minimized. 4. Fire rating of the assembly should conform to BOCA guidelines. 5. Constructability and cost are concerns on any project and should be considered.

Column Layout The column layout is primarily dictated by the architectural floor plan. Columns are placed according to the apartment layouts on each floor and so that they line up between floors even though the apartment layouts change. Because of the complexity of this detail, the column layout will not be changed for the purposes of this report. Ceiling to Finish Floor Section Depth Based on the existing plan the section depth varies from 1’-8” to 2’-8” depending on the depth of the duct running in the space. Therefore the floor-to-ceiling heights vary from 8’-0” to 9’-0”. For this analysis the largest overall depth will be considered with a constant depth of duct equal to 1’-5” and the overall existing section equal to 2’-8”. The existing overall design of the ceiling-to-floor section is Fig. 2.

For the purposes of this structural report most of this section will not change. The slab thickness, however, will be under consideration as this could increase or decrease the depth of the entire assembly. So, every alternate system’s depth will be considered against the existing depth to see whether it increases or decreases the overall section. In addition to slab depth changes, physical arrangements of the structural elements may impact the section depth. Fig. 2

Overall height of the building, including floor-floor height can impact the cost exponentially. Many of the systems in a building cost more as the building increases in height and therefore, volume, including the cladding, stairwells, elevators, and mechanical equipment for example.

Weight of the System The existing system uses normal weight concrete at 150 pcf with an 8.5” thick slab. This results in a distributed load of 106.25 psf over the entire bay. Since a typical bay is being used for all the options this number will be compared to the distributed weights of the other options. All the options will take into consideration the weight of all members including any beams, joists, girders, or decking used. Overall weight of the structural system isn’t the highest priority, but it is something to take into account as it requires other elements to be sized up or down and could impact the seismic loading. Fire Rating According to the drawings, BOCA guidelines dictate that the slabs for this building have a 3 hour fire-rating. This means that it has a 6.2” minimum thickness. The existing system has an 8.5” slab and more than meets this qualification. Other systems alone may or may not meet the 3 hour fire-rating requirement, but this does not disqualify them from consideration, it just means that further design is required to achieve the rating. This may include fireproofing or different ceiling and finished floor materials. Constructability and Cost Although not the highest priorities for this report both are always important considerations in the design of any building. For this report they will be considered generally in terms of the time required for construction, the weight/amount of materials required, and the overall constructability of any of the systems. Design Aids To complete this report three design aids were used. The assumptions and criteria that governed each are listed here. Any other considerations are addressed specifically under each system.

RAM Structural System was used to design the steel systems. Several assumptions went into the design. The criteria used in RAM to design these models that differs from the default settings of the program includes the use of the LRFD code to control the design and the deflection limits were changed to l/360 for all the loading conditions.

The CRSI Design Handbook 2002 aids in designing concrete structural systems. Throughout the tables, fc’ is assumed to be 4,000 psi from normal weight concrete and grade 60 bars are used for all the reinforcing. All the concrete designs from the CRSI Design Handbook are based on factored loads.

The PCI Design Handbook fifth edition aids in designing prestressed or prescast concrete elements. For all of the systems designed using this book fc’ is taken as 5,000 psi, fpu is taken as 270,000 psi, and fci is taken as 3,500 psi. These designs are based on service loads. Systems Nine systems will be considered in this report. They include the existing flat plate system, two different layouts of a steel system of composite framing with composite slab, a waffle slab system, a flat slab with drop panels, a one-way pan joist system, a double-tee precast/prestressed system, a hollow core precast/prestressed system, and a solid flat slab precast/prestressed system. Existing The current system is a flat plate continuous two-way slab. It is 8.5” thick for the tower levels and this particular bay contains reinforcing as follows: 23 #5 in the top and #4 @ 10 in the bottom of the EW

column strips, 14 #4 or 9 #5 in the top and #4 @ 8 in the bottom of the EW middle strip, 9 #5 in the top and #4 @ 10 in the bottom for the NS exterior column strip, 23 #5 in the top and #4 @ 10 in the bottom of the NS interior column strip, and 15 #4 in the top and #4 @ 8 in the bottom of the NS middle strip. The depth of the floor section is currently 2’-8” from the top of the finished floor to the bottom of the ceiling finishes. The total weight of the slab floor system is 106.25 psf distributed over the entire bay. This system more than exceeds the requirements for a 3 hour fire-rating. Steel System – 1 (12’-6” span) RAM is used to design a steel system for the typical section of the tower under consideration. An intermediate beam spans the long direction, dividing the bay in half and creating a span of 12’-6”. Vulcraft’s 3VL composite slab system is used with a 2” top slab for a 5” total slab depth. Light weight concrete with a minimum compressive strength of 3 ksi is the material used for the slab and this leads to an allowable superimposed live load of 45 psf for a 12’-6” span.

The steel framing system consists of composite beams and girders. Analysis of the beams and girders finds them all adequately designed for the aforementioned loading. Beams:

W10x12 Mu = 39.5 k-ft ΦMn = 46.94 k-ft > Mu


W12x16 Mu = 67.6 k-ft ΦMn = 75.37 k-ft > Mu Girders:


This results in about 2360 lbs of steel weight for an overall self weight distributed load of 37.15 psf over the entire bay, including the slab, decking and steel beams.

In the NS direction the deepest steel beam crossed is 13.7” and in the EW direction it is 12.0”. The overall section depth is decreased by 3.5” per floor if the mechanical/electrical equipment never had to move between spans. However, because it does, the section becomes 3’-6.2”, increasing it by 10.2” per floor. The beams however are not deep enough for 1’-5” duct work to fit through the web if the beam was castellated.

The fire rating of this system cannot be determined without knowing more about the ceiling assemblies. However, if the type of protection used is cementitious then a 3 hour fire-rating can be achieved, if not further fire proofing or a deeper top slab will be necessary. The beams and girders themselves will need to be fireproofed, either by containment in the ceiling behind fire-rated materials, or by applying fireproofing directly to them.

Steel System – 2 (10’-0” span) RAM is used to design a steel system for the typical section of the tower under consideration in a similar way to the previous alternative. Two intermediate beams span the 25’ dimension dividing the bay into thirds. Vulcraft’s 3VL composite slab system is used with a 2” top slab for a 5” total slab depth once again. Light weight concrete with a minimum compressive strength of 3 ksi is the material used for the slab and this leads to an allowable superimposed live load of 75 psf for a 10’-0” span.

The steel framing system consists of composite beams and girders. Analysis of the beams and girders finds them all adequately designed for the aforementioned loading. Beams:


W10x12 Mu = 37.9 k-ft ΦMn = 46.94 k-ft > Mu Girders:



This results in about 2240 lbs of steel weight for an overall floor system distributed load of 37.00 psf over the entire bay, including the slab, metal form and steel beams.

In the NS direction the deepest steel beam crossed is 9.87” and in the EW direction it is 13.7”. The section depth is decreased by 3.5” per floor if the mechanical/electrical equipment never had to move between spans. However, because it does, the section becomes 3’-6.2”, increasing it by 10.2” per floor. The beams however are not deep enough for 1’-5” duct work to fit through the web if the beam was castellated.

The fire rating of this system cannot be determined without knowing more about the ceiling assemblies. However, if the type of protection used is cementitious then a 3 hour fire-rating can be achieved, if not further fire proofing or a deeper top slab will be necessary. The beams and girders themselves will need to be fireproofed, either by containment in the ceiling behind fire-rated materials, or by applying fireproofing directly to them. Waffle Slab The CRSI Design Handbook is used to design a waffle slab for the factored superimposed load on the typical bay. A waffle slab is designed that had a 30”x 30” void with 6” ribs at 36” on center spanning the long dimension of 30’. This allowed for 100 psf factored load on the bay with #4, #5, and #6 steel reinforcing bars in the ribs. The total depth was 11” and this included a rib depth of 8” and a slab depth of 3”. Increasing the depth of the waffle slab only decreased the steel slightly and decreasing the size of the voids changed almost nothing.

Based on the 0.545 cf/sf factor the distributed weight of the slab is 81.75 psf.

This bay is not a square, but to use the design handbook it was approximated as a 30’x 30’ square bay, which is slightly conservative since the shorter dimension is 25’, but also demonstrates the point that this system may not work on this column grid. The design loading as well is conservative since the handbook only calculates sizes for 50 psf or 100 psf factored loading cases.

The overall floor section is increased by 2.5” per floor, the difference between the 11” waffle slab and the 8.5” flat plate. Although there are voids in a waffle slab there are no continuous paths to run the mechanical/electrical equipment through so it has to be run underneath the slab.

Since the slab, which covers the entire space, is only 3” thick this does not meet the 6.2” minimum slab thickness to achieve the 3 hour fire-rating. The fire-rating could be increased based on the waffle slab design or if different ceiling and construction materials were used in the rest of the section assembly. This will have to be considered later. Flat Slab with Drop Panels

The CRSI Design Handbook is used to design the two-way flat slab system for the factored superimposed load on the typical bay. Since the system is two-way both an exterior design and an interior design were used to find the reinforcing for the typical bay. To span the 30 feet and carry a superimposed factored load of 100 psf the slab is 10”. The drop panels are 8.5” and are 10’ wide (1/3 of the clear span). The reinforcing bars in the column strips are primarily #5, #6, and #8, and in the middle strips they are #5 and #6.

Although the overall depth of the system is 18.5”, the mechanical/electrical equipment could run between the drop panels and therefore the slab alone can be considered in the depth of the floor section unless the deck section, 18.5”, is larger. The overall depth is 2’-9.5”, 1.5” greater than the current system.

This bay is not a square, but to use the design handbook it was approximated as a 30’x 30’ square bay, which is slightly conservative since the shorter dimension is 25’. This short dimension would also lead to a different size drop panel in that direction so for further investigation this system would need to be fully designed. The design loading as well is conservative since the handbook only calculates sizes for 50 psf or 100 psf factored loading cases.

Without beams and girders the only structural distributed load is due to the slab, the drop panels, and the reinforcing. Using the 25’ x 30’ typical bay the

overall distributed load due to the self weight of the structure is 191.1 psf.

This slab is more than adequate to achieve the required 3 hour fire-rating. The minimum slab thickness required is 6.2” and this slab is 10” at its thinnest. One-Way Concrete Pan Joist

The CRSI Design Handbook is used to design a one-way pan joist system for factored superimposed load on the typical bay. The slab is designed to span the 30’ dimension so it is an exterior span. The 20” form with 5” rib at 25” on center had a capacity of 93 psf. The steel reinforcing is all #5 bars. At the ends of the slab are girders spanning the 25’ dimension between columns. These girders are 12”x18” with 664 lbs. of reinforcing steel.

The overall depth of the one-way slab system is 13”, consisting of 10” deep ribs with a 3” top slab. These sit on top of the beams at either end. So as long as the mechanical/electrical equipment can run between the joists, clear space is 20” between them, then there is a reduction in the overall floor section of 5.5”. If at any point the equipment crosses under the interior beam then the overall depth jumps to 46.5”. This increases the overall depth by 14.5”. The beams at the ends however will control the depth of slab as mechanical/electrical equipment will need to pass underneath them.

The floor system alone results in a distributed load of 39.58 psf. This in conjunction with the weight of the girders and the extra reinforcing steel in the girders results in a system weight of 41.1 psf across the entire bay.

With only a 3” top slab, there are areas of the flooring which do not satisfy the 6.2” minimum slab thickness. More action will have to be taken to increase the fire-rating of this system as previously outlined. Double Tee Beams The PCI Design Handbook is used to design a Double-Tee beam system for superimposed service loads on a typical bay. The Tees are designed to span the 30’ dimension. An 8LDT12 of light weight concrete with a 68-S strand pattern allows a superimposed service load of 47 psf. This section is 12” deep overall with the top slab only 2” deep. The Double-Tee is 8’-0” wide. There are 6 - ½” diameter strands strung straight through the section.

These sections will be supported on rectangular beams at each end spanning 25 feet (26 feet in the design book). The superimposed load on the beams is 2280 plf and this results in a 28IT20 inverted T beam. The dimensions are b top = 1’-0”, b bottom = 2’-4” and h = 20”. There are 9 - ½” diameter strands and the beam weighs 383 plf. This results in a total floor system weight of 54.3 psf over the entire bay.

Even though the top slab is only 2” which should reduce the overall floor section, because there are beams at either end of each span this can’t happen. Using inverted T beams allows the double tee panels to rest on the seats of the beams. This results in concrete floor depth of only 20”. The overall floor

section depth increases to 3’-7.5”, an increase of 11.5” from the existing system. If however raceways or other openings could be cut into the rectangular beams to allow the mechanical/electrical equipment through this section would decrease.

With only a 2” top slab it could be hard to achieve the 3 hour fire-rating required. Other considerations would have to be made similar to the systems above. Hollow Core Planks The PCI Design Handbook is used to design Hollow Core concrete planks for superimposed service loads on a typical bay. The planks are designed to span the 30’ dimension so that the supporting beams need

only span 25’. A 4LHC8 plank of light weight concrete with a 76-S strand layout has a superimposed service load capacity of 62 psf. This section is 8” deep overall without the extra 2” topping option. The plank is 4’-0” wide with 7 - 3/8” diameter strands strung straight through the plank.

These planks will be supported on rectangular beams spanning 25 feet (26 feet).The superimposed load on the beams is 3210 plf resulting in a 12RB28 rectangular beam. The beam dimensions are b = 12” and h = 28”. There are 12 - ½” diameter strands and the beam weighs 350 plf. This results in a total floor system weight of 69.3 psf over the entire bay.

The Hollow Core plank itself is only 8” deep which would reduce the floor section by 0.5”. But once again the end beams control the system and unless they can be cut to allow mechanical/electrical equipment to pass through the floor section depth is going to increase.

Since the plank is 8” deep it more than meets the requirements of the 3 hour fire-rating. It may however need to be investigated further to make sure that the hollows do not affect the fire-rating. Fig. 3 – Plan for Hollow Core and Solid Flat Slab Construction Solid Flat Slab The PCI Design Handbook is used to design solid flat slabs for superimposed service loads on a typical bay. The slabs are designed to span the 30’ dimension so that the supporting beams need only span 25’. A LFS8 flat slab of light weight concrete with a 58-S strand layout has a superimposed service load capacity of 57 psf. This section is 8” deep overall without the extra 2” topping option. The slab is 4’-0” wide with 5 - ½” diameter strands strung straight through the slab.

These slabs will be supported on rectangular beams spanning 25 feet (26 feet). The superimposed load on the beams is 4020 plf resulting in a 12RB28 rectangular beam. The dimensions and other physical properties are the same as those listed above. This results in a total floor system weight of 100.3 psf over the entire bay.

The Solid Flat slabs are only 8” deep as specified, but as stated before the end beams control the floor section depth. Unless mechanical/electrical equipment is allowed to pass through the beams the section depth will increase.

The slab is 8” of light weight concrete. This exceeds the minimum thickness of 6.2” required for the 3 hour fire-rating. The effect that light weight concrete versus normal weight concrete has on fire-rating does need to be investigated. Compare and Contrast System Depth Weight (psf) Pros Cons Flat Plate 2’-8” 106.25 Easy construction

Small floor section depth Good fire-rating

High weight

Steel 1 – composite/composite

3’-6.2” 37.15 Easy construction Low weight

Fire proofing would need to be considered

Large floor section depth

Steel 2 – composite/composite

3’-6.2” 37.00 Easy construction Low weight



Waffle Slab 2’-10.5” 81.75 psf Small floor section depth Complicated construction


High weight Flat Slab w/ Drop Panels

2’-9.5” 191.1 Easy construction Small floor section depth Good fire-rating

Very high weight

One-Way Pan Joist 3’- 8.5” 41.4 Low weight Easy form construction


Double-Tee 3’-7.5” 54.3 Low weight Easy construction


Hollow Core Plank 4’-5.5” 69.3 Low weight Easy construction


Solid Flat Slab 4’-5.5” 100.3 Easy construction Large floor section depth

High weight

Appendix Waffle Slab Design Aid

Flat Slab with Drop Panels Design Aid

One-Way Pan Joist Design Aid

Double Tee Design Aid

Hollow Core Plank Design Aid

Solid Flat Slab Design Aid

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