Executive Summary Pro/Con Structural Study of Non-composite concrete floor system on metal deck supported by steel beams ¾ Non-composite ... 2”, 4000 psi, lightweight cellular

North Carolina A&T State University Mick Leso School of Education Building Structural

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Executive Summary Pro/Con Structural Study of Alternate Floor Systems

Technical Assignment #2 describes the existing floor systems in the James C.

Renick School of Education Building located in Greensboro, North Carolina. A North Carolina A&T State University building, the floor system will see various loadings throughout its existence, most importantly students and faculty. The live loads are a major consideration in a structure of this building use, since rooms could eventually be gutted and changed to be used as storage. This technical assignment contains a complete summary explaining the different structures used to support the floors above grade.

The existing system is a composite system made of a reinforced concrete slab on metal deck. The decking is connected to the supporting beams by welded shear studs that also bind the concrete together with the steel supports to create the composite action.

The other floor structures considered in this report are as follows:

Non-composite concrete floor system on metal deck supported by steel beams Non-composite concrete floor system on metal deck supported by open web joists Reinforced concrete slab with one-way joists supported by concrete girders Prestressed concrete hollowcore slab supported by steel girders

The premise of this report is to find another possible floor system that could

replace the existing arrangement. The characteristics in question include self weight of the structure, cost savings issues, time and ease of erection, and most importantly strength of the system as a whole. What is not considered in full are the affects of lateral loads on these systems. Also, the influence the floors have on the vertical members of the building, including the foundation and soil elements has not yet been accounted for. It has been documented where further research is required to determine the full capacity of the structure due to lateral loads.


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

Table of Contents

1. Existing Floor System………………………………………………3 2. Typical South Building Bays..……………………………………4 3. Alternative Floor Systems…………………………………………6

a. Non-composite with Steel Girders b. Non-composite with Open Web Steel Joists c. Reinforced Concrete Slab with One Way Joists d. Prestressed Concrete Hollowcore Slab

4. Conclusion……………………….……………………………………12 5. Appendix A - Floor Plans………………………………………14 6. Appendix B – Existing Floor Calculations…………………..15 7. Appendix C – Design Loads……………………………………16 8. Appendix D – Open Web Steel Joist Reference…………17 9. Appendix E – One Way Slab with Joists Reference……19 10. Appendix F – One Way Slab Girder Reference.…………20 11. Appendix G – Material Strengths………………………………20 12. Appendix H – Prestressed Hollowcore Reference…………21


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Existing Floor Systems The James C. Renick School of Education Building has a few different floor systems within its walls. The north building has its own separate floor structure, supported by cantilever trusses. The steel frame that connects the two cantilever trusses supports the floor structure. Beams that support a lightweight concrete slab on metal deck come in a few sizes, those being: W21x44, W21x50, W24x55, and W14x22. The slab is 3¼” lightweight concrete, of 4000 psi strength, on 2” – 20GA composite steel deck. The total thickness, of 5¼”, hides the ¾” diameter, 4” long shear studs which provide stability and bind the concrete slab to the steel beams for the extra composite strength. The gravity load from the slab eventually transfers into girders along the truss elevation which are W18x211s at the 2nd floor and W24x55s at the 3rd floor. However, most of the steel frame members in the north building are designed primarily for their strength against lateral loads, due to the large cantilever. For this reason, the alternative floor systems assignment will focus on the south building which has further opportunity for alterations. The north building will be addressed in a later report when there is more information to explain all the forces in the cantilever. At that time, the loads throughout the members of the north building will become evident thanks to computer programs having processed through all the data from different LRFD and ASD loading conditions. The south building’s floor system is just a basic concrete slab poured on composite metal deck, supported by a steel frame. The beams are joined by shear connections and the girders are attached to the columns by moment connections to resist lateral loads. The 3¼” concrete slab on 2” metal deck is the same composite system as in the north building. Beam and girder checks from technical assignment one are listed in Appendix B on page 14 of this report. Shown below is a typical floor layout above the ground floor.

Pros Cons Shallow floor depth Shear stud installation takes time & money 2 hour fire rating without additional fire proofing Concrete poured on site, curing must occur Strength capabilities of a bonded system Fireproofing issues No shoring required


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Typical South Building Bays 2nd & 3rd Floors 3¼”, 4000 psi, lightweight concrete slab on 2” – 20GA composite 40 ksi metal deck supported by wide flange members (typical: W21x44) at 6’3” O.C. with spans of 42’6” & 40’0”. Composite slabs are designed for unshored construction. Reinforcement is #4 top bars @ 1’x6’ centered over all girders running parallel to deck span with ¾” cover from top of slab. Below the top bars at typical depth of 1¼” below top of slab, are sheets of ASTM A185 WWF, placed at a minimum overlap of 8”. The ¾” diameter shear studs, 4” long, will be field welded to the centerline of beams. The number in parentheses next to the wide flange member size on the drawing below gives the number of shear studs on each beam. The studs are spaced uniformly along the length of the beam unless noted otherwise by the drawings.

Typical 2nd/3rd Floor Bay


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Roof 2”, 4000 psi, lightweight cellular concrete slab on 1½” – 22GA 40 ksi metal deck supported by open-web steel joists (typical: 30K9s & 28K8s) at 5’ O.C. with spans of 42’6” & 40’0”.

Typical Roof Bay

Roof structure will not be part of this report.


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Alternate Floor Systems 3¼” Lightweight Concrete Slab on 2” – 20GA Non-Composite Metal Deck @ 2nd Floor South Bldg Remove shear studs from the current system of 3¼” lightweight concrete slab on 2” – 20GA composite metal deck to create a non-composite concrete floor slab. The slab would rest on the existing typical W21x44 beams @ 6’3” O.C. which span 40’. Total slab depth: 5.25” Total weight: 41 psf (1) Pros Cons No shear studs – cost savings Loss of strength against lateral loads No stud field welds – save cost & erection time Less strength against gravity loads Does not require shoring during construction Fireproofing issues Design: Deck: 6’3” spans: uniform service live load Lmax = 400 psf (2)

Max unshored span for a 3 span section: 9.39’ No shoring required Beam check: W21x44 Tributary Area= 6’3” x 40’ = 250 ft2 Dead Load: 55 psf (1)

Live Load (office w/ partition): 80 psf (1) wu = 1.2D+1.6L = 1.2(55) + 1.6(80) = 194 psf w = 194 psf * 6’3” = 1212.5 plf Mu = wl

2/8 = 1212.5 * (402)/8 = 242.5 ‘k Z required = Mu//ΦFy = (242.5 * 12) / (.9 * 50) = 64.67 in

3 W21x44 Z = 95.8 in3 > 64.67 in3

ΦMp = 359.25’k > 242.5’k (3) W18x35 is more efficient Z = 66.5 in3 ΦMp = 249.38’k

1 Appendix C, page 15 2 Pg. 38, 39 United Steel Deck Manual 3 Pg. 5-47 AISC Steel Manual


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Open Web Steel Joists for 2nd Floor of the South Building Remove shear studs from the current system of 3¼” lightweight concrete slab on 2” – 20GA composite metal deck to create a non-composite concrete floor slab again. This time, the slab rests on open web steel joists rather than beams. Joists will also by @ 6’3” O.C. which span 40’. Total slab depth: 23.25” Slab weight: 41 psf (1)

From New Columbia Joist Company’s Steel Joist & Joist Girder Manual pg 52, 54 (2)

Design compares joist’s strength to support total load and a respective live load in lbs/ft Clear span 1: 40’ Tributary Width: 6’3” Clear span 2: 25’ Tributary Width: 5’ Pros Cons No shear studs – cost savings Larger floor thickness No field welding required for studs – cost savings Needs bracing in perpendicular direction Does not require shoring during construction Lateral load resistance depleted Lighter self weight – smaller foundation Stringent fireproofing Allows space for MEP Variability in joist heights

1 Appendix C, page 15 2 Appendix D, page 16


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Design: Tributary Area= 6’3” x 40’ = 250 ft2 Dead Load: 55 psf (2) Live Load (office w/ partition): 80 psf (2)

wu = D+L = 55+80 = 135 psf 135 psf * 6’3” = 1212.5 plf total load 1212.5 plf * 40’ = 48.5k total load

wl = L = 80 psf 80 psf * 6’3” = 500 plf live load 500 plf * 40’ = 20.0k live load

40LH16 with max safe load 53.0k (3)

Try rotating open web steel joist span direction 90° for a 25’ span. Open web steel joist layout is 7 joists per bay, at every 5’ center to center along a 40’ width.Tributary Area= 5’ x 25’ = 125 ft2 wu = D+L = 55+80 = 135 psf 135 psf * 5’ = 675 plf total load 675 plf * 25’ = 16.88k total load

wl = L = 80 psf 80 psf * 5’ = 400 plf live load 400 plf * 25’ = 10.0k live load

18LH05 w/ 2 rows of bridging (3)

Max Total Load: 684 plf Max Live Load: 454 plf Girder Check: Mmax = (16.88k * 5’) +

(16.88k * 10’) + (16.88k * 15’) + (16.88k * 20’) =

Mmax = 844’k Z required = Mu//ΦFy = (844 * 12) / (.9 * 50) = 225.1 in3 W21x44 Z = 95.8 in3 < 255.1 in3

ΦMp = 359.25’k < 844’k Need larger size W30x90

W30x90 Z = 274.0 in3 > 255.1 in3

ΦMp = 1010’k > 844’k (1)

1 Pg. 5-45 AISC Manual of Steel Construction 2 Appendix C, page 15 3 Appendix D, page 16


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Reinforced Concrete Slab with One Way Joists for 2nd Floor of the South Building

The system is poured with 4000 psi concrete interlocked with 60 ksi steel reinforcement. The structure is poured into 30” forms + 6” wide, 20” deep rib + 3” top slab for total depth of 23”. Rib side slope is 1 to 12. The slab is poured directly with the girders for structural consistency, and would replace 3¼” lightweight concrete slab on 2” – 20GA composite metal deck and its supporting beams. Total slab depth: 23” Slab weight: 91 psf (1)

From CRSI handbook pg. 8-13: Ag: 261.3 in

2 Ycg: 15.18” +Mcr: 32.5‘k -Mcr: 63.0‘k Ig: 12,469 in4

From CRSI handbook pg. 8-25: (2)

Clear span: 40’ for an interior span #6 top bars @ 9” & bottom bars are #6’s

Supports a factored usable superimposed load of 204 psf. wu = 1.2D+1.6L = 1.2(14)+1.6(80) = 144.8 psf (1)

Computation of deflection is not required by CRSI note #3 Pros Cons No shear studs – cost savings Larger quantity of concrete No field welding required for studs – cost savings Longer construction time (curing, removal of forms) 2 hour fire rating without additional fire proofing Larger floor thickness Strength of structural continuity Added floor dead load – foundation issues Floor system can all be poured at once Shoring & Formwork

1 Appendix C, page 15 2 Appendix E, page 18


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No lead time Labor Increase Design: From CRSI handbook pg. 8-13: Self Weight: 91 psf Girder design: From CRSI handbook pg. 12-105: (1)

Span: 30’ Tributary Area= 30’ x 40’ = 1200 ft2 Dead Load: 91 psf + Superimposed Dead Load: 14 psf (2)

Live Load (office w/ partition): 80 psf (2)

wu = 1.2D+1.6L = 1.2(105)+1.6(80) = 254 psf w = 254 psf * 40’ = 10.16 klf Rectangular Concrete Girder (h=24.5, b=24) w/ 2 #14 bottom bars & 5 #14 top bars

w = 10.4 klf 4000 psi reinforced concrete 60 ksi steel reinforcement (1)

1 Appendix F, page 19 2 Appendix E, page 18


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Prestressed Concrete Hollowcore Slab with 2” Cast in Place Topping for 2nd Floor South Building 5000 psi precast concrete with 4, ½” diameter, 270 k low-relaxation prestressing steel bars with a composite strength of 3000 psi. 8” deep by 4’ wide precast sections will span direction 90° to current span. This will cover a 25’ rather than a 40’ span allowing not only for shorter span distance but for exact fit of 10 four foot wide precast sections across a 40’ girder span. Also, the W21x44 beams will be removed and the hollowcore slabs will rest directly on the girders. The girders will be evaluated to see if they would need to increase in size to support the excess load. From Nitterhouse specifications (1) Clear span: 25’ for an interior span Low-lax prestressing bars have a 1½” bottom cover UL Fire Rated J917 (2 hours) Allowable superimposed load: 110 psf. Hollowcore slab weight: 82.5 psf Replaces current 3¼” lightweight concrete slab on 2” – 20GA composite metal deck and its corresponding beam layout. Pros Cons No shear studs – cost savings Requires very large steel girders to carry the load No field welding required for studs – cost savings Would increase floor depth 2 hour fire rating without additional fire proofing Altogether heavier system – greater foundation loads Quick and easy erection Design: Superimposed Dead Load: 14 psf Live Load (office w/ partition): 80 psf (2) wu =SDL+L = (14)+(80) = 94 psf Girder Check: wu = (1.2(82.5 psf) + 94 psf) = 193 psf w = 193 * 25’ = 4.825 klf Mu = wl

2/8 = 4.825 * (402)/8 = 965‘k Z required = Mu//ΦFy = (965 * 12) / (.9 * 50) = 257.3 in

3 W30x90 (3)

Z = 274.0 in3 > 257.3 in3

ΦMp = 1010 ’k > 965’k Requires W30x90, an increase in girder size

1 Appendix H, page 21 2 Appendix C, page 15 3 Pg. 5-45 AISC Manual of Steel Construction


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Conclusions

Technical Assignment #2 has laid out a few different floor systems that are in

consideration for the redesign of the Renick School of Education building in Greensboro, North Carolina. However, after all of the calculations and concerns taken into account, it has been decided that the current floor structure is most likely the best arrangement.

Pros & Cons of each system:

Non-composite concrete floor system on metal deck supported by steel beams o This system would save money for a few reasons. First of all, the fact

that shear studs will not be included not only drops the actual cost of building materials, but saves a lot of time and labor during construction. However, the major issue here is that removing shear studs takes away the composite action in the floor system, which makes the slab and girders work together against gravity loads.

Non-composite concrete floor system on metal deck supported by open web joists o The major difference with the open web steel joists is the lighter self

weight of the floor structure as compared to using wide flange members. Unfortunately, the floor depths increase with joists, which do make room for MEP within the depth of the actual structure. Also, this construction needs perpendicular bracing to stop the joists from overturning.

Reinforced concrete slab with one-way joists supported by concrete girders o The reinforced concrete system brings a structural continuity that other

steel structures do not have, especially since it can be poured all at. The 2 hour fire rating that the floor structure itself has saves money and construction time. However, it requires much longer compared to a steel structure in the end, since you have to allow about 28 days for the concrete to cure. Plus, the overall depth of the structure would be much larger which makes for a much larger building dead load. Not only does this increase column sizes, but the foundation as well will see much greater loads.

Prestressed concrete hollowcore slab supported by steel girders o The prestressed concrete systems biggest up side is that the floor can be

poured and cured off site prior to construction time. This way, erection is quick and easy, and the total cost of labor decreases greatly in the end. It also is designed to have a 2 hour fire rating. But, the floor depth increases and makes for a heavier system, once again accumulating more


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dead load into the columns and foundation. In fact, even the steel girders that would carry the prestressed members would need to be much larger, adding extra weight to the structure.

Reinforced concrete slab on composite metal deck supported by steel girders o The existing system of the Renick School of Education Building has a

shallow floor depth, the strength capabilities of a bonded composite system, and does not require shoring. The only down sides are the extra cost in production and installation of shear studs and the fact that concrete must be poured on site.

The building’s current floor system is probably the best you can get for your

dollars in this case. Although, the north building was not included in this design and from further analysis in the future, it will potentially be discussed during another report. Up till this point, the Renick School of Education Building has a solid floor base with its composite system consisting of a reinforced concrete slab and a few steel members, including shear studs, beams and girders.


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Appendix A

Foundation Plan 2nd Floor Plan


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Appendix B


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Appendix C Design Loads

Dead Loads 1st Floor 4" normal weight slab on grade 50.0 psf (4/12)ft * 150 pcf Floor finish 1.0 psf Value from ASCE-07 51.0 psf TOTAL 2nd/3rd Floor

3-1/4" lightweight concrete slab on 2” - 20 gage composite metal deck 41.0 psf

Value from United Steel Deck Manual pg 38

Suspended ceiling grid 2.0 psf Value from ASCE-07 5/8" gypsum board panels 6.0 psf Value from ASCE-07 MEP 5.0 psf Value from ASCE-07 Floor finish 1.0 psf Value from ASCE-07 55.0 psf TOTAL Roof Built-up roofing 2.0 psf Values from ASCE-07 3/4" fiberboard 1.1 psf 1.5 per inch 3" rigid insulation 4.5 psf .75 per 1/2 inch Suspended ceiling grid 2.0 psf Value from ASCE-07 MEP 5.0 psf Value from ASCE-07

2" lightweight concrete slab on

1-1/2" – 22 gage metal deck 31.0 psf Value from United Steel Deck Manual pg 36

45.6 psf TOTAL Live Loads Office 50.0 psf Public areas, lobby 100.0 psf Classrooms 40.0 psf Ground floor corridors 100.0 psf 2nd/3rd floor corridors 80.0 psf Stairs 100.0 psf Storage 125.0 psf Mechanical room 150.0 psf Partitions 80.0 psf Roof 20.0 psf


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Appendix D


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Appendix E


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Appendix F

Appendix G

Materials Steel Structural Steel ASTM A992, Grade 50, Fy = 50 ksi HSS ASTM A500, Grade B, Fy = 46 ksi Misc. Steel ASTM A36, Fy = 36 ksi Reinforcing Steel ASTM A615, Fy = 60 ksi

Concrete Light Weight f'c = 4000 psi (elevated slabs) Normal Weight f'c = 3000 psi (slab on grade, retaining walls, footings)

Bolts ASTM A325 - 3/4" diameter


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Appendix H

Documents

Executive Summary Pro/Con Structural Study of Non-composite concrete floor system on metal deck supported by steel beams ¾ Non-composite ... 2”, 4000 psi, lightweight cellular