Agricultural Hall and Annex East Lansing, MI. Structural Design. Gravity Loads. 1- Based on US Standards

Transcription

1 Structural Design Gravity Loads 1- Based on US Standards Occupancy or Use Uniform (psf) Concentrated (lbs) Office building -Office -Lobbies and first-floor corridors -Corridor above first floor -Partitions -Superimposed Based on British Standards Occupancy or Use Uniform (KN/m 2 ) Concentrated -General Office -Partitions Load Combinations Based on US Standards 2.5 KN/m 2 (52.2 psf) 1 KN/ m 2 (20.5 psf) 2.7 KN ( 607 lbs) 1- U = 1.4(D+F) 2- U = 1.2(D+F+T) (L+H) (L r or S or R) 3- U = 1.2D + 1.6(L r or S or R) + (1.0l or 0.8W) 4- U = 1.2D + 1.6W + 1.0L +0.5(L r or S or R) 5- U = 1.2D + 1.0E+ 1.0L + 0.2S 6- U = 0.9D +1.6W + 1.6H 7- U = 0.9D E H Based on British Standards Design loads Dead + imposed load ( + earth pressure) Dead + wind load ( + earth pressure) Dead + imposed + wind load (+ earth pressure) Condition 1.4 G k Q k E n 1.4 ( G k + W k + E n ) 1.2 ( G k + Q k + W k + E n ) 9

2 Framing Systems All the gravity systems except the one-way skip joist were designed using equivalent frame method as it is described below. ADOSS was used to design the flat slab with drop panels systems and the two-way slab with beams since it uses equivalent frame method in the design. Equivalent Frame Method (EFM) According to ACI : The equivalent frame method involves the representation of the three-dimensional slab system by a series of two-dimensional frames that are then analyzed for loads acting in the plane of the frames. The negative and positive moments are determined at the critical sections of the frame and then distributed to the slab sections: column strips, beams if used, and middle strips. The equivalent frame consists of three parts: Slab beams, columns, and torsional members : Equivalent Frame Each frame contains a row of columns and broad continuous beams. The beams or slab beam includes the portion of the slab bounded by panel centerline on either side of the column, together with the column-line beams or drop panel if used. For the vertical loading, each floor with its columns may be analyzed separately. It is convenient and sufficiently accurate to assume that the continuous frame is completely fixed at support when computing the bending moment at the support. Frames adjacent and parallel to an edge shall be bounded by the edge and the centerline of adjacent panel. Each frame can be analyzed separately : Slab-beams It is permitted to use the gross area of concrete when computing the moment of inertia of slab-beams at any cross section outside of joints or column capitals. Variation in moment of inertia along axis of slab-beams shall be taken into account. Moment of inertia of slab-beams from center of column to face of column, bracket, or capital shall be assumed equal to the moment of inertia of the slabbeam at face of column, bracket divided by (1-c 2 /l 2 ) : Column It is permitted to use the gross area of concrete when computing the moment of inertia of slab-beams at any cross section outside of joints or column capitals Variation in moment of inertia along axis of slab-beams shall be taken into account. 10

3 Moment of inertia of columns from top to bottom of the slab-beam at a joint shall be assumed to be infinite : Torsional Members Tensional members shall be assumed to have a constant cross section throughout their length consisting of the largest of: 1- portion of slab having a width equal to column/capital width, 2- same portion of slab + transverse beam, or 3- transverse beam. Equivalent Frame Analysis by Computer The EFM is oriented toward analysis using the method of moment distribution. Plane frame analysis programs can be used for slab analysis based on the concepts of the equivalent frame method, but the frame must be specially modeled. Variable moments of inertia along the axis of slab-beams and columns require nodal points (continuous joints) between sections where I is to be considered constant. It is also necessary to compute K ec for each column, and then to compute the equivalent value of the moment of inertia for the column. Alternately, a three-dimensional frame analysis maybe used in which the torsional properties of the transverse supporting beam may be included directly. The third option is to make used of specially written computer programs. The most widely used program is Analysis and Design of Reinforced Concrete Slab System, ADOSS, developed by Portal Cement Association (Nilson, Darwin, and Dolan, 2002). All the systems for the submission are designed using ADOSS. ADOSS Analysis Procedure Steps 1- Enter project name and span ID 2- Choose type of slab and frame location, either exterior of interior 3- Number of spans. Spans are measured from center of column to center of column. For 3 spans building, number of spans are 5 because ADOSS considers the projection after column line as cantilever. Also, minimum slab thickness is entered. 4- Choose material properties: 150 pcf for density, 4 ksi for f c, and 60 ksi for f y. 5- Enter slab reinforcement data, can either be accepted or changed if desired. 6- Enter slab geometry. For the end span, the length is equal to the ½ column width. 7- Enter column information, c 1, c 2, and column height above and below slab. This is just an initial estimate for column size. All the columns have the same sizes. Yet, they can be changed if required. 8- Enter the initial size of transverse beams for the end spans or middle spans. Also the right eccentricity value must be entered so the beams edges coincide with column and slab edges. 11

4 9- Enter LL and DL loads. The dead load is only the superimposed load. Partial loads can be entered as well if any. 10- Load factors can be changed if desired. 11- Column fixity factor is 100 % 12- Finally, design the system. 13- Check if everything is ok with the design such as shear, and deflection. 14- Redesign the system if necessary such as increasing slab thickness, and drop panel thickness. 15- Redesign the system 16- Check the system to make sure that everything is ok. ADOSS load patterns: 1- Full dead and 75 % live on adjacent spans 2- Full dead and 75 % live on odd-numbered spans. 3- Full dead and 75 % live on even-numbered spans 4- Full dead and full live on all spans Wide-Module Concrete Joist System (Skip Joist System) Skip Joist System is widely known as Wide-Module Concrete Joist System which a joist system is having clear spacing between ribs of more than 30. Skip Joist is basically designed as T-beams according to ACI Maximum Bar size for single bars in the bottom of a wide-module joist rib Stirrup Style J U No. of Bars Rib Width at Bottom #6 #9 #11 #14 # #4 #6 #8 #10 2 #4 #8 #10 #11 # #5 #8 #9 Maximum Bar size for 2-bar bundles in the bottom of a wide-module joist rib Stirrup Style J U No. of Bars Rib Width at Bottom #9 #11 #11 #11 #11 2 #3 #5 #6 #8 #9 1 #8 #11 #11 #11 # #4 #5 #7 #8 12

5 Design Consideration 1- ACI 7.7.1: Minimum concrete cover: 1 ½ in to stirrups and main flexural bars. 2- ACI : Design shear strength: V c = 2 f c b w d 3- ACI : Minimum shear reinforcement: Av= 50(b w s)/f y 4- Reinforcing steel requirements and recommended details Figure 3 Figure 4 Figure 5 13

6 Openings in Slab Systems Based on US and British Standards Figure 6 Bar cutoff for Wide-Module One way Joist Since all my openings are large and interrupt the steel reinforcement in the column strips, beams shall be designed around the openings to meet the design strength and ductility requirements. Deflection Check For each designed system, the deflection was checked to make sure that it does not exceed the maximum deflection that is set by the US or British Standards. All the deflections that were obtained are mentioned later in the report. The material and reinforcement properties are given below. Appendix A has all the maximum deflections allowed by both US and British Standards. Material Properties and Reinforcement CONCRETE FACTORS SLABS BEAMS COLUMNS DENSITY(pcf ) TYPE NORMAL WGT NORMAL WGT NORMAL WGT f'c (ksi) fct (psi) fr (psi) REINFORCEMENT DETAILS: NON-PRESTRESSED YIELD STRENGTH (flexural) Fy = ksi YIELD STRENGTH (stirrups) Fyv = ksi DISTANCE TO RF CENTER FROM TENSION FACE: AT SLAB TOP = 1.50 in OUTER LAYER AT SLAB BOTTOM = 1.50 in OUTER LAYER 14

7 AT BEAM TOP = 1.50 in OUTER LAYER AT BEAM BOTTOM = 1.50 in FLEXURAL BAR SIZES: MINIMUM MAXIMUM AT SLAB TOP = # 4 AT SLAB BOTTOM = # 4 AT BEAM TOP = # 4 #14 IN BEAM BOTTOM = # 4 #14 MINIMUM SPACING: IN SLAB = 6.00 in IN BEAM = 1.00 in 15

8 Wide Module One-way Joist The wide module one-way joist system for the Agricultural Hall and Annex is designed using the CRSI Handbook. There were several trials of wide module joists in order to match the depth of the girder with the depth of the joist for more economical system and efficient system. The best joist that matched the girder s depth is 66 Forms c.-c, the total depth = 24.5 in (20 Deep Rib Top Slab). All the girders have the same depth as the joists. For the End Span Tabulated Capacity 1002 plf Top Bars # 4 at 10 in Bottom Bars 2 # 7 Stirrups Single leg stirrups, # 3 spaced at 11 in for 123 in For the Interior Span Tabulated Capacity 1774 plf Top Bars # 5 at 11 in Bottom Bars 2 # 7 Stirrups Single leg stirrups, # 3 spaced at 11 in for 125 in Concrete Quantity = 497 plf = 497/ (72 /12 ) = 83 psf Deflection: CRSI: the computation of deflection is not required above the horizontal line (thickness l n /18.5 for end spans, l n /21 for interior spans) 16

9 Figure 7 17

10 Flat Slab with Drop Panel Design Summary (US Standard) Flat Slab with Drop Panels (US Standards) Item Value Comment Slab thickness 12.5 > Minimum Drop panel thickness 3.5 > ¼ slab thickness Drop panel size 10 ft2 Allowable shear stress psi Maximum shear psi << Allowable Maximum joint moment 945 kip Maximum joint shear 169 psi Maximum deflection in << Allowable Figure 8 18

11 Figure 9 E-W Reinforcement for Flat Slab (US Standards) 19

12 Figure 10 N-S Reinforcement for Flat Slab (US Standards) 20

13 Two-way Slab with Drop Panels (British Standards) Two-way Slab with Drop Panels (British Standards) Item Value Comment Slab thickness 13.0 > Minimum Drop panel thickness 3.5 > ¼ slab thickness Drop panel size 10 ft 2 Allowable shear stress psi Maximum shear psi << Allowable Maximum joint moment 1040 kip Maximum joint shear 184 psi Maximum deflection in << Allowable Figure 11 21

14 Figure 12 E-W Reinforcement for Flat Slab (British Standards) 22

15 Figure 13 N-S Reinforcement for Flat Slab (British Standards) 23

16 Two-way Slab with Beams (British Standards) Two-way Slab with Beams (Oman) Item Value Comment Slab thickness 10.5 Drop panel thickness 3.5 > ¼ slab thickness Drop panel size 10 ft 2 Allowable shear stress psi Maximum shear psi << Allowable Maximum joint moment 1040 kip Maximum joint shear 185 psi Maximum deflection in << Allowable Figure 14 24

17 Figure 15 25

18 Figure 16 - Two-way Slab with Beams Reinforcement (E-W) (British Standards) 26

19 Figure 17- N-S Two-way with Beams Reinforcement (British Standards) 27