REINFORCED CONCRETE DESIGN REPORT Proposed Four Story Office Building @ Curtin University
APRIL 21, 2016 J.S.R. JAYARATHNA Y. KRISHANIKA B.A.V.W. FERNANDO
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Executive Summary This report presents the analysis and design of a four-story office building in Curtin University, Bentley, WA, Australia. The building’s plan dimensions are 28 by 53.6 metres, with column spacing of 5-6-5 metres along the short dimension and 6.8-8-6.8 metres along the long dimension. It was designed to meet both strength and serviceability requirements when subjected both to dead loads and live loads. Standard AS3600: Concrete Structures was used to analyse and design the beam for bending and shear.
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Table of Contents Executive Summary ................................................................................................................................ 1 Introduction ............................................................................................................................................ 3 Design...................................................................................................................................................... 4 Slab Thickness ..................................................................................................................................... 5 Loads ................................................................................................................................................... 5 Design for flexure................................................................................................................................ 5 Design for shear .................................................................................................................................. 7 Summary and Conclusion ....................................................................................................................... 8
References ............................................................................................................................................... 9 Appendix. .............................................................................................................................................. 10 A.
Load Path Diagram ................................................................................................................... 10
B.
Calculations. .............................................................................................................................. 10 I.
Slab thickness calculations.................................................................................................... 10
II.
Load estimate calculations .................................................................................................... 10
III.
Design for beam Line E .................................................................................................... 10
IV.
Design for beam Line 3 ..................................................................................................... 10
C.
D.
Design Figures .......................................................................................................................... 10 I.
Design figures for beam line E ............................................................................................. 10
II.
Design figures for beam line 3 .............................................................................................. 10 Action Plan................................................................................................................................ 10
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Introduction This report outlines the design of a four-story reinforced concrete office building located at Curtin University, Bentley, WA, Australia. The building’s plan dimensions are 28 by 53.6 metres, with column spacing of 5-6-5 metres along the short dimension and 6.8-8-6.8 metres along the long dimension, as shown in Figure 1. Typical story heights are 3.75 metres, except for the ground which have heights of 5.8 metres.
Figure 1:Ground Floor Plan View of Office Building
Here are the typical plan view and elevation of proposed office building
Figure 2: Typical Suspended plan view of the office building
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Figure 3: Elevation of the office building
Figure 4: Load paths of the structure
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Design The design of the four-story reinforced concrete structure ent ailed a number of steps and calculations. The design of the building was done accordance with AS3600-2009: Concrete structures. Each section listed below describes one step in the process of the design.
Attached to the end of this report are sample hand calculations for each step in the design process.
Slab Thickness The slab thickness was determined to be 130 mm by general practice of AS3600 (l/d ratio to be 39). The slab thickness was determined using smallest span value. For ease of
construction and economical purposes, a slab thickness of 130mm was used throughout the entire building. Calculations of the slab thickness determination is attached at appendices.
Loads In this design only the loadings on beam was considered. Since the beam system was given as a two-way slab, and it was tedious and complex to do the calculate the loading factors for two-way slab. It was simplified to have an easier calculation and efficient design. The loads were calculated using AS 3600-2009 and load combinations from AS/NZS 1170. The load for the slabs was calculated by multiplying the slab thickness by the unit weight of concrete (25 kN/m³). The load combination from Table 3.1 of AS/NZS 1170 consisted of a load factor of 1.2 for the dead loads and 1.5 for the live loads. Using this load combination. Breakdown of the load design along with the final loading values for beam can be found in appendices.
Design for flexure The T-beams were then designed for the flexural forces they would experience. This design comprised of the determination and selection of the adequate amount of steel necessary in each of the critical T-beam sections. The steel reinforcement is necessary in the portions of the T-beam that are in tension because steel is strong in tension while concrete is very weak 5
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and brittle in tension. However, the T-beam sections cannot have too much steel or they become over-reinforced and the failure mode of an over-reinforcement beam is very sudden. The T-beam should be under-reinforced so there is warning before a failure would occur (under a loading condition that was not designed for). There two unique beam lines were considered to analyze when designing the T-beam for flexure. Beam lines 3 and E were considered for the design. And assumed to continue with same design for the lines B, C, and D using design for Line E. Similarly, for the lines 2,4,5,6 and 7 using the design for line 3. The T-beam width was taken assumed to be 400mm to match the column widths in order to make construction easier. The first step in determining the T-beam reinforcement was to calculate the governing T-beam depth. Using AS3600 code, Interior spans were checked and found the values for the interior T-beam depth for beams with positive bending (tension is in the bottom of the T-beam), it was assumed the rectangular stress block (which is correlated to the portion of the beam in compression), was fully comprised in the flange (i.e. slab). For beams with negative bending (tension is in the top of the T-beam), the rectangular stress block was assumed to be in the stem (i.e. web). Both of these assumptions would be checked in the design process. Next, the effective width of the slab was calculated according to AS3600 8.8.2. The effective width of the slab is the portion of the T-beam flange that contributes to the strength of the T-beam. After the effective width was calculated, the effective depth was then found. For the positive bending sections, the effective depth was the beam depth minus the cover distance, the diameter of the stirrup bar and half of the longitudinal rebar diameter. For the negative section, the effective depth was the T-beam depth minus the cover, the transverse rebar and half of the longitudinal rebar diameter. The distributed load that the T-beam supported was then found by multiplying the tributary area of the T-beam (especial case -calculated in appendices) by either the floor. This value was added to the self-weight of the beam stem for the total line load. Then using the corresponding AS3600 moment coefficients, the moment for each section was found. Using the moment for the section along with the effective depth of the section, the width of the T-beam and an assumed reduction factor (ϕ) of 0.8, the area of steel required in each section was found and a combination of bar sizes was selected. The effective depth was then check again using the same methodology (but using the actual value of half the diamete r of 6
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the longitudinal steel) to make sure it was approximately the value that was assumed. The extreme tension strain and the reduction factor (ϕ) were then verified to be the same as the values that were assumed. The clear distance spacing of the bars was also checked. Finally, the minimum requirements were verified according to AS3600 8.1.6.1, design strength of the T-beam was checked. The reinforcement details (elevation and cross-sections) for floor beam lines in the Appendix. The T-beam flexural reinforcement calculations can be found in Appendices. It should be noted that only one steel reinforcement design was used between the section that requires the larger amount of steel will control the steel region at the first interior support.
Design for shear Next in the design process of the shear reinforcement. Without shear reinforcement the beam would have a catastrophic failure due to shear-web and flexure-shear cracks. These cracks would form due to the shear forces in the beam and cause equivalent tension stresses that would cause failure in the beam since concrete is very weak in tension. This failure would be sudden and extremely dangerous and must be designed against. Additionally, this is incredibly important because this failure occurs substantially before the flexural strength of the beam is reached. Therefore, stirrups at a determined spacing are used to provide a source of tensile strength against these shear forces (and equivalent tensile stresses). As was the case with the flexural design, there are 2 unique beam lines t hat must be designed for shear. Additionally, like the T-beam flexural design, beam lines B, C, D and E; 2,3,4,5,6 and 7. The shear forces at the critical locations were determined using the shear coefficients from AS3600 with the same line load that was used in the flexural design. The effective depth was also calculated using the most conservative value f rom the positive moment sections in the flexural design. The shear diagram was then constructed by applying the shear coefficients from AS3600. The shear at the columns was truncated at a distance d away from the support. The strength of the concrete in shear was then calculated with a reduction factor (0.7). The portions of the beam where the reduced strength of the concrete itself was greater than the factored shear force on the beam are required to have the minimum web reinforcement.
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Summary and Conclusion Using AS3600, a preliminary design of a four-story office reinforced concrete office building was completed. Overall, the structure is a very efficient building with only several edits needed in future iterations of the design. It was determined that the design did not fully comply with AS3600 code, but that these flaws would be revised in future edits to the overall design. The loads for the structure were determined from AS/NZ 1170 with the load combinations from AS3600. The columns were specified to be 400mm by 400mm with a slab thickness of 130 mm and T-beam depths that varied from 465mm to 475mm for two unique beam lines. The chosen T-beam flexural reinforcement was verified through strength checks, as was the T-beam shear reinforcement. The next step in this design project would be to complete a number of iterations on the design until it complies with AS3600.
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References Australian Standard: Concrete Structures (AS 3600-2009) Australian/New Zealand Standard: Structural Design Actions (AS/NZS 1170.0.2002)
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Appendix A. Load Path Diagram B. Calculations I.
Slab thickness calculations
II.
Load estimate calculations
III.
Design for beam Line E i. ii. iii.
IV.
Preliminary sizing of RC beams Calculations for Bending Calculations for Shear
Design for beam Line 3 i. ii. iii.
Preliminary sizing of RC beams Calculations for Bending Calculations for Shear
C. Design Figures I.
Design figures for beam line E
II.
Design figures for beam line 3
D. Action Plan
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