CVE20003 Analysis of a multistorey concrete building
CVE20003, Design of Concrete Structures
Project 1 – Analysis of a multistorey concrete building
A five story building is to be designed and constructed in Melbourne’s eastern suburbs. A preliminary assessment by the engineer has nominated that a concrete framed building would be most suitable. The engineer has nominated the geometry of the structure as per figures 1-4 below. It is proposed that each floor be comprised of a suspended beam and slab system consisting of reinforced concrete (RC) beams (1200mm wide x 600mm deep along grid C and 1200mm wide x 450mm deep along grids 1 & 2) spanning between 550mm x 550mm RC columns. The beams will be supporting a one way spanning RC slab. It is proposed that a 180 mm thick RC slab is required to span between the beams (refer figure 1).
The beams and columns will be rigidly connected using steel reinforcement and should be designed as a moment resisting frame to resist the horizontal forces which will be imposed on the structure due to wind actions (Wu) in the east-west direction (parallel with grid lines 1-3). The wind loads will be applied to the external faces of 250mm thick precast concrete shear walls in the easterly direction as shown in figures 1 and 2. The precast concrete 250mm thick shear walls will be design to take the horizontal loads from wind actions in the north-south directions. As a result, the beams, slabs and columns are not required to be designed for horizontal loads in the north-south directions (parallel to grid lines A-D). The 250mm thick shear wall will be connected to the concrete beams and slabs using steel dowel bars at each level (refer detail A). There is a 100mm gap between the walls and the face of the edge columns, therefore the columns will only experience horizontal loads from the beams connected to them and the walls will not transfer loads to the columns directly along their height. The walls will not support the beams or slabs vertically and it can be assumed that all the vertical loads will be transferred from the beams to the columns. However, the walls are capable of supporting their own self-weight since they are tied into footings at the base and can be assumed to have no out of plane flexural stiffness to resist horizontal loading in the out of plane east-west direction. I.e the wall can only transfer horizontal wind loads in the east-west direction to the beams at each level and do not contribute to providing any resistance the horizontal loads in the east-west direction. The columns are fully fixed into footings at their base and footing to column connection can be assumed as rigid for design purposes.
Each level is to be designed for retail loads (shopping areas, classification D in AS1170.1 table 3.1) using respective dead and live loads extracted from AS1170.1. The superimposed dead load including finishes is to be taken as 1.0 kPa and the live load should be extracted from the AS1170.1 table 3.1. A wind loading analysis conducted by the engineer has determined that a constant positive wind pressure (Wu) of 1.5 kPa is to be applied to the western wall of the building in the easterly direction. The wind pressure is to be applied uniformly to the entire area of the western wall as a pressure loading. This results in a series of horizontal point loads applied to beams at each level.
Figure 1 – Structural floor plan
Figure 4 – Beam to wall details
Prior to the design of the structural members, an analysis is required to determine the critical design actions such as bending moments, shear forces and axial forces in all members of the frame.
The self weight of the RC floor system should be considered in addidtion to dead and live loads derived from AS1170.1 at each floor. The gravity loads should include self weight of the beams, slabs and columns.
As the cadet engineer on the project, you are requried to perform the following tasks:
- Determine the vertical design actions (self weight, dead load, live load) to be applied to beams along grid lines 1 and 2. Use the tributary area method to determine the load on each beam in kN/m. [5 marks]
- Determine the external wind actions to be applied to the 5 story frames comprisiong beams and columns along grids 1 and 2. The design actions should include horizontal wind loads only. [5 marks]
- Determine the vertical design actions for a typical slab panel along grid line C (self weight, dead load, live load). The width of the panal should be determined using the triburary area method and the load expressed as a kN/m line load along the full panel width. [5 marks]
- Based on AS1170.0, the designer has determined that two load combinations for ultimate limit state should be used to determine the critical design actions on the structure for later use in design: Ed1 = [1.2G, Wu, 0.4Q] and Ed2 = [1.2G, 1.5Q]
Using the unfactored design actions determined in steps 1 to 3, determine the factored design actions applied to the frames on grids 1, 2 and the typical slab panel along grid C. [5 marks]
- Using the portal method, determine the factored bending moments, shear forces and axial loads for the frame along grid 2. The bending moments, shear forces and axial loads should be determined independantly using both load combinations for [Ed1 & Ed2] specified in part 4. Note that to determine the moments and forces using load combination Ed1, the moments, shear forces and axial loads due to wind loads and gravity loads will need to be determined seperately and combined using the principal or superposition. [20 marks]
- Using the catilever method, determine the factored bending moments, shear forces and axial loads for the frame along grid 2. The bending moments, shear forces and axial loads should be determined independantly using both load combinations for [Ed1 & Ed2] specified in part 4. Note that to determine the moments and forces using load combination Ed1, the moments, shear forces and axial loads due to wind loads and gravity loads will beed to be determined seperately and combined using the principal or superposition. [20 marks]
- Assuming that the one way spanning slab panel along grid C is pin supported at all support locations, use approximate methods of analysis to determine the bending moments, shear forces for load combination Ed2 = [1.2G, 1.5Q]. Use the vertical loads calculated in part 3 in this section. [10 marks]
- Using Space Gass, determine the factored bending moments, shear forces and axial loads for the two frames along grids 1 and 2 using both load combinations [Ed1 & Ed2]. [15 marks]
- Compare the column reactions, maximum and minimum bending moments, shear forces and axial loads obtained for frames along grid 2 from the portal method, cantilever method and spacegass. Use tables to discuss any differences in accuracy between these methods. Note that the comparison only needs to be made at the maximum and mininum moment/shear force locations at the worst story/locations. [10 marks]
- The final project report must be a concise, organized, and easy-to-read document. The exact layout and table of contents of the final design project report is up to the individual. The report should have enough information to assess the accuracy and the approach used for the analysis and appropriate outputs from spacegass and hand calculations illustrating the results. [5 marks]
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