3.2.7 System over strength factor, ππ
For different Structural system, System over strength Factor varies. The System over strength Factor, Ξ©π for different frame system are given in BNBC (2020). In the case of this study, for Intermediate reinforced concrete moment resisting frame system, the values of System over strength Factor, Ξ©π has been considered 3 for this study.
3.2.8 Deflection amplification factor, πͺπ
For different Structural system, Deflection Amplification Factor, πͺπ varies. Deflection Amplification Factor, πͺπ for different frame system are given in BNBC (2020). In the case of this study, for Intermediate reinforced concrete moment resisting frame system, the values of Deflection Amplification Factor, πͺπ has been considered 4.5 for this study.
3.2.9 Site classification, S
Site Classification is determined based on the soil properties of upper 30 meters of the site profile. Average soil properties are determined. The site profile up to a depth of 30 m is divided into n number of distinct soil or rock layers. Where some of the layers are cohesive, π is the number of cohesive layers. Hence βππ=1ππ = 30 m, while βππ=1πππ <
30 m if π < π in other words if there are both cohesionless and cohesive layers. The standard penetration value directly measured in the field without correction will be used. The site classification should be done using average shear wave velocity if this can be estimated, otherwise the value of standard penetration may be used. The site class S for different soil condition is given in appendix. In the case of this study, for Intermediate reinforced in the case of this study, as the soil condition of Dhaka city indicates Deep deposits of dense or medium dense sand, gravel or stiff clay with thickness from several tens to many hundreds of meters, the values of Site Class has been considered Sc for this study.
3.3.1 Drift
Story drift is the displacement of one level relative to the level above or below due to the design lateral forces. The provision of adequate stiffness, particularly lateral stiffness, is a major consideration in the design of building for several important reasons. In terms of serviceability limit state, deflections must be maintained at a sufficiently low level to allow proper functioning of nonstructural components and to prevent excessive cracking and consequent loss of stiffness. One simple parameter that afford and estimates the lateral stiffness of a building is the drift index, defined as the ratio of the maximum deflection at the top of the building to the total height. Design drifts index limits have been considered in this study as per BNBC, 2020.
3.3.2 Overturning
Every building has been designed to resist overturning caused by wind or earthquake, which ever governs. Moments can be calculated from the triangular load distribution formula. However, the loading is based on an envelope of maximum shears and may be less at any moment in time. Overturning moment due to lateral load is checked at the base of a structure. In this study overturning moment due to lateral load in x and y direction are checked from ETABS analysis result.
3.4 Loads For Analysis
Prior to structural analysis it is essential that the loads that may act upon a building during its lifetime be duly considered and incorporated in the analysis. The loads that may act upon the factory building are considered as follows:
3.4.1 Dead loads (D)
Dead loads (D) are those gravity load which remain acting on the structure permanently without any change during the structures normal service life. These are basically the loads coming from the weight of the different components of the structure. For the sake of convenience in the analysis, sometimes this kind of loads are divided into two types, namely a) self-weight of the structure (SW) and b) the weight coming from the non-structural permanent components of the building (SDEAD). In concrete building the weight of slabs, beams and columns etc. which form the main structural system is considered the self-weight (SW). The weights of floor finish, water proofing layer, partition walls and other non-structural permanent components generally
constitute the rest of the total dead load, i.e., SDEAD. For the analysis and design checking of the building, following are the values of dead loads,
Unit weight of reinforced concrete = 150 pcf Unit weight of brickwork = 120 pcf
Floor Finish = 25psf Partition wall load = 50psf 3.4.2 Live load (L)
Live load is the gravity load due to non-permanent objects like machines, furniture, and human. Analysis has been carried out based on load recommended by BNBC (2020). Live load for light work room without storage is taken 50 Psf.
Earthquake load (E)
Although there has been no major incident of earthquake hazard in the recent past of Bangladesh, earth quake is not uncommon in this area. Scientific geological study of the earth crust below Bangladesh shows that Bangladesh does fall in moderate to high seismic risk zone. Statistical evidence from past major and minor earthquake incidents shows that a major earthquake is overdue in the recent times of geological scale. Therefore, it is necessary to prepare against any possible earthquake hazard.
Regarding the earthquake resistant structural design, it essential that the specific design code is followed. For the analysis and design checking of these structuresβ
main considerations for calculation of earthquake load are given below.
Zone co-efficient, Z = 0.2 (zone 2, As Per BNBC 2020)
Structure importance co-efficient, I = 1.00 (Standard Occupancy, Table 6.2.17, BNBC 2020)
Response modification co-efficient, R = 5.0 (IMRF, Table 6.2.19, BNBC 2020) Site co-efficient, S=1.15 (Table 6.2.16, BNBC 2020)
3.4.3 Wind Load (W)
Proper structural design of any building structure must include loads due to wind also. For the analysis and design checking of these structuresβ main considerations for calculation of wind load are given below. Bangladesh is typically a storm prone area where consideration to the thrust due to storm must be given in the analysis and design of building and structures. Wind load due to storm is typically modeled as
lateral thrust force causing sway or overturning of the building. Detailed specifications on wind loading on buildings are outlined in BNBC (2020). The structure is located in Dhaka for which the following basic parameters are used in wind load calculation,
Basic wind speed, Vw = 62.5 m/s (Dhaka City, As per BNBC 2020)
Structure importance co-efficient, I = 1.00 (Standard Occupancy, Table 6.2.17, BNBC 2020)
External Pressure Co-efficient, Windward, Cp = 0.8 (Figure 6.2.6, BNBC 2020) External Pressure Co-efficient, Leeward, Cp =0.2 & 0.3 (Figure 6.2.6, BNBC 2020)
Exposure Type = A (BNBC 2020) 3.4.4 Loading and load combination
The basic sources of loads are described in earlier section. These loads are applied on the model in seven basic categories. These are as followed:
Load Case 1: Self-weight of structure (SW).
Load Case 2: Floor finish (FF) Load Case 3: Partition wall (PW).
Load Case 4: Live load on roof (LL)
Load Case 5: Earthquake load on East-West Direction. (Ex) Load Case 6: Earthquake load on North-South Direction. (Ey) Load Case 7: Wind load on East-West Direction. (Wx) Load Case 8: Wind load on North-South Direction. (Wy)
These eight basic load cases are analyzed by ETABS-V9.7.1. When these eight basic load cases are combined accordingly considering the direction of lateral loads, we obtain, after simplification, the following combination cases:
Combination Case 1: 1.4 D
Combination Case 2: 1.2 D + 1.6 L + L Combination Case 3: 1.2 D + 1.6 L +0.5Lr Combination Case 4: 1.2 D + 1.6 L + 0.8Wx Combination Case 5: 1.2 D + 1.6 L - 0.8 Wx Combination Case 6: 1.2 D + 1.6 L + 0.8 Wy Combination Case 7: 1.2 D + 1.6 L β 0.8 Wy Combination Case 8: 1.2 D + L+ 0.5Lr + 1.6 Wx
Combination Case 9: 1.2 D +L + 0.5Lr - 1.6 Wx Combination Case 10: 1.2 D + L+ 0.5Lr + 1.6 Wy Combination Case 11: 1.2 D +L + 0.5Lr - 1.6 Wy Combination Case 12: 0.9 D + 1.6 Wx
Combination Case 13: 0.9 D - 1.6 Wx Combination Case 14: 0.9 D + 1.6 Wy Combination Case 15: 0.9 D - 1.6 Wy Combination Case 16: 1.2 D + L + Ex Combination Case 17: 1.2 D + L - Ex Combination Case 18: 1.2 D + L + Ey Combination Case 19: 1.2 D + L - Ey Combination Case 20: 0.9 D + Ex Combination Case 21: 0.9 D - Ex Combination Case 22: 0.9 D + Ey Combination Case 23: 0.9 D β Ey
3.4.5 Application of load and analysis
A static analysis is performed using the loadings and combinations of loads (mentioned earlier) for this building.
Figure 3.1: Applied Live load on floor