A thesis submitted to the Department of Civil Engineering, Bangladesh University of Engineering and Technology, Dhaka in fulfillment of the requirement for the degree of. One first-order effect is the initial deflection of the structure in response to the lateral load. A basic structural analysis model of the building was prepared with the dimensions and details obtained from preliminary design strategies.
The magnitude of earthquake load is a result of the dynamic response of the building to the shaking of the ground. The seismic design forces are a fraction of the actual forces that a structure may experience during an earthquake. They are expected to resist limited inelastic deformations in their members and connections when subjected to the forces resulting from the ground motion of the design earthquake.
Ordinary moment resisting Frames (OMRF) 17
Moment Resisting Frame System is a structural system with an essentially complete space frame that provides support for gravity loads. Moment-resisting frames provide resistance to lateral loads primarily through bending action of members (Schueller 1977). It is a moment-resisting frame specially detailed to provide ductile behavior and to meet the requirements given in BNBC 2020 Sections 8.3.3 to 8.3.8.
In Seismic Zones 2, 3 and 4, reinforced concrete frames resisting the forces caused by seismic movements must be Special Moment Resisting Frames (SMRF) for Seismic Construction Category D. In Seismic Zones 2 and 3, reinforced concrete frames exposed to the forces , caused by seismic movements must be at least intermediate moment resisting frames (IMRF) for seismic construction category C.
Seismic Effect and Force on Structure 18
- Inertia forces in structures 18
- Horizontal and vertical shaking 20
- Flow of inertia forces to foundations 21
- Response of tall buildings 21
- Three-dimensional analysis 30
Earthquake causes ground shaking in all three directions – along two horizontal directions (say X and Y), and vertical direction (say Z) Figure 2.3. Under horizontal ground shaking, horizontal inertial forces are generated at the mass level of the structure (usually located at floor levels). The magnitude of the inertial forces induced in an earthquake depends on the mass of the building, the acceleration of the ground in Figure 2.4, the nature of the foundation and the dynamic characteristics of the structure.
The time taken (in seconds) for each complete oscillation cycle is the same and is called the fundamental natural period T of the building. Fi = Part of the seismic base displacement, V induced at level i hi, hx = Height from base to level i or x. For example, a concentrated vertical load applied to the top of the structure and the weight of.
Loads And Load Combinations 30
Loads 30
Three-dimensional analysis: When three-dimensional analysis is performed, ground motions should consist of pairs of appropriate time histories of horizontal ground motion acceleration (in two orthogonal horizontal directions) that should be selected and scaled based on individual recorded events. Appropriate ground motions must be selected from events with magnitudes, rupture distances, and source mechanisms consistent with those controlling the maximum earthquake considered. Where the required number of recorded ground motion pairs is not available, appropriate simulated ground motion pairs shall be used to determine the total number required.
For each pair of horizontal ground motion components, an SRSS spectrum must be constructed by taking the square root of the sum of the squares of the five percent damped response spectra for the components (where an identical scale factor is applied to both components of a pair). Each pair of motions must be scaled so that for each period between 0.2T and 1.5T (where T is the natural period of the fundamental mode of the structure) the average of the SRSS spectra of all horizontal component pairs is not less than 1, 3 times is not the corresponding ordinate of the actual design acceleration response spectrum. Although there has been no major incident of wind hazard in the recent past of Bangladesh, wind pressure is not uncommon in this area.
Load combinations 31
For use in load combination 5 in the allowable stress/strength design method or load combination 5 and 6 in the strength design method, E shall be determined in accordance with the following equation. For use in load combination 7 in the strength design method or load combination 8 in the allowable stress/strength design method, E shall be determined in accordance with the following equation. The horizontal seismic load effect, Eh, shall be taken as the horizontal load effects of seismic base displacement V or component forces Fc.
The directions of application of horizontal seismic forces for design will be those that will produce the most critical load effects. The combination that produces the most unfavorable effect for the effect of the particular action will be considered. The maximum vertical ground acceleration shall be taken as 50 percent of the expected peak horizontal ground acceleration (PGA).
Combination of Earthquake Loading with Other Loadings 34
The value of Emh should not exceed the maximum force that can be developed in the structure or element as determined by a rational plastic mechanism analysis or nonlinear response analysis (static or dynamic) using the expected real values of material strength. Allowable Stress Increase for Overstrength Load Combinations Where allowable stress design methodologies are used with the effect of seismic loading applied to load combinations 5, 6 or 8 of the allowable stress/strength design method, the stresses of allowable are allowed to be determined using an allowable stress increase of 1.2. This increase shall not be combined with increases in allowable stresses or reductions in load combination otherwise permitted by this standard.
In structures classified in Seismic Design Category D or E, horizontal cantilever structural members shall be designed for a minimum net upward force of 0.2 times the dead load in addition to the appropriate load combinations allowed for the stress/strength design method and the strength design method.
Drift And Deformation 35
The deflection in the plane of the diaphragm, as determined by engineering analysis, shall not exceed the allowable deflection of the attached elements. Seismic zones express the propensity of a region to occur earthquakes in the historical past, including the expectations in the future. For proper guidance in the design of structures, it is essential to have the fullest possible understanding of the seismicity and tectonics of the land.
The seismicity at a site is partly determined by the distance of any fault and partly based on the frequency and magnitude of shocks that have occurred in the past. The seismicity of Bangladesh is deeply related to tectonic behavior in and around Bangladesh caused by the subduction of the Indian plate under the Tibet subplate to the north. The major earthquake events in India adjacent to Bangladesh have caused damage in the region and therefore the seismicity of Bangladesh needs to be considered regionally.
Wind Load 39
- Materials Property 43
- System over strength factor, Ω 45
- Deflection amplification factor, Cd 45
- Site classification, S 45
In the case of this study, for intermediate reinforced concrete moment resisting frame system, the values of R, have been considered as 5 for this study when only columns are used and 5.5 when shear wall is used along with columns. These are defined in the figure given in chapter two of a seismic zoning map in the code. In the case of this study, for the location of the building in Dhaka City, which is located in Zone-2 of the seismic zone map of Bangladesh, the values of Z have been considered as 0.2 for this study.
In the case of this study, for building use category-2, the values of Importance Factor I have been taken into account for this study. In the case of this study, for the intermediate reinforced concrete moment-resistant frame system, the values of System above strength factor, Ω𝑜 have been taken as 3 for this study considered. In the case of this study, for the intermediate reinforced concrete moment resisting frame system, the values of deflection strengthening factor 𝑪𝒅 have been considered as 4.5 for this study.
Structural Stability 45
- Live load, L 47
- Earthquake load, E 47
- Wind load, W 47
- Loading and load combination 48
- Application of load and analysis 49
Site classification is determined based on soil properties in the upper 30 meters of the site profile. In the case of this study, for the 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 a thickness of several tens to hundreds of meters, the city class values were for this study treated as Sc. Ensuring adequate stiffness, especially lateral stiffness, is an important factor in building design for several important reasons.
A simple parameter that copes with and evaluates the lateral stiffness of a building is the displacement index, defined as the ratio of the maximum deflection at the top of the building to the total height. In this study, the overturning moment due to the lateral load in the x and y direction are checked by the result of the ETABS analysis. Prior to structural analysis it is essential that the loads that may act on a building during its lifetime are considered and properly included in the analysis.
These are basically the loads that come from the weight of the various components of the structure. For ease of analysis, these kinds of loads are sometimes 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). For analysis and design control of the building, the following values are for dead loads.
Although there have been no major earthquake incidents in Bangladesh in the recent past, earthquakes are not uncommon in this area. Scientific geological research of the earth's crust beneath Bangladesh shows that Bangladesh falls in a zone of moderate to high seismic risk. Statistical evidence from previous major and minor earthquake incidents shows that a major earthquake in recent times of geological scale was already overdue.
Bangladesh is typically a storm-prone area, where shocks due to storms must be taken into account when analyzing and designing buildings and structures.
Software 50
Models 50
