Effects of Static and Dynamic Loads on analysis and design of footings
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Date
2013
Authors
Alaa Abd El-Haq Majdi Al-Ghouj
Rawan Shakarneh
Montaser Abu Khaled
Majdi Al-Ghouj
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Abstract
The project basically is a design for a commercial building foundation, taking into account earthquake loads. Earthquake loads are calculated based on the most probable earthquake that may occur in the region. Through this project static and dynamic loads that affect the footing with variation to the number of stories is studies. SAP2000 software is used to carry out static and dynamic analysis and SAFE software is used for design. It was found that the thicknesses of mat foundation from dynamic load models are less than those of the static loads case, especially for low number of story building and the thicknesses become almost equal as number of stories increase above 12. However, more research is needed to compare the thickness of mat foundation for high rise buildings (above 16 stories). The foundation is the most important element in the structure which carries out any engineering construction. The foundation is the part of an engineered system that transmit to, and into, the underlying soil or rock the load supported by foundation and its self-weight, the resulting soil stresses except at the ground surface in addition to those presently exist in the earth mass from its self-weight and geological history. Foundations have the function of spreading the load from the superstructure so that the pressure transmitted to the ground is not of a magnitude such as to cause the ground to fail in shear, or to induce settlement of the ground that will cause distortion and structural failure or an acceptable architectural damage. This project will study the different types of loads effects on footing with the variation to the number of stories, also will focus on the geotechnical properties of the soil, and design the foundation of the proposed building. SAP2000 program will be used basically to analyze the building to get the loads on each footing and later on designing the foundation. The foundations are a particular, significant, artificially designed built model part of a structure which must have: To receive a structural static loads and dynamic loads (wind, vibration, and earthquake) externally applied to the superstructure, and transmit these loads to the soil (rock) at a given depth below the ground water surface, whichever is the case. To distribute stresses at the base of a footing of the foundation, to an allowable intensity on the soil (rock). A foundation is interfacing element between the superstructure and the underlying soil or rock. The loads transmitted by the foundation to the underling soil must not cause soil shear failure or damaging settlement of the superstructure. There are different types of foundations to be used for such a building and not all the types of foundation can fit each structure, it depends on other factorsthat should be taken into consideration such as bearing capacity and loads that acting on the foundation itself. There are two types of foundation namely, shallow foundation and deep foundation. Static loads represent true physical loads on model objects to represent weights or forces acting on the structure. Structural loads or actions are forces, deformations, or accelerations applied to a structure or its components. Loads cause stresses, deformations, and displacements in structures. Assessment of their effects is carried out by the methods of structural analysis. Excess load or overloading may cause structural failure, and hence such possibility should be either considered in the design or strictly controlled. For design process, after the analyzing process has completed using SAP 2000 software for both static and dynamic condition, the obtained loads used for the beginning of design process. The design will be performed using SAFE V12.3 software which is known with its capabilities to design different types of slabs and foundations The design procedure for this project is, the analysis were performed for multistory building with variation of number of stories four, eight and twelve respectively. However, before starting the design process using SAFE, a preliminary stage was used to perform certain calculations that are related to checks of soil pressure under footing and thickness checks to get the premier thickness to start design stage. The design for dynamic loads, the design of this case will be performed using SAFE V12.3 as design software exactly like static loads case. But by different way the model of the mat foundation was first prepared by SAP2000 by introducing new area section of the mat with proposed thickness, and then it is drawn under the model after removing the supports. Then adding of springs with stiffness of 15000 KN/m under the mat. The model is then run until the analysis was performed. After that, the mat foundation is exported to SAFE with all the loads of the building to be designed by SAFE. The results and conclusion were as follow: 1) The thicknesses of mat foundation from dynamic load models are less than those of the static loads case, especially for low number of story building. The reason is due to lateral loads of the earthquake that relief the vertical loads and this leads to lesser thicknesses than static conditions. Hence, the thickness of the mat was found from static loads since they are larger than dynamic loads. It is found also that the thickness form static and dynamic loads become almost equal as number of stories increase above 12. 2) It is found that concrete volume is greater for static loads than those for dynamic loads, because the thicknesses for the dynamic are less than the static.