12 November 2009

Analysis မ်ားႏွင့္ ဆက္စပ္လ်က္

စကားဥိး။

Software ေတြ အေၾကာင္း ေရးျဖစ္ေတာ့ ဒီ Software ေတြကို ေမာင္းႏွင္မယ့္ အသိပညာ တစ္ခုေတာ့ လိုတယ္ေလ… ဒီေတာ့ ဒါေလးေတြကို ေလ့လာျဖစ္ခဲ့တယ္…။ အသိတစ္ခုကို ထြန္းညွိေပးခဲ့တဲ့ GTC က ဆရာေတြနဲ႔ ဆရာ ဦး၀င္းေအာင္ခ်ိဳကို ဒီ စာေလးနဲ႔ ဂါရ၀ ျပဳလိုက္ပါတယ္…။ တစ္စံုတစ္ခု လြဲမွားခဲ့လွ်င္ေတာ့ ဒါဟာ ကၽြန္ေတာ့္ရဲ႕ ညံ့ဖ်င္းမႈသာ ျဖစ္ပါတယ္….

Equivalent Static Analysis

This approach defines a series of forces acting on a building to represent the effect of earthquake ground motion, typically defined by a seismic design response spectrum. It assumes that the building responds in its fundamental mode. For this to be true, the building must be low-rise and must not twist significantly when the ground moves. The response is read from a design response spectrum, given the natural frequency of the building (either calculated or defined by the building code). The applicability of this method is extended in many building codes by applying factors to account for higher buildings with some higher modes, and for low levels of twisting. To account for effects due to "yielding" of the structure, many codes apply modification factors that reduce the design forces (e.g. over-strength factors).

(ဒါကေတာ့ Earthquake အတြက္ Equivalent ျဖစ္တဲ့ Static lateral Load ကို Ground motion မွာ apply လုပ္ၿပိး စဥ္းစားတာပါ... Base Shear V = ma နဲ႔ လြယ္လြယ္ကူကူ စဥ္းစားတယ္လို႔ နားလည္မိပါတယ္... Building weight ကို proportion အမ်ိဳးမ်ိဳးနဲ႔ lateral မွ သက္ေရာက္ၿပီး approach လုပ္တာပါ... building ဟာ regular shape ရွိသင့္ၿပီး the whole structural ဟာ twist မျဖစ္မွ ပိုသင့္ေတာ္မွာပါ...)

Response Spectrum Analysis

This approach permits the multiple modes of response of a building to be taken into account (in the

frequency domain). This is required in many building codes for all except for very simple or very complex structures. The response of a structure can be defined as a combination of many special shapes (modes) that in a vibrating string correspond to the "harmonics". Computer analysis can be used to determine these modes for a structure. For each mode, a response is read from the design spectrum, based on the modal frequency and the modal mass, and they are then combined to provide an estimate of the total response of the structure. Combination methods include the following: * absolute - peak values are added together square root of the sum of the squares (SRSS) * complete quadratic combination (CQC) - a method that is an improvement on SRSS for closely spaced modes It should be noted that the result of a response spectrum analysis using the response spectrum from a ground motion is typically different from that which would be calculated directly from a linear dynamic analysis using that ground motion directly, since phase information is lost in the process of generating the response spectrum. In cases where structures are either too irregular, too tall or of significance to a community in disaster response, the response spectrum approach is no longer appropriate, and more complex analysis is often required, such as non-linear static or dynamic analysis.

(RSA ဟာ peak phase shaking မွာ အခ်ိန္ ဘယ္ေလာက္ ၾကာသြားမွာလဲ ဆိုတာ မစဥ္းတာႏိုင္တာကေတာ့ သူ႔အားနည္းခ်က္လို႔ ဆိုႏိုင္ပါတယ္.. သူစဥ္းစားတဲ့ force က frequency နဲ႔ modal mass မွာ သြားမူတည္ေနပါ္တယ္...သူက mode ေတြကို combined လုပ္ၿပီး စဥ္းစားေပးပါတယ္.. building ဟာ သိပ္ကို irregular ျဖစ္ၿပီး သိပ္ျမင့္လြန္းရင္ RSA နဲ႔တင္ ဒီဇုိင္း မလုပ္သင့္ပါဖူး...)

Linear Dynamic Analysis

Static procedures are appropriate when higher mode effects are not significant. This is generally true for short, regular buildings. Therefore, for tall buildings, buildings with torsional irregularites, or non-orthogonal systems, a dynamic procedure is required. In the linear dynamic procedure, the building is modelled as a multi-degree-of-freedom (MDOF) system with a linear elastic stiffness matrix and an equivalent viscous damping matrix. The seismic input is modelled using either modal spectral analysis or time history analysis but in both cases, the corresponding internal forces and displacements are determined using linear elastic analysis. The advantage of these linear dynamic procedures with respect to linear static procedures is that higher modes can be considered. However, they are based on linear elastic response and hence the applicability decreases with increasing nonlinear behaviour, which is approximated by global force reduction factors. In linear dynamic analysis, the response of the structure to ground motion is calculated in the time domain, and all phase information is therefore maintained. Only linear properties are assumed. The analytical method can use modal decomposition as a means of reducing the degrees of freedom in the analysis.

(ဒါကေတာ့ နမူနာ ယူထားတဲ့ graph တစ္ခုခုကေနရတဲ့ Ground motion ကို apply လုပ္ၿပီး စဥ္းစားတာပါ... Example စဥ္းစားထားတဲ့ ငလ်င္ pattern နဲ႔ ဆင္တဲ့ earthquake အတြက္ပဲ သင့္ေတာ္ႏိုင္ပါတယ္.. ကြဲျပားတဲ့ effect ေတြ ခံရရင္ေတာ့ safe နည္းသြားႏိုင္ပါတယ္... ဒီ analysis မွာ stress နဲ႔ strain ဟာ proportional ျဖစ္ေနေပမယ့္ loading ေတြကေတာ့ dynamic ျဖစ္ပါတယ္..)

Non-linear Static Analysis

In general, linear procedures are applicable when the structure is expected to remain nearly elastic for the level of ground motion or when the design results in nearly uniform distribution of nonlinear response throughout the structure. As the performance objective of the structure implies greater inelastic demands, the uncertainty with linear procedures increases to a point that requires a high level of conservatism in demand assumptions and acceptability criteria to avoid unintended performance. Therefore, procedures incorporating inelastic analysis can reduce the uncertainty and conservatism. This approach is also know as "pushover" analysis. A pattern of forces is applied to a structural model that includes non-linear properties (such as steel yield), and the total force is plotted against a reference displacement to define a capacity curve. This can then be combined with a demand curve (typically in the form of an acceleration-displacement response spectrum (ADRS)). This essentially reduces the problem to a single degree of freedom system. Nonlinear static procedures use equivalent SDOF structural models and represent seismic ground motion with response spectra. Story drifts and component actions are related subsequently to the global demand parameter by the pushover or capacity curves that are the basis of the non-linear static procedures.

(ဒီ analysis ကေတာ့ earthquake ေၾကာင့္ ျဖစ္လာတဲ့ frame ရဲ႕ displacement မ်ားလာမႈကို member ရ႕ဲ capacity မွာ ultimate အထိ plastic ဆန္ဆန္ pushover နဲ႔ စဥ္းစားတာ ျဖစ္ပါတယ္... ဒီေနရာမွာ Force ဟာ Displacement နဲ႔ တိုက္ရိုက္ အခ်ိဳး မက်ေတာ့ပါဖူး )

Non-linear Dynamic Analysis


Nonlinear dynamic analysis utilizes the combination of ground motion records with structural model, therefore is capable of producing results with relatively low uncertainty.nonlinear dynamic analyses, the detailed structural model subjected to a ground-motion produces estimates of component deformations for each degree of freedom in the modal responses are combined using schemes such as the square-root-sum-of-squares. In non-linear dynamic analysis, the non-linear properties of the structure are considered time domain analysis. This approach is the most rigorous, and is required by some for buildings of unusual configuration or of special importance. However, the calculated can be very sensitive to the characteristics of the individual ground motion used as therefore, several analyses are required using different ground motion records.

(ဒီ analysis မွာ ေတာ့ Force ဟာ displacement နဲ႔ တိုက္ရိုက္ အခ်ိဳး မက်တဲ့ အျပင္ displacement ဟာ nonlinear ျဖစ္လာပါတယ္... Stress ေၾကာင့္ ျဖစ္တဲ့ strain ဟာလည္း တိုက္ရုိက္ proportion မျဖစ္ေတာ့ပါဖူးး loading ဟာ dynamic ျဖစ္လာၿပီး loading ေၾကာင့္ျဖစ္တဲ့ displacement ဟာ ဆင့္ကဲ ဆင့္ကဲ ကို စဥ္းစားေပးရပါမယ္...)

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