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*SEFP Consistent Design* *UBC Seismic Drift Limits* *Doc No: 10-00-CD-0003* *Date: June 04, 2013* The goal of this tutorial is to demonstrate how to evaluate building drifts and story drifts using UBC 97 guidelines. The philosophy behind Story Drift Limits is “Deflection Control”; In UBC 97, deflection control is specified in terms of the story drift as a limit on the lateral displacement of one level relative to the level below. The story drift is determined from the maximum inelastic response, ΔM. Let’s start by defining the design-level response displacements. The elastic deflections due to strength-level design seismic forces are called design-level response displacements. These are denoted by ΔS, where the subscript ‘s’ stands for strength design. Design level response displacements are what you get out of your software, when you run analysis. Please note that structural analysis softwares may provide these values in different formats; say a percentage of height or a direct output. Well, to calculate your story drifts, first you need to find maximum inelastic response displacements from your design-level response displacements. The maximum inelastic response displacement is defined as: ΔM = 0.7RΔS Where, R is the structural system coefficient, the subscript ‘m’ in ΔM signifies that we are calculating a maximum value for the deflection due to seismic response that includes inelastic behavior. Seismic drift values are much larger than wind values. UBC uses maximum inelastic response displacements rather than the design level displacements to verify the performance of the building. Seismic drift limits are 2% & 2.5% of the story height for long and short -period buildings. For a floor to floor height of 12 feet the max., allowable inelastic drift value would be 2% of 12 feet= 0.02*12*12 in=2.88 in. For wind for a 12 story height, drift would be L/400=12*12/400 =0.36 inches, A comparison of both wind and seismic drift limits shows that earthquake inelastic displacements are quiet large compared to wind displacements. That is why proper detailing is emphasized in seismic design. When calculating ΔS for seismic, make sure: You have included accidental torsion in your analysis. Use strength design load combinations: 1.2D + 1.0E + 0.5L & 0.9D + 1.0E. You are using cracked section properties for reinforced concrete buildings. Typical values are Icr walls= 0.5EcIg, Beams = 0.5EcI g & for Columns 0.5 - 0.7 EcIg. Use a reliability/ redundancy factor= 1 to calculate seismic forces. Whenever the dynamic analysis procedure of §1631 is used, story drift should be determined as the modal combination of the story drift for each mode. Determination of story drift from the difference of the combined mode displacements may produce erroneous results because maximum displacement at a given level may not occur simultaneously with those of the level above or below. Differences in the combined mode displacements can be less than the combined mode story drift. Example: A four-story special moment-resisting frame (SMRF) building has the following design level response displacements.(See attached Image) R= 7.0, I= 1 Time period= 0.6 sec (See the attached image for Story Information) Calculate: Maximum Inelastic response displacements. Story drift in story 3 due to ΔM. Check story 3 for story drift limit. Maximum Inelastic response displacements ΔM = 0.7RΔS ΔM = (0.7) (7) ΔS = (4.9) ΔS (See the attached image for Maximum Inelastic response displacements) Story drift in story 3 due to ΔM Story 3 is located between Levels 2 and 3. Thus ΔM drift = 5.39 - 3.43 = 1.96 in. Check story 3 for story drift limit. For structures with a fundamental period less than 0.7 seconds, §1630.10.2 requires that the ΔM story drift not exceed 0.025 times the story height. For story 3: Story drift using ΔM = 1.96 in. Story drift limit = 0.025 *(12*12) in = 3.6 in. > 1.96 in. Therefore, Okay.

You can start by selecting a design code that would help you establish the definition of soft story. After that you would need to proportion your structure so that you end up with a soft story (using thinner columns on lower floor which you want to behave as soft story etc). So, for example you want to use UBC97 as your design code, the first thing to do would be look up in that design code what qualifies as a soft story and then you go from there. If you are new to seismic, going through the following might help. Thanks.

Yes rana bhai.. u r right. i realized that after rechecking the model.. 😀

It looks like you have defined them as 'columns'.

Please share the bending moment figure also with values shown. Maybe there is difference in material properties between both models?

Rivit can be take replacement of Auto cad in firm. It will reduce discrepancy of architectural and structural drawings.

We use REVIT structure everyday for structural drafting and structural information. We don't however use ROBOT structural analysis or REVIT to export because making analytical model personally for me in REVIT is useless.

Autodesk Revit is in fact not a structural software. Rather, it is a software used mainly for saving architecture & construction related building information (for example, 3-dimensional building layout, wall & floor types, finishes etc.), in order to visualize the building geometry, preparation of bills-of-quantities and control construction related activities. Revit does not carry out structural analysis or design directly. The geometrical & loading information from Revit however can be exported to Autodesk Robot software for structural analysis. Revit may be very useful in architecture, estimation and construction fields, but, not in the structural analysis or design. Because, the quantum of structural output obtainable from Revit is not worth the time and effort required to model the whole structure in Revit for getting some structural output. Structural modelling otherwise can be done more easily & speedily. Therefore, IMHO, there is no use of wasting time on learning Revit, in case you are interested in structural analysis & design only. HTH Regards

That is very nice of you umaaa to making such an awsum plateform , i know very less about structural engineering bt i am proud of you