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In etabs we define mass source frm there we select from load and then we define differnt loads like dead , live etc my question is that what is use of this "mass souce" we donot apply it in the structue then where we need it?? and which loads are defined here means dead, live and which are not defined?? Thanks

I have observed few issues while getting the design results of reinforced concrete members from ETABS (v 9.7.4). I wanted to share this issues with you and get your input and observation on it. ETABS always considers the minimum eccentricity for selecting the design moment of columns irrespective of the probable behavior of the column, whether short or long column. See section 10.10.6.5 and its commentary of ACI 318-08 which deals with minimum eccentricity of long columns. You should always check the design moments that ETABS uses for columns if you want to bring down the cost of construction. If your model has lateral loads, ETABS will give you design moments in column irrespective of its status as braced or un-braced as per ACI 318 criteria. You should investigate if the storey under consideration is braced, or un-braced (10.10.5.2), and decide appropriate design moments of columns. ETABS has a tendency to select a time period of the building that is considerably less than the value obtained by the approximate method, Method A, of the section 1630.2.2 of UBC 97. To quote the FEMA 451 document: ''Because this formula is based on lower bound regression analysis of measured building response in California, it will generally result in periods that are lower (hence, more conservative for use in predicting base shear) than those computed from a more rigorous mathematical model". So, there is no need to use the value of time period that is lot less than Ta. One should always check the time period used by the software; ETABS can overestimate the seismic force by more than 2 times. Method A gives lower T and higher V, so FEMA 451 has advised not to use the value of time period less than this value even if rigorous analysis gives a lower value. I have seen the results where Etabs have use the value of time period less than Ta; in-fact as low as 0.5Ta, which can increase the base shear two times.

Assalam o alaikum 1. I want to ask how ETABs considers the sway conditions in concrete frame design? I designed an RC structure in which most structural members fail but when I change the preference to sway intermediate most of them pass the design. Is there any way to define prior to design for IMRF input? 2. What is the maximum allowable deflection for an RC structure? As in codes it is usually mentioned for structural members but not for the whole structure like at top story for seismic design.

I think there is no absolute max requirement for overall building drift for strength seismic zone. All you have to check is inter-story drift. For wind yes, there is this usual H/400 requirement where H is the overall building height. And in your case it is 202x12/400 = 6.06" = 6" (Use 0.7W to check drift here). You can also check human comfort criteria and acceleration at top floors. Yes 20" seems excessive, and you must put more strength and stiffness to your building. The building is very flexible.

You can see the following topics to calculate your allowable displacement/ drifts. If your displacement is greater than what is required, check your input and add more shear walls. http://www.sepakistan.com/topic/1341-ubc-seismic-drift-limits/ http://www.sepakistan.com/topic/109-building-drifts-in-etabs/ Thanks.

W.salaam. Since we normally don't consider soil structure interaction in structural analysis, therefore I am quite certain that the statement quoted by you about choice of pin or fixed based on soil type is flawed. Anyhow, for concrete columns unless special detailing is done at coulmn joints, they would always transfer moment that should be accounted in the design. Using fixed based would likely result in conservative design for structural members than using pinned base. The real response would stand somewhere inbetween a pin and fixed base. Thanks.

Just to correct you here (and my self too) that mass source command is used for calculating Center of Mass of the diaphgram. So this option is used even for wind load or any other load where we need these two centers. Pardon for my old post which was a mistake

thanks rana bro its cleared now u defined it in simple way

its the seismic weight according to code. The load you put here will be used for Seismic calculations. This is W in IBC, UBC or other american codes... Select "From Loads" option and put all dead loads with factor = 1 and all live loads with factor = 0.25 this is to consider 100%dead loads + 25%live loads for seismic calculations....