Expert: Arshad Khan - 2/5/2010

Question
QUESTION: Dear Sir,
It it the first time I ask you so I hope you willing to answer me!
1/ I have a difficult in analysis dynamic of building. Now I'm design the building with podium(4 story) and 2 blocks above this podium. One block with 19 story and one with 15 story (please see my attached image). It is the first time I design the building with 2 blocks. I do model of this building in 1 model. So I define dynamic analysis as eigen vector with 24 mode. After analysis, i see that the first, the second and the third modal of the bigger block are mode 1,8,14(for X direction) and mode 2,7,13 (for Y direction). And the first, the second and the third modal of smaller block are mode 3,10,19(for X direction) and mode 5,12,15 (for Y direction). So can I use the result for analysis wind and seimic?
 I assign semi-rigid DIA for all story, D1 for story 1,2,3, D2 for story 4 to roof of bigger block, D3 for story 4 to roof of smaller block. I don't know what i do right or wrong. Can you give me an advice?
2/ I use membrain for the slabs, so in some slabs with complex geo metry, if i use auto mesh membrain as DEFAULT, the loads transfer to the beam and moment in beam are not normally. Would you like to give me an advice for it?
3/ I define the pilecap with the shell element, and the result i have Mmax in shell element (at any point). Can I use this moment for designing reinforcement concrete? and how to use the moment at point?
Thank you very much!

ANSWER: Hi Khanh,

You appear to have done the modelling OK and can use the 2 sets of modal info to analyse for wind/earthquake.

As for the slabs, membranes are fine so long as you do not need to take out-of-plane stiffness into account for your analysis/design. This will be fine so long as the slabs are relatively flexible compared with the beam stiffness.

ETABS is not very good for design of pile caps- you should extract the column forces and design the pile caps manually or by using other software such as PROKON, or spreadsheets such as available from the website below:

http://www.engineering-international.com/

---------- FOLLOW-UP ----------

QUESTION: Dear sir,
Thank you very much for your helping!
About slabs with membranes, I want to ask about how to mesh membrane in ETABS  to have the exact moment in beams. Because I use auto mesh as DEFAULT, so the moment in beams is not normally. Thus, I very worry about it. Can you help me?

Answer
Hi Khanh,

I don't know what the precise problem is, but use the Help menu to find the solution. Maybe you need to use one-way loading distribution? I cut/paste below something that may be of help:

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Use Line for Floor Meshing
During analysis, ETABS automatically meshes (divides) area objects that are assigned deck properties or slab properties with membrane behavior only. Meshing helps distribute loads realistically. See Area Object Mesh Options for more information.

The Assign menu > Frame/Line > Use Line for Floor Meshing command provides you with additional control over meshing of floor objects on a frame-by-frame basis.

Apply this assignment to a line object as follows:

Select the line object(s) to be affected by floor meshing.

Click the Assign menu > Frame/Line > Use Line for Floor Meshing command.

Select the desired option from the drop-down list:

Yes Tells ETABS to include the selected line object(s) in automatic meshing of associated floor objects.

No Tells ETABS to exclude the selected line object(s) from automatic meshing of associated floor objects.

Cancel Exits the command without changing the assignment.

View assignments to the element by right clicking on it to bring up the Line Information - Assignments Tab form.