Expert: Arshad Khan - 4/16/2011

Question
QUESTION: dear sir
aoa
i hope you will be fine...thank you for the guidance you've been giving me till now....please suggest me on the following:-
i am to model a 11 storey RCC structure in ETABS...
do i have to provide the stiffness modifier for all the members from the start?...i,e assume the sections to be cracked?....or first do i have to perform an analysis to check whether the cracking moments are being exceeded or not then decide about the stiffness modifiers?....

secondly ill be needing to design the foundation in SAFE for all the seismic and gravity forces....how will i export the foundation model from ETABS....will i have to model the raft in ETABS first?....

thirdly, how will i design the lift wells in the model and what will be the concept for it.....

also there is a parking floor in the basement i have to design and its  a grid of 9.2m x 9.2m ...what framing system do i have to adopt for it and is it recommended to input a moving load for the design of ramps or should i just apply a unform pressure on the entire floor and ramp?.....
please suggest me in a bit detail...
thankiing you in anticipation
Regards..

ANSWER: Hi Kamran,

1. First do an analysis without modifiers and then adjust just as you have noted.

2. Yes, you can model the raft in ETABS and then the export to SAFE will be straighforward.

3. Assuming that your raft will drop down under the lift wells (with a transition zone like a 45 degree soffit splay for the drop), you can ignore the effect of the pit as far as the global foundation design is concerened. Then you can reinforce locally to suit the stresses from the analysis.

4. For a grid pf 9.2x9.2m, you can either adopt a solid 2-way slab, or use a 2-way coffered slab to reduce weight/volume of concrete. There is no need to apply moving loads, but do apply pattern loading in adjacent spans so as to capture the worst moment/shear envelopes.

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QUESTION: thanx sir, if im not mistaken u mean a flat slab by saying Solid 2 way slab ryt??.....
my building is 305m long and it is difficult for me to provide expansion joints that should have vertical continuity because of the geomtery of the building....should i carry out an analysis using temperature loading as well?....if yes please guide me hoe i do it....

ANSWER: Hi Kamran,

1. If you do not ahve a headroom problem, it will eb better to have downstand beams, with 2-way slab (solid or coffered). Flat slab will be quite heavy, unless post-tensioned.

2. 300m is far too long without an expansion joint- your thermal stresses will lead to an uneconomical structure, so try to break it down to maximum lengths of around 75m, or 100m at the very most.

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

QUESTION: Dear Sir,
please guide me how should i decide the thickness of the lift well in the 11 storey building if:-
1. it is possible to connect the lift well to the building framing from all sides
2. it is possible to connect it from 2 sides of the framing.

Answer
Hi Kamran,

This really depends on the relative size and location of the liftcore to the rest of the building and whether you plan to resist all lateral loading by the core, or by a dual system with moment resisting frames.

For an 11-story building, I would start by assuming 250mm thick walls and then run ETABS to see if that is stiff enough to limit drift to H/600 for initial design, and to limit 1st mode period to around 1.0 second.

Yes, you can frame beams into the RC core, form all sides or from 2 sides- make sure they align with the walls, and if the beams are wider than the walls, then they should continue through the walls, but the inner face should be flush with the liftshafts.