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Re: How to medel temperature effect in RCC building using STAAD?

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Ashraf,
 
        I can not understand how you are getting the results you are getting.
 
        The expansion of a beam due to temperature should be calculated from the formula
 
            delta(L) = L*alpha*delta(T).
 
            For a 10 meter (30 foot) long steel beam this should give
 
            delta L = 10,000 mm * .0000117*20 degrees C = 2.34 mm (about 0.1" for 36 degrees F).
 
Since, presumably, half would go to each column the resulting moments should be very small.
 
        I suspect you have something wrong with your model.  Two possibilities come to mind: you have a substantial mistake in numerical data entry; or there is something wrong with the basic philosophy of the computer modes you are analyzing.  Such things can be extremely tricky and difficult to find.  I have had similar problems in the past myself and had to get help sorting it out.
 
        Step one: take a close, critical look at your deflections.  This may locate the problem very quickly.  I was once designing a guyed stack and spent hours trying to correct ridiculous results.  Finally, I reviewed the deflections and found that the deflection at the top was several thousand MILES!!  After that it took less that two minutes to find that the nodes for the bottom ends of the guys had not been anchored.  Problem solved.
 
        Step two: check your data entry.  I suspect a decimal or a units problem in the thermal loading data.
 
        Step three: review your model philosophy.  You may be making your model too complicated.  A 2 mm displacement applied to the top of a 5 meter (16+ feet) high column will have trivial results; a 2 mm displacement applied at a 100 mm (4") end offset at the base of a column will cause just the results you're experiencing.
 
        If your model is too complicated one thing you might try is to change all of your fixed reactions to spring reactions.  In reality there is no such thing as a "fixed" reaction; any foundation will move or rotate a small but measurable amount when subjected to loading.  If your model is too detailed, especially in the reaction area your computer's matrix inverter will be "applying" whatever force it needs to force the deflections to zero at that point.
 
        Good luck.
 
Regards,
 
H. Daryl Richardson
 
----- Original Message -----
Sent: Saturday, January 28, 2006 11:26 AM
Subject: Re: How to medel temperature effect in RCC building using STAAD?

Mr. Daryl,
Thanks for your lengthy mails. I am getting very high moments on the column due to temperature effect. Even the moment at base of footing due to temperature is much higher than moment due to seismic loads. I am designing a two storeyed building for an oil company. There specification says to consider +/- 20 degrees change in temperature. The grade beams are restrainig columns too much. As you said the effect of temperature on the upper part of structure is negligible. So I want to confirm whether its realistic or not. I am incorporating only axial elongation in the frame elements.
Thanks and regards
Ashraf

 
On 27/01/06, Daryl Richardson <h.d.richardson(--nospam--at)shaw.ca> wrote:
Mr. Ashraf,
 
        +/- 20 degrees would be negligible for the types of structures I work with; never the less, I will try to answer your question.
 
        I think the answer to your question is changes in axial length alone is sufficient for your purposes but the following paragraphs might elaborate on this a bit.
 
        Building materials, and most other materials  for that matter, expand fairly uniformly in all directions due to temperature increases (and also the reverse for temperature decreases).  Your question will be better answered by answering the question "Does this change in element dimension have any effect on your structure?"
 
        For the most part there is virtually no stress effect in building framing members due to temperature change when all of the members are at the same temperature (with two exceptions).  Columns connected to grade (which, presumably does not move or shrink due to temperature changes) may experience bending due to becoming out of plumb due to length changes in beams connecting them.  X bracing elements connected to the base of columns discussed in the last sentence, will probably undergo a change in length which may be different from other framing elements, hence stress change due to an axial change in length.  I think STAAD should take care of all this automatically.
 
        Elements that are very wide, such as floor plates, metal decking, and wall cladding require consideration.  Usually this is taken into account by appropriate detailing to prevent leaking and other undesirable situations.  In most cases this is accommodated by the Architect rather than the structural engineer (except for bridge decks).
 
        Hope this helps.
 
Regards,
 
H. Daryl Richardson
----- Original Message -----
Sent: Friday, January 27, 2006 11:40 AM
Subject: How to medel temperature effect in RCC building using STAAD?

 
Hello all,
I have to include temperature effect in a RCC building for +/- 20 degrees temperature change. I am using member elements in STAAD for modeling the building . Is it correct to apply temperature effect for axial elongation alone?.
Thanks in advance
regards
Ashraf

--
S.M.Ashraf
Structural Engineer
Al Habshi Consultants Office
P.O.Box 27154
Safat 13132, Kuwait
Tel. 965-2430103
 



--
S.M.Ashraf
Structural Engineer
Al Habshi Consultants Office
P.O.Box 27154
Safat 13132, Kuwait
Tel. 965-2430103