Temperature forces on concrete structure 1

[kol99]

I am using STAAD pro 2004 to analise and design a 2 storey R/C house. I have a question regarding the loads/forces in the system. Made all the loading case : self, live, wind, earthquake, temperature, etc. It seems like un-usuall to me to have design results with the temperature loading component (dead+live + temperature), as a critical case?! Normally I would have expected to have D+L+W, or D+L+E, as critical cases on all concrete columns. Is it possible to have temperature loading, bringing such internal stresses as to be critical? I filled the temperature loading pattern as regular, with the difference between element faces 20 degree celsius. Did anyone had a similar condition or design issues? I desinged this structure in Metric units.
Thanks
Kol

 

[civilperson]

Temperature is not a load case, CHANGE of temperature does produce stress and strains.

 

[corus]

Differential thermal expansion produces stress and strain. This may be caused by a difference in temperature within the structure or a difference in material properties at a uniform temperature, or both. If the design code you use doesn't specify thermal loads (ie. for a house) then I'd think you don't have to include it. If it's for a nuclear reactor, for instance, then include it. Please.

corus

 

[kol99]

It's not the code that specify the load combinations. I am using Euro codes 2, and there is a case to load the structures with streses for temperature difference from inside to outside of structure. Here is not the metter of designing a nuclear reaktor, but the simple design of a 1 to 2 storey R/C framed house. My question is that I didn't expect to have a load combination including temperature to be a ruling one, especially for some of the column reinforcemt. Normally the interior forces coused by wind or earthquake would be ruling factor on reinforcent design of columns. I haven't used the american codes of program STAAD Pro 2004, but I believe there is a case too for temperature.

 

[Philip86]

Be sure to only include the temperature difference on certain elements.

For example, in the australian code we have a case can have cases of say +22 or -20 degrees for exterior elements. But in interior buildings, there is only the need for crack control reinforcement, which is usually secondary to primary load cases. Our Bridge Code specifies a load combination of Perminate Effects + Thermal (for those whos code doesnt specify a temperature case). But since most buildings do not have concrete facade. There is little area of conrete which is acutally exposed to the exterior of the building.

So if you are applying a blanket + or -20 temperature gradient on all elements this will not reflect the actual temperature effects occouring on the building.

I know in bridges Temperature effects (Creep and Shrinkage) are very large and are a major design consideration. But I would assume that on a building like yours it shouldnt be as critical.

 

[kol99]

Philip. thanks for giving me a clear idea and guide. Yes, I should load only the exterior columns with Temperature stresses, in fact I applied the street on the whole structure. When I have a chance I will attached a file with the results of these load cases, in regard to reaction on ground(foundation), so you can view the effect.
Hej Philip, how Austalia is copping with this wilde fires? Might have been a hot sommer there!?

 

[Dinosaur]

Yes, the effects of temperature can govern in low frames. I believe it will be hard to decide what range of temperature to design for; however, since buildings are fairly protected. Your support conditions will be very critical to the solution. consider pinning some columns at the base and see how things change. It could require you to make more substantial moment resisting connections at the top of the first floor though.

 

[kol99]

Yes, your first statement is tru. I tried the structure as 1 floor and it produced a higher moment from temperature. The same structure I made 2 levels, and the moment from temp' combination come lower. This is strange , but it needs analizing various effects. The structure is designed as columns, two shear walls, flat slab supported on columns. I considered the columns as fixed ones. At an early stage I modeled the structure without shear walls, which meant the columns had to be lateral resitant. Then I remodeled with 2 shear wall perpendicular. So dinosaur, you say that if I make some of the column support as pinned ones, then I will have a different result?! Well, I will and thanks for the advice. The thing is that I am a srtuctural engineer, but I don't design everyday. One have to be everyday structural designer, in order to know all this hints, possibilities, options, etc.
The last sentence: Do you mean, providing some concrete beams from column to column? In fact this I didn't like to have.

 

[Dinosaur]

If I understand the problem, modeling the base of the columns will allow rotation at the base to alleviate the movements from temperature. But if the other end of the column has no support, then the whole thing is unstable. As long as you design and detail the columns to match your FE assumptions, then things should work out.