Fully fixed supports for RC Frame - Not true in real life....

[kellez]

Hi,

I am designing a 2-storey RC frame with fully fixed supports at the base, and i am thinking that this is not the case in real life, even in RC frames the supports are never fully fixed. Therefore i just wanted to see what other experienced professionals choose for the supports at the base of RC frames. My opinion is that it could be acceptable to decrease the stiffness of the supports to lets say 80% instead of choosing a pin support.

In my case a foundation slab 45cm thick with foundation beams is used, very strong design.

The reason that got me thinking is that I am getting very high forces which in turn demand very large cross sections and this is just a 2 story house

pictures of the type of foundation used

 

[klaus.]

Well...any 'Model' is an approximation of the real life
nothing is 'exact'
If you are uncertain, than you need to verify the assumption ( e.b. by calculation the rotations) or do an additional calculation with different stiffness
Say an upper bound (high stiffness) and a lower bound (lower stiffness)


best regards
Klaus

 

[kellez]

Hi,

Thats a very reasonable suggestion and something i am planning to do,

however what i want to know is, do professional engineers take this into consideration, is it something well known and adopted?
is it common practise to reduce the stiffness of supports?

I am sure there are research articles that address this, and i know for certain there are research papers on steel frame models
using modal experiments to identify structural properties (such as youngs modulus and connection stiffness)
its called Finite Element Model Updating technique.

But again my concern is if this is a common practise between professionals

 

[kellez]

I just had a thought, eurocode8 suggests reducing the stiffness of the elements by 50% for seismic analysis due to cracking.
Which i did, Dont you think that is reasonable to assume that the stiffness of the supports also reduces during an
earthquake due to cracking therefore it should be considered in the model?

 

[Settingsun]

Hi,
This is from steel portal frame design guidance (Steel Construction Institute publication P164, referencing British Standard BS5950), but might give you an order of magnitude:

For 'fixed' bases, it gives column base stiffness about equal to the column stiffness = 4EI/L.

 

[kellez]

Ok but what are you actually referring to? It could only be a theoretical value which you can find in every structural engineering text book.

What is this value? and what is it based on? is it based on experiments? Does the book actually refer to this value as the stiffness of the supports in real life cases?

 

[Settingsun]

BS5950 commentary might have full details. I don't have a copy.

In the document I do have, it's referred to as a 'convenient' way to 'avoid soil mechanics calculations'. I presumed this is what you're after, given you were talking about an arbitrary 80% value.

I'd compare it to the fully-rigid results and see if it makes much of a difference. If you're uncertain, make sure your reinforcement quantity is light so the sections remain ductile and therefore tolerant to analysis inaccuracy.

PS: 80% of what? If we're talking about the ideal perfectly-rigid assumption, 80% of infinite stiffness is still infinity. If it's a 20% reduction of bending moment at the column bases, that's within the usual redistribution range, and especially so since we know the bases aren't actually perfectly rigid.

 

[SlideRuleEra]

...do professional engineers take this into consideration, is it something well known and adopted?

Once-upon-a-time (before software), engineers would occasionally alter the coefficients of equations to account for "imperfect" conditions. This was done based on the performance of similar structures, not on guesses or estimates. Also, bounding the results, as recommended by klaus, would be mandatory before considering a modified coefficient.

With sophisticated software available today, I doubt this procedure is widely used or accepted.

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