Long Term Deflection of Reinforced Concrete

[jfmann]

I am interested in learning of any experience with long term deflection of reinforced concrete beams or slabs that has resulted in problems, even if only appearance related. The ACI Code clearly requires consideration of long term deflection when deflection calculations are required. However, the primary reference also states that there is very limited data on deflection of real structures.

 

[Lionpride]

I am also anxious to see real world long term deflections on a project I recently helped design. With 36" deep beams and a 62 foot clear span, the parking garage pushes the envelope of conventional reinforcing and I was hard pressed to develop a design that works economically within such constraints. I found that with nearly maximum tension and some compression reinforcing and a camber of approximately 4 inches, short and long term deflections could meet our deflection criteria of L/240. I found trying to predict all the many variables for deflection, including actual live load and concrete strength, almost impossible and consider this design almost more of an art form.<br>
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As this project is constructed over the next few years, I will be happy to update you on any findings. I know you are looking for designs that have resulted in problems - and to that end, I hope I won't have too much to report.<br>
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Please share any information you've found on this subject.

 

[ishvaaag]

I have found problems related to incompatibility between the deformations of the structure and attached masonries.

Some of these problems are not so likely to happen today, these were structures of the early 80's in Spain, when our commonly available means of calculation -I refer mainly to deflections, even for mere elastic consideration- did not include for common practices such thing as a FEM analysis program.

Others are consubstantial to natural processes and construction procedures and will recurr as nearly the same rate than now.

Much of these serviceability failures (in strength never have had to meet one) are related to scarce stiffness of the floors due to the owner-builder insistence on using flat beam plus one way joist construction at depths a bit scarce for the spans.

Dishing of the foundations I have seen to cause very patent cracks in attached partition walls even in the concurrence of piling foundations. Related to dishing I have also seen cracks in junction in plan zones of irregular plan buildings. Also related I have seen the triangular &quot;cut&quot; cracks at corners of floors upon loading, even when unidirectional, here you see the 2 way behaviour even when reinforced 1 way referred recently is for real.
Unnoticeable deflection in other cases was enough to pass significant load to badly executed and then fragile masonry floor facing. The crackability of attached brick (coocked clay) masonries starts at about L/2000 or even less, being noticeable at L/1600. No code exacts as much.

On the different compressibility of masonry and concrete I have also seen the unsupported but attached veneer of a column to shear at the junction of the supported and unsupported parts.

For what I have seen of old concrete, these buildings when century old should give further problems, for really I have seen examples in continuous foundations and beams that concrete CREEPS (over 10 cm deflection in about 5 cm spans!)
Not all cases are to be so severe, nor seem to be being, but
the unreliability issue stands.

What is much needed is an array of good and clear books on time dependent effects on RC and PC/PT structures. Some of the books I have bought I have seen to be in error even to my modest understanding! Hope I will be able to buy and understand well one these coming years.

Whilst the more close thing in this regard must be the programs being sold for bridges or so that include &quot;thorough&quot; account of time-dependent effects.

Article

Serviceability Limit States Under Wind Load
Lawrence G. Griffis
NSCC 1994 proceedings

is very good on serviceability items. No creep in it, a steel item...but serviceability does not reckons material.