Suspended roof floor slab cracked at supports
Suspended roof floor slab cracked at supports
(OP)
Hi I would appreciate if someone could advice me on this problem. I have a situation where a driveway suspended slab at the roof level cracked at every supports (spaced up to 5m apart). The crack pattern seems continued and extending over the length of the beams underneath. Is this type of crack associated due to thermal movement? Or poor construction esp. reinf placing? I have noticed there is no expansion joints despite the slab spanning more then 45m.






RE: Suspended roof floor slab cracked at supports
Do you have the structural drawings of the driveway? Do you know what the design loading was? And do you know how the slab has actually been loaded?
With a bit more information, we may be able to give you a bit of advice, but in the end you should have a structural engineer in your area assess the structure.
RE: Suspended roof floor slab cracked at supports
In terms of loading, they have used 5kN/m2 (i.e. standard loading for vehicles) and have considered all the dead load. The total service design dead load is 8.5kN/m2. As such there seems satisfactory in term of loading calculation. Well to answer your last question, i don't really have a complete report but it looks to me they have designed in very simplified method such as wl2/10 for bending moment and M.F for deflection checking, which is very conservative, I think.
Any help? Appreciate.
RE: Suspended roof floor slab cracked at supports
1) The 125 to 150 slab is a bit light for the 5000 spans. L/D is normally limited to about 28 for end spans and about 33 for internal spans.
2) The cracks definitely sound like flexural cracks rather than direction tension cracks. This could be due to either inadequate reinforcement quantity or improper placement of the reinforcement (too low).
3) 5kPa is more than adequate if just for automobile loading, but if there are any trucks or other heavy vehicles using the driveway, overload could be an issue.
4) Is there any cracking on the slab soffits near midspan? If not, this probably indicates the top steel has yielded at the supports, but the bottom steel has not yielded.
5) If both top and bottom steel was designed for Wl/10, and the live loads have not exceeded the 5 kPa design load, then the design is conservative and the cracks can be considered a serviceability issue rather than compromising the strength. As you said this was a roof driveway slab, you probably have a waterproofing issue.
I still recommend that you have a structural engineer assess the slab on site.
RE: Suspended roof floor slab cracked at supports
You have cleared most of my doubts. Actually I agreed with your advise as stated in item 2. It is a combination of poor workmanship esp. on the placement of top reinforcement and the design calculation with minimum thickness. Bytheway do you have any idea on the pattern of crack that likely may form in a suspended slab if it is cast without any movement joints? Your immediate reply and advises are very much appreciated.
RE: Suspended roof floor slab cracked at supports
RE: Suspended roof floor slab cracked at supports