huangyhg

concrete roof slab load
hello,
my father being a civil and structural engineer thought i should post this question in eng-tips and see if the results were consistant with his.
i've purchased a house built in th 50's which has a 4" thick horizontal concrete slab as the garage roof (fall out shelter i guess). the size of the slab is 22'x14' with minor spalling occuring. the slab is bisected equally on the underside with 1 concrete crossbeam whose thickness is 6.5"x6.5". there is minor cracking in the beam. the whole slab is supported on the perimeter (garage walls) by standard cinder block and an outer layer of your typical red looking concrete house brick. my question is(finally here), what is the maximum load this concrete slab will support? there's a door that leads from the 2nd floor out to it but i have yet to venture out. your answers would be much appreciated as my father graduated queen's university in 1956 and it would be of interest to see your results compared to his. i would also be grateful of the advice. thanks for your help eng-tips!!
spinner
wow! that's a tall order with the information at hand. first, do you have plans of the as-built slab? presently all we know is the area of slab and that it is supported on all four sides by concrete block faced with ordinary clay brick. moreover the slab has a small beam for additional support in the middle. which way does the beam run north and south, east-west, or both? what about the reinforcing? do you know the amount of reinforcing and what spacing? what about the strength of concrete and that of the reinforcing? there are just too many unknowns to even begin this problem. although we could all make some educated guesses, that's all the end result would be is a guess.
sorry not to be of further help, but if there was more information...
where are you at ron? i'm anxious to see what you think of this problem!
qshake
hey q! i read this one last night but didn't take the time to respond. thought is would be a good one for the forum to take on as a challenge.
i agree with you that we need more info, so let's ask a few investigative questions to see where we are so that it doesn't cost spinner1 an arm and a leg to get an answer, albeit a very qualified answer!
spinner1...
what is the pattern of the cracks in the slab?
what is the pattern of the cracks in the beam?
what is the orientation of the beam?
is the slab covered on top by a roof membrane?(you mentioned a door for access)
how large is the spalling and can you see any rebar in the spall? if you can see rebar, scrape around it a bit and see if you can discern the size. see how much cover is on the rebar.
what is the aggregate in the concrete? is there a reasonably uniform distribution of coarse aggregate and fine aggregate showing in the spalls?
is there a distinct coloration difference between the surface of the concrete and the deeper concrete? if so, how thick is the discoloration?
are there any rust stains? if so, where do they occur?
look at the underside of the slab and see if you can determine a pattern to the rebar. sometimes you can.
qshake....anything else?
ron, i think, as always, you've covered the situation with the right questions. if we could have some answers to those i think spinner1 will know whether or not to hold any dance parties atop the old car shed!
its really interesting to find an old concrete roof at least on a residential structure. i wonder (i hope anyway) that the roof slopes in one direction. i onced evaluated a building roof that was composed of four monolithic hyperbolic paraboloids (a wind blown umbrella for lack of better description)each one supported by a cruciform column, with a drainpipe right at the column. the four birdbaths covered a masonry building and only that portion of the roof over the building was waterproofed at all. what was there was great stuff too. but as you can imagine, the section of the roof that wasn't covered was critically deteriorated. what gets into someone's head that we need only to cover half the building? having covered the other half would have saved the owner my fee and the work necessary to correct the problem! and that architecture...a concrete birdbath...you don't see to many more of those...i wonder why! just a story to share about concrete roofs.
yeah, q...it is interesting to see a concrete roof on a residence. i have an old friend who came from cuba in the 60's. he is a contractor and his dad was a contractor/engineer in cuba. he told me about their house in cuba with a concrete roof, monoslope, no roof membrane, no coating, no cracks, no leaks. i told him he was nuts...too much cuban coffee. he swears its true!
about those architects....what are some of those guys thinkin'? i've seen some of those screwball roofs, too!
spinner1...sorry for the dialogue. if you can answer those questions, maybe we can come up with something. q and i get invited to the party on the roof, though.
in my view you simply have to know how much steel in this structure. you'd need to know the code or building requirements in the 1950's when the house was put up; assume minimum steel and analyse beam and slab it based on that, which is a simple thing. i'd do it but in malaysia i can't quickly put my hand on the right reference!
by the way, in asia every darn thing is concrete, and has been for a lot of years; you won't even find many 1 & 2 storey houses without a concrete frame!
anthony tugwell
project director & consulting engineer - currently in kuala lumpur. malaysia
beep! wrong answer adt. we must know more than how much steel is in the slab. for one thing we don't know the strength of concrete and while concrete gains strength with aging there is a big difference between 3000 and 4000 psi concrete especially when we know nothing about the structure. which brings me to my next point...if the structure is badly deteriorated, what's the point in guessing the capacity based on uncracked concrete? no, i would rather no more about it than just the amount of steel.
i appreciate your comments on the material of choice in aisa. why is that? i would think that concrete is a hard-to-come-by material in many parts of asia. in addition, i can't say i would be sold on the labor practice with concrete there either. just not enough inspectors. by the way, how are additions handled? i can hear that concrete saw warming up now!
sorry qshake didn't like the last answer! i didn't propose that all else was irrelevant. yes, of course the concrete strength is important and the capacity will be dependent on this and all the other factors. it's old and one would have to assign a figure, which i would say would be 2500 or below based on the age and type of structure.
however, if this thing is fairly cracked top and bottom perhaps analysis as a catenary is the only way to go!! yes, ok i know, there's no edge restraint!
just on that however, maybe we need to ask how much central deflection there is presently as measured across the slab/beam. you could then calculate back (with some creative assumptions about neutral axis location etc), to get a "concrete-based" moment capacity to compare with the "steel-based" one in my previous message. the governing one will then give the maximum total applied udl. subtract dl and hope the number is not negative!
another point; this is not much of a beam is it! only 10.5" overall depth (267mm) across (i'm guessing) the 14' span. l/d = 16.
re. asia: there is little by way of trade capability, eg. bricklaying, blocklaying etc. so concrete is convenient but, yes, the quality can vary. but that's why i'm here because my people do a lot of that sort of inspection work as part of our overall project management work. and there's never an inspector to be seen, that's true.
we're lucky to have reasonably good pre-mix concrete plants however. cement availability is ok here but is very variable in other parts of the region.
anthony tugwell
project director & consulting engineer - currently in kuala lumpur. malaysia
thank you spinner1 for the topic! it lead me to reminisce about all the thin-shell structures we did in the 50's and 60's. there was a mexican architect of that era, felix candelera i think, whose "birdbath" structures were primarily steel nets covered with concrete.
i don't think measuring the central deflection does you any good at all. you can certainly measure the different elevations across the slab (as compared to the mid-point of the slab) but that doesn't tell you anything as the builder could have built the slab in any original condition (i.e. camber or sag). thus, you do not know what the deflection under dead weight really is.
you could, however, do a load test on it to verify a total safe capacity. this would involve choosing a preferred load capacity for the roof (i.e. 20 psf or higher with snow) and load testing it up to your load times a safety factor of perhaps 2. you could also measure the deflection change under load to determine a stiffness for serviceability checks.

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