Water Ponding at Ground Storage Reservoir Roof

[quicksilver01]

What options do I have to improve the roof's pitch for better water drainage. The structure is almost 50 years old and is experiencing sagging at middle spans where water ponds. I have no knowledge of the steel grade used for the reinforcement either. We are trying to get the pitch to 1/4" per foot using lightweight grout,or some sort of cementitious foam. This means a little over 24 inches extra at the center of the structure.
Are there any standard practices for this particular situation? Any direction or help will be much appreciated.

Extra info:
Slab Thickness: 6.75 in (d=5.5)
Diameter of GSR: 224ft
Roof Supports 14 in columns in 20ft grid.

http://i683.photobucket.com/albums/vv199/v-acuy/Project Photos/GSRRoof.jpg

 

[Ron]

I would question the decision to use lightweight grout fill as even this will add a lot of weight, causing progressive deflection.

I would take the tank out of service, cut and jack the columns, and splice the columns to add height. That way you can control the slope in any way you want.

 

[MiketheEngineer]

Look into wood trusses with new roof?? Or just simple stud walls with new roof.

Done all the time.

MANY years ago I helped out at on the military academies. We retrofitted literally hundreds of roofs.

 

[JedClampett]

You might want to do a little cost benefit study for your client. Is there really a problem here? Puddles? Is any rainwater contaminating the reservoir?
Any fix will be expensive and difficult. Adding 24 inches of insulation or lightweight concrete just to slope the roof seems excessive. How about filling the depressions with a leveling compound?

 

[quicksilver01]

A lot of these alternatives do sound expensive. We have consider filling the depressions with a leveling compound. In fact, that's what we would probably end up doing, based on the calculations we have been running that pretty much tell us that we don't have very much room to increase the slope.

Anybody has a comment about what steel grade was standard in the early sixties? I haven't been around this world that long to know and there is not any documentation available that provides this information.

Thanks for the responses, it looks like we are headed in the right direction.

 

[MiketheEngineer]

Probably A36 - 36 ksi

 

[JedClampett]

In the early sixties, 40 ksi reinforcing was more common than 60 ksi. Unless you want to have testing done, assume 40 ksi.

 

[MiketheEngineer]

Are you talking steel beams or rebar??

 

[quicksilver01]

Rebar.

 

[dik]

Assuming the rebar is in good shape, and is capable of supporting the load, as noted, it is likely 40 ksi stuff; in the late 60's, 60 ksi was commonly used in our environs with 40 ksi stirrups/ties. As noted, you can take a sample and test it. You might want to check for corrosion.

If capable of supporting the added load, you will gain a little more deflection. A 6-1/2" slab spanning 20' is about as thin as you want. If you can get the topping to act compositely with the slab, the effect will be minimal due to the added depth of structure. The slab must be shored.

Where the structure is not capable of supporting the load, the use of wood beams and trusses to bring the loads to the column locations is good... just a 'superimposed roof'.

Depending on your height, 14" columns may be marginal, in particular for added loading.

Dik

 

[msquared48]

With that slab thickness to span ratio, L/35, it's no wonder you are seeing ponding and creep. Should have been a 10" slab. Oh well.

I agree with the wood roof option here. At about 10 psf it will only add about 4 Kips to the columns without loading the slab anymore. You will just have to provide adequate venting. It will not habe to be taken outof service either.

Mike McCann
MMC Engineering