Water Tank Roof Loading 3

[waytsh]

I want to get some second opinons on what would be the acceptable loading for the roof of a water tank. Normally I would think that the Roof Live Loads or Snow Loads as specified in AWWA and ASCE would be used. However I have seen some situations where because the roof is accessible by ladder the ASCE live load for promenades of 60 psf was used. Is this reasonable? My roof snow loads in this area are only going to be around 25 psf so I will be more than doubling the design load on my tank roof.

Just curoius on how everyone else is handling this.

Thanks,

 

[MotorCity]

waytsh,

You will need to look at a number of load combinations. Perhaps the most significant load will be the negative suction pressure on the roof if the tank is drained. This force is usually compensated by providing relief vents. However if these vents become blocked or malfunction, the roof could collapse if it is drained. The worst case may be if the tank is being drained with snow on the roof. Good luck.

 

[JAE]

I don't think a promenade live load would be appropriate. Do you really think you'd get a large group of people up there at one time enjoying cocktails?

The loading on a water tank would be more for maintenance and access where it would be more similar to a catwalk live load (per ASCE 7) at 25 psf....which sort of matches the snow load anyway.

 

[JStephen]

Normally, you design for the snow load (25 PSF typically, although it can be reduced). You also design for a concentrated load- think it's 500 lbs, but check your pertinent AWWA tank standard to see. You don't combine the two loads, and in most cases, it's the snow load that governs the design.

Normally, venting vacuum is not added to the snow load. For most water tanks, the vents are substantially oversized, and venting pressures are minimal. You don't design for blocked vents- if that's a concern, you reconsider your vents, not the roof. Full vacuum is 2,116 PSF, and it's unreasonable to design a roof or shell for that.

 

[waytsh]

Thank you all for your input. I feel better about sticking with the roof snow load and leaving it at that.

Motorcity and JStephen, just out of curiosity how do you calculate the pressure from the potential suction force? Is there a reference to this somewhere?

JAE, you conjured up a pretty funny mental image in my head of all these hoity-toities sipping their champagne on the roof of my water tank. I suppose a more plausible situation would be a bunch of high school kids with something to prove. Either way I agree that it would be extremely unlikely to have this many people on the roof of the tank.

Now that I am sticking with just the snow load however, another question comes up. The unbalanced snow loads called out in ASCE and picked up by the IBC code do not specifically mention the type of roof that I have. My roof panel wedges are flat and have no curve. At the center apex they bear on an interior column. So in profile it would appear to have a gable configuration, but in actuality it is a type of flat dome. I'm sure there is a more technical name for it but at this hour of the night it eludes me. "Cone" I guess. Do you think this would fall under the domed roof or the gable roof equations for unbalanced loading?

I would be very interested to hear your opions on this as well.

P.S. Does ASCE have a point of contact for questions like these?

 

[JStephen]

To calculate pressures from venting, you need some information on flow characteristics of the vent.

You can treat it as a orifice and apply Bernouli's equation, for starters.

ASHRAE Handbooks have some information on flow hoods, screens, etc., that can be applied to tank roofs.

Vent manufacturers can furnish venting information, if you're using any kind of factory-made unit.

API-650 states that those tanks are good for up to 1" water column of vacuum, or 5 PSF. On water tanks, assuming 5 PSF and designing the vents based on that will give you very small vents, compared to inlet and outlet sizes. Typically, tank vents are sized the same as inlets and outlets, and you might have 1 PSF actual pressure.

On unbalanced snow load, check the AWWA standard to see if that loading is even required. If it is required, it helps to minimize the column cap size, which minimizes the moment arm of the eccentric loads. You may also get eccentric column loads in the erection process that are greater than snow loads.

A common tank roof shape is just called "cone roof" or "low cone roof". On steel tanks, these would most often fall under the "flat roof" category, with slope less than 5 degrees.

 

[JAE]

I would at least study the effect of an unbalanced load, whether your applicable code requires it or not. In reality, unbalanced snow loads do occur on pitched roofs.

For your "cone" roof, you may have some form of unbalanced snow occurring, but with the conical shape, there may be more shedding of the snow than on a straight pitched roof condition. But the unbalanced snow load called for in ASCE 7 doesn't add a huge penalty to the design, just creates a load case with a heavier drift on the leeward sides.

 

[waytsh]

The slope on my roof is 3%. It looks to me like the only way ASCE would require unbalanced snow load for this slope would be under the category of a hip and gable. Does anyone know if the the ASCE Design Guide For Snow Loads addresses conical roofs?

 

[JStephen]

Here in Texas, the state currently requires a minimum 3/4"/foot slope on tank roofs. Make sure you have it steep enough to meet applicable codes & standards.

 

[gwt1]

Good point. I could not find anything in IBC, but AWWA did mention a minimum 2% roof slope so I think I am ok.