Maintenace Roof Access

[goMSUfootball]

I am trying to define a safe loading for Maintenance Roof Access to a domed tank.
API 650 3.10.2.1 states that all roofs will have a minimum roof live load of 25 PSF.
IBC also has a minimum roof live load of 24 PSF for Maintenace Access of roofs.
ASCE 7 will allow for a corrleation between uniform loads and a concentrated load by multiplying the uniform load by a contributing area of 2.5'x2.5'. This would result in a concentrated load of 156 pounds.
Does this mean I have to find a leprechaun to send up on the tank roof for "maintenace access"?
Really though - Does the 25 PSF uniform load limit acess to 1 person?
Some work would require 2 guys to change motor parts on an agitator.

 

[JedClampett]

Almost all roofs, maintenance or not, are designed for 20 psf. There is a concentrated load of 300 lb for areas subject to workers, which I've never seen used.
I guess the inherent conservatism of designing a whole roof for 20 psf when there is only occasionally 2 workers on it, creates the acceptable design. As far as the ASCE conversion, I think it's very general. If you had workers shoulder to shoulder, maybe, but these guys (or gals) tend to give each other more than a couple of feet daylight.
But if you don't feel comfortable with the 20 psf (or 24 psf) loading, design for two morbidly obese, 600 lb., workers. There's nothing saying you can't design for more.

 

[goMSUfootball]

Where did you get the 300 lbs? It would be interesting if that is for an entire roof. One of the codes had broken it further (IBC?) by working in a 200 sqft reference area.

This is for an existing self supporting tank roof. I'm really concerned the uniform loading is based off a 175 lb person. I'm pretty sure my millwrights bench that with one arm.

I've seen break downs where snow load is developed. I am really looking for something to see how 25 PSF has been determined to be a safe uniform load for "maintenance access". How did the code developers average a steel worker, millwright and electrician getting access to a roof within 25 PSF when other planned accesses such as industrial catwalk is 100 psf.

sorry for the rambling response terying to put toddler to bed at same time.

 

[JStephen]

I think the current API 650 specifies 20 psf roof live load but also requires 40% of 1" of water column vacuum.

As to your question, the formulas used for designing self-supporting roofs are based on uniform loading and corresponding buckling. If they are not corroded, they should be adequate for several hefty people to walk around on. That's how they get built and painted, after all. I don't remember if the current API-650 specifies a concentrated roof load or not- if so, it's not included in the design formulas for self-supporting roofs, but presumably, the roofs designed by those formulas are adequate for it. AWWA D100 specifies a 500 lbs concentrated roof load. It never controls unless you have very small rafters. Usually, self-supporting roofs are hoisted into place, and the lug loads are far higher local loads over a smaller area than personnel will ever generate.

Note that when you talk about a concentrated load on a self-supporting roof, the failure mode is going to be a local dent in the roof, not falling through the roof or the whole roof collapsing. You could possibly evaluate the roof using the formulas for local loading on vessel walls. But this is just normally not considered at all.

 

[JedClampett]

The 300 lb. concentrated load is in ASCE 7-05, Table 4-1.

 

[goMSUfootball]

Thanks for the help guys.

I understand the tank will in reality support several hefty guys. I've been up on them myself.
I'm looking for the reasoning behind the code so I can cite it in the development of a safe load practice for our maintenance guys.

I think I'll cite both concentrated load references and take the lower of the two for good measure.

 

[goMSUfootball]

I also found this in a meeting notes post for an update to API 650 in the future. It's for aluminum roofs, but It's the kind of explanation I've been looking for.

G.4.2.5.2 Concentrated Load The roof shall
be designed to support two concentrated
loads 1100 N (250 lbf), each distributed over
two separate 0.1 m2 (1 ft2) areas of any
panel

My ASCE 7 is from 98 and they refer to roof formulas without giving the concentrated load.

 

[MiketheEngineer]

Don't forget guard rails - if required