Max. allowable load on building rooftops

[Qubed]

I need to determine the max. allowable load (in pounds per square feet) on rooftops of buildings for a project I am working on (involving heavy machinery). I need to determine this for various countries. I will start with the U.S (which may differ from state to state). Does anyone have any recommendations as to how I can get started with this? Should I look into documents similar to NBC (National Building Codes)? Thanks!

Soh

 

[cpiedra23]

max. allowable loads, considering gravity only, are normally split between dead (permanent) loads and live (occupancy) loads. for a roof, dead loads would be the weight of the roofing, insulation, attached ceiling, structural weight (of the deck and beams), and other loads. live loads for a roof would be for, say, someone performing roofing or maintenance on the roof.

the max allowable load (dead+live) on a roof depends on the structural design (the beams, deck, etc.), and really can only be said on a roof by roof basis. an engineer can't blanket all, say, heavy machinery buildings, and say that type of bldg has a max. allowable load on the roof of ___psf. one would have to calc out the numbers to see what the roof could take as a max. live loads on roofs are code mandated minimums (see linked table, from the 2012 International Building Code/2013 Calif. Building Code, Table 1607.1).

hope this helps.

 

[JStephen]

I don't think you'll find a good concise answer.
Roofs in general are designed for the anticipated snow loads, wind loads, live loads, etc. But that all varies depending on location and year built, and older buildings aren't required to meet current building codes.
That topic has come up in some other threads where people wanted to mount solar panels on the roofs, you might read through some of them.
And, you do have me curious as to what kind of "heavy machinery" you're planning on. Very few building roofs are designed to support bulldozers. I've seen some videos of excavators being used on the top of buildings for demolition (but I'm not sure if that was a roof or a floor)- regardless, someone either did a lot of extra analysis specific to that project, or it was just folly, hard to tell by watching until this happens:

https://www.youtube.com/watch?v=Wxe588uU1S0

 

[Ron]

Are you looking for post-construction installed machinery or will the accommodation of the machinery be designed into new buildings?

 

[Qubed]

cpiedra23, Jstephen, Ron, many thanks for your replies.

This mechanism will be installed on pre-existing buildings. It will contain a refrigeration cycle and weigh more than 2000kg (nothing at all like a bulldozer). It will be more similar to a solar panel. So in this case, it will be considered a 'live load'.

It appears as though the best approach would be to look into building code documents for different countries. And yes, it looks as though location and year built will be big factors.

 

[Ron]

You will also need to check the existing structure to make sure it can structurally support the additional machinery AND check deflections so that serviceability is not negatively affected. You need to consider where each will be put on the roof, relative to the primary structural components.

When you add load to a roof structure, both capacity and deflection are affected. If deflection occurs and impedes drainage on the roof, it can lead to overloading the structure and in some cases, structural collapse.

Most manufacturers of rooftop equipment provide the loads that the equipment will induce and then a LOCAL structural engineer must decide if it will work on the roof and where it can be located. For the equipment manufacturer to try and meet all potential codes and structural configurations is ludicrous.

 

[TLHS]

A 20kN load isn't small, especially if it's fairly concentrated. You're going to have to investigate and likely reinforce any roof you install it on that doesn't have some sort of design allowance for a machine room or something similar.

 

[hokie66]

Qubed,

I agree with the others that you are approaching this from the wrong angle. From a study of roof loading requirements in different countries, you will gain little information about how a piece of equipment can be supported on any given roof.

 

[racookpe1978]

Think from the bottom up: Most of the weight (structure weight) of the building is, well, holding up the building above the member itself. Plus, as mentioned the dead load (constant) and live load (changing load). Then the walls support the next level, that level supports the next higher, then the upper walls are "just" strong enough to hold the roof structural and the roof waterproofing (tiles or shingles or sheetmetal) and the "known" live loads (snow, ice, wind, etc.)

But 2000 kg (4400 lbs) is large. And live loads are by definition "unknown" margins against likely failure due to unforeseen causes - you cannot say "Because I have put a 2000 kilo load on my roof, I will now longer have any ice storms in my country." Or, "Because i have a 2000 new kilo load on my roof, it will only snow 1 meter the next 30 years."

There will be very, very few designers who deliberately over-build their structural roof supports "just in case" somebody wants to add an extra 2000 kilo load. Far-sighted engineers of commercial or fabrication plants who need the stronger roof and upper beams to hold piping? Maybe.

 

[chicopee]

If you can get the original drawings, then you may have a shot at determining roof loading. Your best approach is to hire a structural engineer for these projects.

 

[dhengr]

Qubed:
2000kg (4400 lbs.) is one hell of a bunch of solar panels, which would be spread over a much larger area than your equipment at a rate of 5 or 10 lbs./sq.ft. You will see hundreds of variations in the exact structural design of the roofs you are dealing with, and you will have to look at each one to see if there is any excess cap’y. there to carry your load. Generally speaking, you can not just plunk this 4400 lb. load down on a 10'x10' sq. area on just any roof and expect it to work. You would do will to design your equipment with a base structure (a skid, four beams, two beams in two different directions, or some such) which will span 20 or 25' so that you can support it directly over the bldg. columns. And, even then, you will have to design to get these concentrated loads through the roof structure and down into the columns, which may or may not have the extra cap’y.