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Roof Top Unit Point Loads

Roof Top Unit Point Loads

Roof Top Unit Point Loads

(OP)
Some manufactures do not provide corner point loads for roof top units or sometimes this information is not available and only the total weight of the unit is known. When it is know that the center of gravity is not dead center what would be a good factor to increase the point loads by? For example 1000 lbs/4*1.35=337.50 lbs. Is 1.35 to conservative? Thanks!

RE: Roof Top Unit Point Loads

You're not building a watch...total load over number of supports should suffice.

RE: Roof Top Unit Point Loads

Somewhere in the IBC, there is a statement about a 30% increase. Once you do this, no need to worry about corner point loads.

Sometimes the 30% increase becomes outrageous that I don't factor it in if the plans reviewer will go along. I use 2/3 of the total weight on 1 side. I was told way back that the motor side would be about 2/3 of the weight of the unit. I use that as my worst case.
 

RE: Roof Top Unit Point Loads

Heh...somewhere between what Ron says and RacingAZ says is the truth.  I would bet on it.   smile

 

RE: Roof Top Unit Point Loads

A 30% increase puts 65% on one side.  2/3 will put 67% on one side.  Same thing.

I agree with Ron.  If 87.5 pounds makes a difference, there are other problems.

RE: Roof Top Unit Point Loads

For the last 48 years I have used about 60% (not that far from the 65% suggested in some of the earlier replies)on one side. This is what my mentor told me to do back then, for equipment supports. I suppose though that if you have the centre of gravity of the load in both directions, from the manufacturer, of the operating weight of the unit, you could design for that. I cannot think of why not...can you? Of course if there is a Code requirement, then you should abode by that unless there are sound engineering reasons not to.

RE: Roof Top Unit Point Loads

If you have a four rail curb, then based on the location of the CG of the unit in the horizontal plane, and using two-way span coefficients for a two way slab, compute the line loads to the four rails of the curb and go with that.  

If you only have two rails, it's even simpler, it's just a simple beam reaction analysis.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Roof Top Unit Point Loads

I increase the weight by 20% per ASCE7 4.7.2.

Also, when you apply the 1.9 factor to the wind loads on the unit (ASCE7 6.5.15.1)- you will get some overturning that needs to be accounted for.

RE: Roof Top Unit Point Loads

Hawkaz, that 20% is for impact.  That doesn't address the distribution of the weight among the support points.

RE: Roof Top Unit Point Loads

For RTUs its common to have one end heavier then the other, specially the end with compressor. So I have always used a factor of 1.5 when I have only the total weight of RTU.

RE: Roof Top Unit Point Loads

BTW the 1.5 factor that I indicated is actually not that conservative. I remember using that factor for an RTU and later on the contractor provided a more detailed load layout of the unit, and the actual loads at one side of rtu came out to be little bit higher then what I had previously calculated.

RE: Roof Top Unit Point Loads

If it is very heavy and will effect the system because it is near the max allowable loading, then you either need to get more info from the supplier.  If your system has sufficient margin, then take it as evenly distributed over 4 corners.

If you can justify that there is sufficient margin to make a slightly unconservative assumtion, then you are okay.  If you cannot do so, then you need to get more info.

Like Ron said, you are not building a watch.   

RE: Roof Top Unit Point Loads

Pounds are for aeronautical and mechanical engineers, I don't break a sweat until 1 kip :)

So I agree with Ron.

Only if you are checking existing roof joists would I get this accurate, and I have been there before. If its new, use KCS joists and a conservative load, or if its a WF beam assume 1 kip and keep chugging. The MEP may change the damn thing 4 times before you go to 100% CDs anyway so I overestimate MEP loads, saves you a lot of work and hassle!

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