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<5% Gravity Alteration

<5% Gravity Alteration

<5% Gravity Alteration

(OP)
Hello,

Quick question I have my thoughts about, but got to thinking and wanted to bounce it off the community...

Currently analyzing the addition of some dead load to a roofing system. Want to make use of the <5% increase in demand to make this easy for everyone.

My question...do I compare the new dead load to the overall load (dead, live roof), or only dead to dead...I can see how you could argue both ways in various circumstances...

If we have a system designed for minimal dead but a ton of live or roof live, then a minimal dead load addition would be fine, but maybe not <5% if you only compare dead to dead. On the other hand, if you compare the new dead to everything else, where do you stop (dead, roof live, snow...I personally wouldn't use snow/rain/etc in my capacity)? Thoughts?


Thanks!

RE: <5% Gravity Alteration

I've always looked at the controlling LC to determine the % increase. IBC Section 3403.3 states that any gravity load increase of more than 5% triggers analysis/replacement/strengthening. Why wouldn't you use snow? I have used snow multiple times when evaluating rooftop equipment alterations.

RE: <5% Gravity Alteration

(OP)
and this is where I start thinking too much...normally we would look for the largest load when designing as the "controlling case", but in this particular situation where I am going to compare 5%, I guess one could argue that I should take the smallest load resulting out of the LC's (which would be dead alone in my case) as the "governing case" since the smallest load generated would be the most conservative...

At the end of the day, if I compare to Dead+Roof Live I am fine, but if the city rejects me and I can only compare dead to dead...well...no bueno...

RE: <5% Gravity Alteration

Why couldn't you use the largest load combination? That's what the member had to be designed for in the first place. If you double the dead load in the dead load only case but only increase the load in the controlling case 5%, you're only increasing the member utilization 5%.

RE: <5% Gravity Alteration

(OP)
Ah, well I suppose there is a bit more I didn't describe for this specific instance.

I do agree that if I knew what loads were applied, etc, I would use the largest since that is what the system was designed for. But, alas, was designed and built in the 70's; were aren't sure what the original loading was so I am trying to be very conservative and use as little load as possible for the comparison.

Here is more detail:

Have a truss system in a mall atrium. Owner wants to hang a ridiculously large LED billboard type of sign from the truss system. I have no idea what loads were originally used, so I am trying to use only the bare minimum that will still keep me under the 5% so I can avoid analyzing this roofing system (it's more complicated than just "a truss" as I have let on).

RE: <5% Gravity Alteration

In my professional opinion, your system should probably be analyzed regardless if you can stay within the 5% provision. A "ridiculously large LED billboard" could weigh a significant amount. Because the point loads from the system are more localized than the distributed rooftop loads, it is fully likely that a member that passed for the distributed load could be significantly overstressed from the point load. I don't know what the loads of the billboard are, but I would have a hard time trading localized BC point loads for TC distributed loads.

RE: <5% Gravity Alteration

I think something that is often neglected here is that an increase in DL to the roof has a larger effect on seismic force distribution than it does for vertical loads. a 5% increase to DL at the roof could jump the seismic load at the roof more than that - and increase forces in the LFRS more than the 5-10% limit per the code. If the building is close to a square, and in a low seismic region, it may not matter. It may be an issue for long/skinny buildings though.

RE: <5% Gravity Alteration

Yeah I wouldn't apply the 5% here, unless you're adding the billboard at a point where a point load 20x the load of the billboard exists. Might feel justified looking at the effect of the point load and saying the system is okay if it utilizes less than 5% of the member capacity.

RE: <5% Gravity Alteration

My view is that you would look at all applicable load combinations and determine what the controlling gravity load combination was. Per the code language, this is the "design gravity load".
Then look at your new load condition, with that combination, and verify that the total load isn't more than 5% over.

For uniform loads, this can be done by comparing the loads.

For your case, with the new concentrated load you can't just look at loads.
You'd have to look at comparisons of shears and moments to see that you haven't increased them more than 5%.

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RE: <5% Gravity Alteration

some thoughts:
1. as i understand it, what your referencing says a 5% increase in forces due to gravity loads, not simply a 5% increase in loads.
2. agreeing with others above, worst case seems appropriate, and others observing local vs global conditions.
3. " in a mall atrium. Owner wants to hang a ridiculously large ..." are there people below it?, be careful.
4. "a ridiculously large LED billboard type of sign from the truss system...." sounds like significant point loads on a truss, which "I have no idea what loads were originally used". Suppose the trusses were originally designed for a nice uniform load. It's not unreasonable to anticipate that, although your atrium truss' overall moment capacity and shear capacity remain within our magical 5% envelope, nevertheless, local webs and local web connections may experience design forces exceeding their original design by 100% or 50% or 500%.
5. above was math, now some philosophy. recently looked at a retail store switching out 3 RTUs, "like for like" as the contractors like to say. total weight change was from about 1250 lbs to 1350 lbs each. problem was, roof designed originally for only 2 RTUs at 1250 each. someone had installed a 3rd sometime back when, unengineered and unpermitted. The replacement of the 3rd RTU could have been justified in the present based upon the 5% related to 'existing' 1250 lbs, but not 5% related to 'original' 0 lbs.
6. 5% increase ok, but what about when the allowables are less today than we thought back then, like bottom chord tension in a bowstring truss? by the way, what material is your truss made of? Then there was the in-grade lumber testing propgram that adjusted lumber allowables again.
7. a steel truss roof was just installed but unoccupied when the owner wanted to add some RTUs. The design engineer, who had exact knowledge of design criteria, wanted to use the "5% rule". Somehow I think that this was not the intent of the 5%, but rather perhaps to give engineer's some room to exercise sound engineering judgment for buildings whose past is shrouded.

RE: <5% Gravity Alteration

Here's what I'd recommend for a simple span roof truss with snow loads dominating:

1) hang from the truss panel points.

2) make a reasonable estimate of your loads.

3) pick a sensible load case for comparison. Probably just 1.25D + 1.50L.

4) work out your truss shear and bending diagrams.

5) work out your truss shear and moment diagrams with the new load included.

6) Ensure that your shear and moment diagrams have nowhere grown by more than 5%

The trouble with a truss will be, if your new loads occur near med span, your original shear diagram may take on a rather low values there which could neuter the efficacy of the 5% method in that area.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: <5% Gravity Alteration

(OP)
Thanks all for the input. I sincerely appreciate it.

If you are curious still, in this instance, the truss span is 80'. It is actually two trusses side-by-side that are low...Altogether, we have roof trusses spanning 80' @ 10' o.c. bearing on two lower 80' trusses that run orthogonal.

All members are 5-1/8" x 21" glulams, with joints every 10'. The sign will hang from these two glulams (one each truss) at midspan of the 80', but will mount near the joints, so the induced moment and shear on the chord it is actually hanging from will be very small - all of the load will (almost) be imparted to the nodes, so that's good news...

Anyway, thanks for the insight here! I love this place.

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