Uninhabitable Attics 3

[jike]

In ASCE 7-98 and IBC 2000 under Residential, it gives loads of 10 psf and 20 psf for unihabitable attics without and with staorage, respectively. A wood truss supplier told me that this is a non-concurrent loading, in other words he does not have to add it to roof live load. I see no reference stating this in the codes.

Am I missing something or is this a mis-interpretation on his part?

 

[SperlingPE]

I believe this load would hbe added to the bottom chord (colatteral bottom chord live load). Should this load be concurrent with the roof live load? Yes, I believe it should.

 

[wiktor]

This is definitely a misinterpretation by supplier (I guess that he has typical trusses designed for roof load, and he doesn't like to alter his/her calcs and design. The loads are concurrent.

 

[AlohaBob]

The truss supplier spoke correctly. The 10 psf is for design of the bottom chord only. Basically, it provides for a load for bending between panel points. There's also a prescription for a point load anywhere along the chord, so Joe homeowner can walk on the truss chords to reach his box of ornaments.

Otherwise, he must design the truss for a live load of 20psf basic roof live load, or a snow load - should that govern.

That's not to say you can't design for a greater load. If the owner knows he may wish to store more than christmas ornaments in the attic, you can specify a greater than minimum design bottom chord load and applied concurrent with the roof live load.

The truss manufacturers don't normally use a higher load than code prescription since their business is so competitive.

But being a structural engineer, I look for the opportunity to reinforce trusses to lift M-1 tanks to change their tracks in the garage. Most people don't have such an appreciation of reinforced trusses.

 

[jike]

I just had an email from our State Plan Review Dept. (Wisconsin) and he confirmed that the loads should be combined with the roof live load per the IBC.

I believe there may be other states interpreting this requirement like AlohaBob stated.

Frankly, I cannot see anything in IBC that allows this interpretation unless the state has modified that particular requirement.

 

[AlohaBob]

You got me going. I had to research this years ago.

I checked the IBC ....

Uninhabitable Attics without 10
storage ( My book has a misprint)

Scuttles and Accessible ceilings 200 lbs

Load combinations 1605.3 Formula 16-9 appear to make snow and roof live load exculsive of each other.

Exclusion note of the UBC not here any more. I wonder if it will come back??


I checked old UBC's

'88 Table 23-B Ceiling Live Load 10 footnote 5 Need not be considered with other live loads....

'94 Table 16-B Similar

 

[jike]

Formula 16-9 was exactly what I was looking for!

Thanks, AlohaBob!

 

[LPPE]

I respectfully disagree.
Equation 16-9 from 1605.3 states (at least in my copy)-

D+L+(Lr or S or R)


D is Dead Load
L is Live Load
Lr is Roof Live Load
S is Snow
R is Rain.

Now, AlohaBob is correct that Roof Live Load and Snow are exclusive of each other. However, I would say that the attic load is a live load, not a ROOF live load. It is applied on the attic floor, which as far as I understand, is not a roof. Therefore, I would say you should DEFINATELY combine your attic load on the bottom chord with your Roof Live Load on the top chord.

 

[CSEllc]

LPPE you are correct.

The 10 psf for non-habitable attics without storage (or 20 psf for non-habitable attics with storage)is applied to the bottom chord as a live load.

The top chord of the truss is still designed for the ROOF live load (typically snow in the NE).

Look at trusses as "pre-made" rafters and ceiling joists. Wouldn't you design the ceiling joists for the 10psf (or 20psf) and the rafters for the ROOF live load. Well now...the top chord is the rafter, the ceiling joist is the bottom chord.

You include both loads in the design.

Now that we have that straight.....I really doesn't matter. If on your structural drawings you specify to include both loads, then the truss supplier should include both loads in the design. There is nothing in the code that says you can not exceed the minimumns.

AlohaBob, I disagree with you comment regarding truss suppliers not using higher loads than the code requires since the industry is very competetive. Shouldn't WE,as structural engineers who are ultimately responsible for the project, be TELLING the truss supplier what to design the trusses for. The DESIGN LOADS FOR should be clearly indicated on our drawings. Every supplier would (should) be providing a similar bid based on the specified loading. If they don't look at the drawings for the loadings, and use the "minimumns" to bid the project and underbid the project that is their problem.

On many projects, the truss supplier submited copies of their shop drawings with different loads than I specified. I return with drawings a big rejected note across them and tell them to look at the drawings for the specified loads. They always resubmit with the corrected loadings.

 

[DaveAtkins]

IF one requests that roof trusses be designed for BOTH snow load or live load on the top chord simultaneous with attic load on the bottom chord, at least use the 0.75 multiplier on the live loads, as permitted by Code. I still think a bottom chord will never see anything close to 10 psf, if it is an uninhabitable attic. The truss webs are too close together, and there is no flooring to allow for storage. And the occasional person walking in the attic won't be anywhere close to that load, either. I am okay with interpreting the Code liberally, and saying attic load is a roof live load.

DaveAtkins

 

[CSEllc]

Dave,

Who said the attic load is a ROOF Live load?

I know that I didn't say that. LPPE didn't say that. I don't think Aloha Bob said it was a roof live load either.

 

[AlohaBob]

Yes. No. Yes. No.

I interpret the code to say the bottom chord must be designed for bending 10 psf or the point load between panel points.

The code basic minimum roof live load is 20 psf. It seems reasonable to me that you choose how to apply that. 10 psf bottom chord + 10 top chord. Then 20 applied to the top chord. So the top chord would be designed for greater bending than the bottom. Maybe the 10/10 is a condition the roof might see after construction, but during construction the 20 on the top chord is most appropriate.

I believe the live load in the attic is a roof live load and not required to be applied simultaneously with the design snow load.

I also think there should be plenty of discretion here. I don't think the truss manufacturer must design for something beyond explicit loads in the code. I think that creates an uneven playing field in this competitive market. And I see reason for a specific owner to request greater design criteria if he knows he wishes to load up the attic.

People want things cheap or free. Everything has a cost however and there's a limit to how far you can lean out a structural system to save on cost. So unless a person specifically designs their own roof to carry an excess storage load, bet that that space was never designed to carry it and the cost to make it so is something that will cost extra.

It would be real nice if people opt to spend their extra dollars in my back yard, but so long as a minimum threshold of safety is maintained, I can't fault someone for deciding to spend their money elsewhere. And I do what I can to honestly interpret a minimum standard.

I think everyone here has valid points. And I agree whatever the minimum explicit loads and how they are to be applied need to be published on the design drawings so competitive bids are for the same required work and the homeowner have it in writing exactly what he is paying for.

 

[ronster]

The 20 psf minimum roof live load is applied to the top chord and is independent of the bottom chord load. If the roof trusses do not have strip of plywood applied to the top of the bottom chord and access for storage is not provided then the 10psf live load applies(this is not a roof live load and not part of the 20psf minimum).

 

[AlohaBob]

It seems the dispute is over whether the bottom chord is required by code to be designed for 10 psf storage in addition to 20 psf live load.

For the sake of this discussion, I read the commentary. 1607.11.2 Minimum Roof Live Loads

There it states: ....the minimum roof live loads typically occur during roof maintenance, construction or repair.

Then looking at Table 1607.1 If the attic space is uninhabitable without storage, use 10 psf load. So that applies to the bottom chord for who knows what purpose? But that chord or ceiling joist must be designed to carry that minimum load.

Now move from the attic to the whole truss. Now the loads prescribed have a purpose - maintenance, construction, or repair.

Unless otherwise specifically stated for that purpose, the ceiling space for the truss is not designed for storage. And if it is specifically to be designed for storage, then it is to be designed for a minimum of 20 psf. And if that were how it is to be designed, then it would include being in addition to the roof live load of 20 psf for the purpose of maintenance, construction or repair, if it is part of a truss since this is an ongoing design load criteria which must be considered. The roof will need to be repaired even while there's stuff up there.

So what's the minimum standard? I think we have one without penalizing anyone unnecessarily. If someone wants a storage space, declare it, and pay the extra cost for it understanding that the code does not require it to be provided as a minimum roof function. If the space is designed for storage, then do so safely and without need to worry about crashing through the ceiling. If it's not designed that way, be careful, you take a risk.

 

[AlohaBob]

I'm from Wisconsin.

The code reviewers probably write the code as well as interpret it. But what do they really know?

Now you can tell I've been here long enough.

At any rate, if the code reviewer says combine a 10 psf bottom chord and a 20 psf top chord live load, that's what you got to do. I doubt they all say this though. I have practiced here just several years. And I never specialized in residential truss design.

I didn't study the Old Wisconsin Code for this specific issue. Maybe the reviewers interpretation has some bearing there.