Components and Cladding 3

[jike]

ASCE 7-98 requires that wood roof trusses be designed for C&C loading with Zones 1, 2 and 3 and overhangs. It seems impractical for the EOR to list all the loads for the truss designers because of the various effective areas for different trusses. About the best that we can do is to reference that the truss designer needs to comply with the C&C requirements of ASCE 7-98. Additionally, our drawings must list: Basic Wind Speed, Importance Factor, Exposure Factor and Internal Pressure Coefficient and minimum dead load (to determine net uplift).

Does anyone else specify C&C loads differently?

Does the typical truss designer know how to handle this loading or is it done automatically for him? Does the typical truss program take these C&C loads into account?

 

[JedClampett]

I recently ran into the same problem with a light gage steel truss roof. I added a sketch defining and dimensioning zones 1, 2 and 3 with overhangs. Next to it I added a little table with upward and downward wind pressures. I realize this is somewhat of a bailout, but I really didn't know any other way to make sure the design was done for the loads as I understand them.
Also, the wind pressures vary as tributary area changes. I gave the maximum. I'd rather have a design for the highest wind loads than have a mistake on the low side. How can you clearly explain that a truss with a 50 sq. ft. tributary area has more load than one with 100 sq. ft?

 

[TFL]

I recently ran into a problem with a recent design were the person doing the design really did not understand components and cladding.

We are transitioning to IBC in a couple months and i have noticed that IBC requires you to list the components and cladding loads you want the structure designed for, if the design is being done by a different engineer.

Next time I think I would do exactly as Jedclampett suggests and list pressures in the notes based on zones and delineate the zones either in a note or on plan

 

[jike]

Wouldn't the same type of analysis be required of the steel joist manufacturers? Granted, most of the joist roofs are flat, but they still must comply with Figures 6-5c, 6-7a or 6-8. Does anyone know how they handle C&C loads?

We should expect no less engineering from the wood truss and light gage metal truss designers.

 

[boo1]

Consider adding if the structure is designed as partially or totally enclosed.

 

[JAE]

We do what JedClampett suggests - developing roof key plans that outline various areas/zones on the roof.

We do this for steel joists, pre-engineered steel roof trusses and wood roof trusses.

The trib. area issue is a bugger but we simply identify the components that we are dealing with and their general areas and provide loads with respect to the worst case.

You can also include at least a short breakdown on your diagram such as: Area A C&C uplift pressure = x psf for trusses spanning < 12 feet, uplift pressure = y psf for trusses spanning 12 feet or more. This at least breaks it down into two designs - but you need to review the members that apply to this C&C table and act accordingly.

But keep it simple - complexity breeds mistakes by others.

 

[Lutfi]

I have been adding an isometric sketch of the structure showing all roof and wall zones. I also add a table with C&C loads for various areas (10,20, 50 and 100 and over 100) square feet.

I do show the "a" dimension and list it on the plans as well. Besides all of this, I state the wind speed, exposure, importance factor and building category.

I think this is a good practice. Architects like it because it gives the C&C pressures for windows, doors etc.

By the way, read the ASCE guide (Q&A section) for treatment of trusses as MWFRS and C&C.

Regards

 

[kvillebasser]

We have had trouble with C&C loading on wood roof trusses.

I would recommend that you pay close attention to the notes in the truss shop drawings. We've have come to realize that the truss manufacturers in this area are using C&C loadings to design the chords and webs, but are using MWFRS loadings to develope the uplift anchorage forces.

While I know the at the roof system is part of the overall lateral stability of the building, as it acts as a diaphragm, its my opinion that the uplift anchorage should also conform to the C&C loadings.

We ended up adding an additional set of notes to the "load" portion of our truss design criteria notes such that the truss manufacturers would have to determine uplift forces per C&C requirements.

What gets me is that the cost of uplift anchors is very very small, yet in my conversations with the truss manufacturers, they state that they're trying to save a dollar on anchorage requirements. On a branch bank for example, we might have 100 trusses.....if we provide Simpson H2.5 anchors they cost about 33 cents each....but if we provide Simpson H2.5A's or H10's, we might add 50 cents an anchor.......or add ($0.50)(100)(2) = $100.00
Seems like a "nickle-dime" issue.....

 

[JAE]

For very large trusses, the MWFRS winds (or "primary") winds may actually govern as the At = 1000 sf limit kicks you out of the C&C winds and allows you to use the primary.

But this is for very large At trusses (like a 200' span @ 5' spacing.)

 

[Lutfi]

kvillebasser, which part of the country do you live?

I agree with you assessment regarding the anchors. I require that the truss anchors be designed to resist the following reactions/loads:

1. The uplift reaction from the truss
2. The load parallel to the wall/support
3. The load perpendicular to the wall/support

Often times, I get these anchors selected purely based on uplift by the truss supplier and or contractor. They totally disregard my cross sections and notes. It is always struggle! My point is the anchors are so cheap compared to other components.

I also like to point out that many truss fabricators employ technicians/drafters who will do all of the input and modeling. They also read and interpret the plans and load requirements. The software dumps out “what ever” and some moonlighting engineers will sign and seal these plans and calculations for them. I often get the wrong wood species that I specifically requested. I get wind loads based on SBC not ASCE 7-98 that I asked for. Even at times I do get the wrong wind speed. When I call to talk with the engineers, they are not there. All I get is a salesman who tries to BS me. Fellows, do your job and please review the submittals because it is your professional duty as EOR and you it to the owner/public.

kvillebasser point is well taken.

 

[TFL]

I CAN'T EVEN GET THE TRUSS SUPPLIER TO DESIGN THE HOLDDOWNS. THEY REFUSE TO DO IT IN THE NORTHEAST