Wood Truss Connection Question

[structuresguy]

I am wondering what everyone uses when designing the connection of wood trusses to the main structure: Component & Cladding wind pressure or Main Wind Force wind pressures?

My personal interpretation is that the top chord of the truss would be designed for C&C loads, but the bottom chord and webs would be designed for MWFRS loads. I came to this conclusion after reading the commentary in ASCE7-98. It would then follow that the connection of the truss would use MWFRS loads, which results is a large difference for uplift loading. I live and work in Florida, so this is critical for me. Sometimes, it is very difficult to make a connection work using C&C loads. This was always a source of discussion amongst my co-worker, so we used C&C loads typically.

However, I recently discovered that the Wood Truss Council of America has taken the position that truss reactions be based on MWFRS, as well as the design of webs and bottom chords. So they are interpretting the ASCE in the same manner as I am.

Any thoughts, or code requirements, which you may contribute are greatly appreciated. Thanks in advance.

 

[UcfSE]

Try a search on this topic. It has been discussed many times.

I use C&C for uplift and lateral out-of-plane, MWFRS for lateral in-plane, and MWFRS for uplift combined with lateral in-plane. If the tributary area is large enough I use MWFRS for everything.

 

[JAE]

I concur with UcfSE -

Technically speaking, for ANY connection or member, they must be designed for BOTH MWFRS and C&C. This I know sounds confusing but know that each of the two (MWFRS and CC) are based on the same wind criteria, but are different in magnitude based on the tributary area of surface that is providing the wind.

Both must be checked for the applicable wind load combinations but only one will control the design. As the tributary area for a structural element increases (whether its a nail, a hanger, a truss member, a truss, or a shearwall) the total applied wind will statistically grow smaller.

As wind is applied to a structure, there a wide variations of pressure that occur. Its sort of like your stereo EQ bars that dance up and down as your music plays. These little spikes and valleys of wind pressure are higher in some area (building corners) and lower in others (fields of large walls) and so the pressures in ASCE 7 are adjusted accordingly.

But if you have a very small trib area - there is a much higher statistical chance that the WHOLE tributary area will have a high spiked pressure at any one time than if you had a large tributary area where the valleys help temper the spikes.

For a truss:
The overall truss has a set tributary area so it would first be checked for C&C loading (for uplift, connection to the building, etc.) as a "component" of the structure.

Now if the truss is set on top of a shearwall, and serves as an element on the lateral main wind force load path, then it also needs to be checked for the condition of MWFRS wind as a collector.

Its not either/or - it is AND/BOTH.

 

[DRsquare]

I know this has been discussed many times, and I have read all of those discussions I could find. I agree with the same method as JAE and UcfSE. I often use buildings of all heights method because many residential buildings aren't rectangular. In those cases I also look at Case 3 shown in Figure 6.9, as per Section 6.5.12.3. This considers the forces as if the wind is coming both directions and sums 75% of the forces from both directions.

I've never seen Case 3 mentioned in these threads (that I recall). Is this appropriate? Do others look at this? If not why not, or when should it be used? Often this comes out to be similar to C&C loads.

Take pity on me, I'm one of those worthless mechanical engineers who can't even design brake drums.