Uplift on Wood Roof Trusses 2

[abusementpark]

I've seen where some pre-engineered wood roof truss engineers will provide uplift reactions based on MWFRS pressures, and consequently, the hurricane ties at each truss support are sized based on MWFRS pressures.

What you do you guys think about that? I really wish ASCE 7 would further clarify the differences between MWFRS and C+C. I think this is one of those gray areas.

 

[OHIOMatt]

The commentary to ASCE7-05 list trusses a component. I really do not see any gray area. At most, a wood truss would see something like 200 sf of area (but most would be significantly less).

The hurricane ties, would have essentially 1/2 the area of the truss they support (I am simplifying, but this would not be too far from reality).

I would send the shops back and make them correct the trusses.

 

[JAE]

<700 s.f. per ASCE 7 is C&C.

Anything with larger tributary area is MWFRS.

 

[abusementpark]

What if the trusses collect load from more than one surface (i.e. a gable truss)? There are some people who think that is an important factor in MWFRS vs. C+C.

 

[JAE]

It is not - the wind values are based on statisitics relative to tributary area.

Wind pressure doesn't "know" whether the truss has one or twelve layers between it and the truss - the wind would be the same regardless of layers.

 

[OHIOMatt]

In the gable truss example, it will see wind in both the vertical and horizontal directions. For pressure in the horizontal direction, the individual members would need to be designed as components as well.

If you read the comentary to ASCE7-05, they give the example of a long span truss with the individual components designed based upon their tributary area. Then the chords, would be designed based upon the overall tributary area. If the area is large enough this may well be MWFRS>

 

[woodman88]

Typically, in Arizona and California, the pre-engineered wood trusses are designed after the building has been approved by the building department and are sent in as a deferred submittal. Per the IBC and IRC the truss engineer is only responsible for the truss to truss connections, not the truss to plate. So the building designer should have already calculated and detailed for the C&C uplift connections from the trusses in the building plans.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[abusementpark]

It is not - the wind values are based on statisitics relative to tributary area.

Read the wording in ASCE 7. There is something about MWFRS collecting load from more than one surface.

Wind pressure doesn't "know" whether the truss has one or twelve layers between it and the truss - the wind would be the same regardless of layers.

But it is affected by how long of a continuous surface it can attach to. With C+C loading, you are looking at peak pressures over a localized area. There is a difference if there are multiple surfaces within an area under consideration.

 

[abusementpark]

<700 s.f. per ASCE 7 is C&C.

Anything with larger tributary area is MWFRS.

So if I have a moment frame with <700 sf of tributary area, I need to design it for C+C loading? For example, for a 15' foot tall pre-engineered metal building with 20' frame spacing, each frame would have a maximum tributary area of 600 sf. You are going to make them design it for C+C loading?

 

[abusementpark]

Per the IBC and IRC the truss engineer is only responsible for the truss to truss connections, not the truss to plate. So the building designer should have already calculated and detailed for the C&C uplift connections from the trusses in the building plans.

A lot of engineers in my area make the truss engineer design the truss to plate connections.

 

[JAE]

abusementpark - the commentary in ASCE 7 does speak of multiple "layers" but setting aside the code for a second and thinking clearly of what wind actually "does" to a surface, the variation of wind across small areas provides a higher chance of peak loads for any kind of structural element (screw, girt, cladding, column, footing) with small areas. This is not a code thing or a layer thing. It is simply common sense and has been stated in numerous articles and publications.

The commentary is attempting to offer users a feel for elements which may receive load from multiple connections and "layers".

If you have a vertical column and footing that support only 100 s.f., I would use C&C wind load (roof uplift) on that column and footing, despite the fact that the column receives the wind load through roofing, insulation boards, screws attaching the boards, metal deck, joists, beams, bolts connecting beams, etc.

These layers do not PHYSICALLY change anything about what sort of wind pressure arrives at the top of the column.

For an element that is part of a MWFRS, you would design the brace system for MWFRS loads, AND, if the area on the brace is low, I would also design check for C&C as well. These are not mutually exclusive necessarily.

 

[spats]

Keep in mind that for C/C, effective area need not be considered to be less than L^2/3.

 

[woodman88]

abusementpark

Quote: "A lot of engineers in my area make the truss engineer design the truss to plate connections."

Are you sure about this?

I have heard of truss manufacturers typing on hangers and anchor clips onto sealed drawings without the truss engineer knowledge. And that others will mark up unsealed truss layout drawings with connections and send them out with the truss package.

Hopefully, the truss/layout drawings you are getting have not been changed like this.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[abusementpark]

I have heard of truss manufacturers typing on hangers and anchor clips onto sealed drawings without the truss engineer knowledge. And that others will mark up unsealed truss layout drawings with connections and send them out with the truss package.

Hopefully, the truss/layout drawings you are getting have not been changed like this.

I hope that hasn't been occurring. I usually spend time critiquing the selection of ties to make sure they meet the reported uplift loads and to make sure they are providing something that is compatible with the structure at special cases like heavy girder truss connections.

 

[a2mfk]

1. I have heard arguments for MWFRS of trusses but I have never been convinced, and agree they should be C and C as explained in detail by JAE- it is related to tributary area, and because an individual truss may be subject to localized zones of higher pressures (think of a jack truss at a hip). And from a common sense standpoint, WHO CARES?!! Use the higher pressure when in doubt, especially when it comes to roof trusses and connections, which have proven over and over again to be a VERY vulnerable part of a wood-framed structure when it comes to wind. The cost to the overall structure is negligible, and the gain of strength in an often weak area of the structure is advantageous. Pre-eng truss manufacturers will always try to supply the bare-a$$ minimum since they get the job based on an up-front bid.

2. Truss to structure connections should ALWAYS be designated by the EOR, we are the only ones with the overall structural knowledge of that particular building. Many people, including some SEs, neglect to design truss connections for the combined forces due to uplift:
>>> shear from the roof diaphragm to the wall (unless you transfer it via blocking)
>>> out-of-plane shear due to lateral forces on the wall (transfer mechanism for lateral forces from the wall into the roof diaphragm).

Simpson for example requires this check in their tech notes and provides an interaction formula. These are often listed as F1, F2, and F3 and diagrams are provided in the manual. I know, probably preaching to the choir....

 

[woodman88]

a2mfk - Until the C&C uplift reaction is greater than half the truss (live load plus dead load) design reaction, there can be no truss manufacturing (lumber or metal plates) cost increase in the truss design. Typically, the uplift would have to reach two thirds the design reaction before a minor increase in cost may occur. With hip trusses and small girder trusses due to the minimum plate sizes and standard lumber grades will typically have no cost increase again from C&C loading.
So why do the truss programs show the MWFRS uplifts? They do because the truss program C&C uplift is often very conservative and greater than the building requires. If the C&C uplift is sent to the building department they may require the building designer to not only increase the building holddowns (which is not required per the builder designer) but in some cases have wanted the building designer to redesign the building for this higher uplift. Rather than trying to make the truss program more accurate for C&C loading, the truss plate companies has allowed an option to show only the MWFRS reaction (the truss is still designed for the C&C loading) and leave the required C&C reaction and connection up to the building designer.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[abusementpark]

Use the higher pressure when in doubt, especially when it comes to roof trusses and connections, which have proven over and over again to be a VERY vulnerable part of a wood-framed structure when it comes to wind. The cost to the overall structure is negligible, and the gain of strength in an often weak area of the structure is advantageous.

The MWFRS vs. C+C decision has implications on more than just the truss to structure connections. What about top plates to stud, stud to bottom plate, bottom plate to anchors, etc.??? It is all part of the same load path with similar tributary areas. Do you design all that C+C loading?

If I understand JAE's interpretation, all this would be designed for C+C loading, simply by virtue of its tributary area, regardless of how far it is down the load path.

Truss to structure connections should ALWAYS be designated by the EOR, we are the only ones with the overall structural knowledge of that particular building. Many people, including some SEs, neglect to design truss connections for the combined forces due to uplift:
>>> shear from the roof diaphragm to the wall (unless you transfer it via blocking)
>>> out-of-plane shear due to lateral forces on the wall (transfer mechanism for lateral forces from the wall into the roof diaphragm).

What else does the truss engineer need to know?

With regard to other forces at that joint:
- We provide blocking whenever there are significant diaphragm shear forces to transfer. How else are you going to get the load down from the roof diaphragm to top of the wall without inducing bending about the truss to wall connection?
- We call for a few toenails at the truss to wall connection to account for the out-of-plane loading separately.

 

[a2mfk]

Abusement- I do very little wood framed design anymore, but when I did I would not recalc for wall design. Just use C and C. Its a pittance in the overall cost and the extra insurance is worth it. And guess what, nobody will ever know but you and whoever (if anyone) that reviews your calcs. This is a common discussion on structural message boards and to me its and academic one. I am not sure why someone would argue very strongly for MWFRS unless that is the way they have always done it and they want to justify their method. Economically its negligible in the cost of a structure...

Also most structures I have designed are in Florida where CMU is predominant, so wind uplift transfer is usually an afterthought. In a really high uplift situation that effects walls and foundations I would recalc for MWFRS.

Not everyone specifies blocking for shear transfer, but I usually do. Unless you have a small enough heel height, then there are connections that can transfer the roof diaph. shear load into the top of the wall.

And for out of plane I suppose toe nails would work, I just don't like them so I never specify them. And many times that out of plane force can be quite critical. Also, can't toe nail into a bond beam if its a CMU wall with no plate.

Different regions, different methodologies, as long as it is thought out and somebody uses engineering judgment... Feel like I am beating a dead horse here. Sorry.

 

[ron9876]

I agree with JAE. What would be the logic for using smaller than C&C loading for truss connections to walls. If both the wall and the truss are designed for C&C then why would the connection between the two be designed for MWFRS.

a2mfk I am in South Florida and I find that truss to wall connections typically require the heaviest connectors that suppliers produce. Before some of the heavier connectors came on the market (Simpson DETAL for instance) it would often require more than one connector per truss.

 

[dhengr]

A2mfk:
That’s not beating a dead horse. That’s just applying a little common sense, and engineering judgement, and realizing that the client probably wants the design done during your lifetime, and his. All this despite the current state of the building codes. Soon the wind loading design on a small building will take longer and involve more pages of calcs. than an entire warehouse building design did 20 years ago. And, surprise, surprise that warehouse is doing just fine after several significant wind events, it still has its roof, and doesn’t leak or lean too much.