Lateral Wind Loads - Residential Construction

[FESI]

Pertaining to the connection between the top of wall and roof structure - mainly between wood or concrete masonry walls and pre-engineered roof trusses - I have noticed that the emphasis has been made on uplift when it comes to proprietary connectors. It seems that most connectors such as "twist" straps and many truss girder connectors are not rated for lateral loads. In addition, the proprietary connectors that are rated for lateral loads typically do not have near the uplift capacity as other connectors. When a connection is evaluated for a load combination (uplift, lateral parallel and perpendicular to wall) I have found that many connectors are very dificult to justify especially in high wind speed areas.

My question is: What are other engineers doing? I know that if I specify two connectors on one truss to resist uplift and lateral loads, the contractor never seems to call and say "thanks".

 

[Taro]

I might not understand your question, but isn't it customary to add blocking between the trusses to transfer lateral loads from the roof to the wall?

 

[FESI]

Yes, that is typically part of the roof diaphram, but there must be some connection between the bottom of truss and top of wall to transfer the loads.

 

[DaveAtkins]

I design the hurricane ties for uplift only, and don't worry about lateral loads parallel or perpendicular to the wall (not a good answer, I know).

I believe most of the time each truss is toenailed to the top plate before the hurricane ties are installed. That should help or even take care of lateral loads perpendicular to the wall. And as mentioned above, blocking will transfer loads parallel to the wall.

And I take solace in the fact that most roof failures in design wind events are due to uplift, not the other load conditions.

DaveAtkins

 

[FESI]

In my opinion, wood blocking is part of the roof diaphram and does not directly transfer lateral loads to the top of wall unless said blocking is somehow attached to the wood truss and wall below and in my experience, this blocking is only provided between the trusses and not connected to the wall below. The only connection that I have seen typically used in residential construction is proprietary connectors attaching trusses to the top of wall - and of course the toe-nail method

BTW - I have never myself or heard of anyone pre-drilling a hole through a truss to install a toe-nail. I have split many a rafter and trusses (younger years) nailing toe-nails. I have also been in search for hours on end for the elusive box of toe-nails and of course the "sky-hook".

I have noticed that some engineers in South Florida (high end residential) have specified a steel angle and steel plate welded assembly that is attached to a cast-in-place concrete beam with expansion anchors.

Has anyone noticed this practice?

 

[JAE]

FESI - you seem to be dismissing the concept of nailing the blocking to the top of the wall and then connecting the diaphragm to the top of the blocking. This is what we specify always for wood construction for the lateral forces. The ties are simply used for uplift per DaveAtkins.

Your statement "unless said blocking is somehow attached to the wood truss and wall below" ... I would say that trying to transfer lateral shear from the roof sheathing through the truss isn't a good idea. The trusses aren't designed to take this lateral load and as such, the blocking is the only competent mechanism to transfer the force.

 

[FESI]

JAE
An Example - A 12ft high wall in a 140mph wind zone - exposure C. Wood and concrete masonry walls are typically desinged as simply supported and therefore there is a lateral load being transferred at the top of wall through the connection to the truss through the diaphram and ultimately to the shear walls to the foundation.

Shear forces are being transferred between the bottom of truss and top of wall parrallel to the wind load and chord forces are being transferred perpendicular to the wind load.

Pre-Engineered roof trusses should be designed for lateral buckling and all required permanent component bracing to achieve this should be shown on the truss shop drawings. This is the responsibility of the truss engineer and should be stated so on the contract drawings along with the lateral load values specified by the EOR.

I realize that typical wood frame construction for the majority of the US is fine, but what I'm asking is the connection of wood roof members to wood or concrete masonry walls for high wind zones - lets say 130-150mph exposure C.

 

[RARWOOD]

Contact Simpson or USP. I believe both of these companies have brochures which were developed for use in Florida and other high wind areas.

Some codes require a continous load path from the roof to the foundation. In high wind areas to meet that requirement requires a lot of straps and detailing. In the past both Simpson & USP had brochures to assist designers to meet the above requirements.

 

[ctcray]

Sounds like you are talking about an axial load into the truss. I thought like everyone else when you said "shear" you were talking about transferfing loads parallel to the wall. I'd say that most trusses around the country aren't in a 140mph exposure C zone, and the toe-nails plus uplift connector are addequate. In your case, you might have to take it a step further, ie. add another type of connection. That's like telling a framer he needs 2x12s for 100psf when he's used to building with 2x8s for 40psf, they're not engineers and don't always understand.

Still, like you said, the load must be spec'd by EOR to the truss manufacturer so that the truss is designed to handle the load. However, permament bracing for trusses is designed by EOR, not truss designer. The truss drawings will only tell you where bracing is required in order for the truss to work.

 

[FSS]

There is also the disconnect between the truss engineering and the building engineering. The truss design package is ultimately just engineering through software of the metal plates connecting a bunch of 2x4's or 2x6's into truss shapes. Their designs have nothing to do with an actual building, and the fine print in the individual sheets will tell you that.

Its up to the building engineer to use common sense and/or engineering judgement to make sure load paths are taken care of.

 

[JAE]

FESI, like ctcray, I also had my head at 90 degrees to what you were talking about - my apologies.

Yes, for the windward/leeward walls the lateral shear is driving straight into the trusses and thus the blocking would be oriented in the wrong way to adequatly resist this direct shear.

I'm looking at Simpson's cataloge - for the "H" ties and they all have some type of lateral load capacity in addition to the uplift.

Also, in another thread I previously posted a new format required in Idaho for wood truss designs. It may not completely resolve your comments about the truss engineer-building engineer disconnect, but I thought it was a good start:

[blue]Idaho proposal for a more "proper" format for wood trusses (whether gluelam or pressed gangnail):

Basis of Design

1. Premanufactured roof trusses are an engineered system - that is, each truss has a specified location in the overall structural system.

2. The premanufactured roof truss system is both a vertical and lateral load system.

3. The lateral load forces must be developed by the EOR and given to the truss engineer to design (drag truss load, collectors, etc.).

4. All truss-to-truss connections are the responsibility of the truss engineer.

5. All drag load (horizontal truss transfers) connections between the truss and the structure are the responsibility of the EOR.

6. All truss-to-structure (walls or beams) connections are the responsibility of the EOR.

7. It is the responsibility of the truss engineer to develop the loading required to implement the requirements of snow load Appendix A.

8. It is the responsibility of the truss engineer to review all of the EOR’s design specifications, roof truss support, and drag details and to incorporate these requirements into the engineering design of the roof truss system.


Required Information from Truss Engineer:

The truss engineer should develop a truss layout plan for the truss system that clearly indicates the truss vertical support conditions, truss-to-truss connections, drag trusses and collectors, and any other field-installed reinforcement, including field-installed top chord reinforcement at eaves necessary to execute the truss system design. The truss roof framing plan should be sealed by the truss engineer and be included with the individual truss cut sheets. The truss engineer should also provide proper supervision of any truss company technicians.[/blue]

 

[FESI]

Way to go Idaho. Thank you for the information.

 

[FSS]

Where can I find more information on the Idaho language/rules? Are they in effect? I tried through their engineering board website and could not find anything.

 

[JAE]

Oops my mistake - it was NEVADA, not Idaho.

In their July 2004 issue of "The Nevada Register".

Here's the link:

Nevada Register July 2004

 

[FSS]

Way to go Nevada is more like it. Wow. Is this actually in effect for them? I can just imagine the screaming that would go on if that approach was taken everywhere.

 

[FESI]

Back to the original question - the connection between the top of wall and roof structure - what are other engineers calling out for? My opinion is that connectors must be rated for uplift and lateral loads and therefore - are there any engineers that have a problem with specifying one proprietary connector for the truss to wall connection?

 

[COEngineeer]

I just figure out the uplift force from IBC. Open my simpson catalog and pick one appropiate hurricane tie and put it at each truss/rafter.

In colorado the uplift is not really a concern unless you are designing a barn like building or long lookouts. So I usually just specify simpson H5 at every connection to the exterior wall. So I only use the hurricane for uplift. Lateral loads will be taken cared by the toe nails into the top plate. I dont think the shear would be that much? Just vertical cross section of the roof profile? There is a lot of toe nails into the plate!

 

[RARWOOD]

I have specified Simpson H-1, H-10 & h-14 as truss hold downs in applications where I was required to resist both uplift and horizontal lateral load. For cases where multiple loads in different directions are resisted by a single connection a unity equation is used. See Simpon's 2006 catalog page 164.

If your loads are too high for a single connector you will either have to combine two different types of connectors or use one on both the inside and the outside of the wall.

 

[FESI]

Thanks RARSWC - high wind loads may require multiple connectors. What about truss girder connectors that have no lateral load values??

 

[samdamon]

In practice here on NE coast I have found that providing one prefabricated connector (for example a 10-nail simpson H2.5) at a truss heel will often (but not always) safely carry both the uplift on the truss and lateral force (parallel to the truss) on the top of the wall combined. Connector literature usually provides both lateral and uplift allowable capacity ratings for ties, and the interaction of both loads really should be considered in the selection of the connector. Designing for uplift alone is not correct imho.

For carrying wood-framed roof diaphragm loads into a shear wall that is perpendicular to roof trusses, 2x eave blocking should be typically used imho to provide a real load path into the walls. Using a hurricane tie or a few toenails into each truss bottom chord aren't going to do this job properly because you will have then have an overturning moment perp to the truss, cross grain bending on the truss heel components, and uplift on the toenails. Undesirable!

The eave blocking connections to the top of the wall and to diaphragm should be sized and specified to carry the required shear forces. If the truss heel is greater than 11 1/2"+- high as it crosses over a supporting wall, you will need to use manufactured timber for the blocking because a 2x12 will not be not tall enough. Builders hate having to rip manufactured timber to height because it tends to eat sawblades. The top cut on the blocking should be beveled to allow good nailing from the diaphragm into the blocking. The eave blocking may need vent holes or slots to allow air circulation, but its not a hard thing to rationally design.

Unfortunately, Architects and builders rarely understand the need for eave blocking. Its something that is frequently neglected by designers and the builders are not used to seeing it. There is a fair amount of labor required to install it, so its usually a fight to get it put in. Try to check in the field that its going in before they start sheathing the roof, otherwise you will have an interesting problem to solve. Regards