After the Storm

Building for High Wind Resistance

This course has been approved by AIA (1 HSW/LU) and ICC (0.10 CEU).
This webinar includes an overview of high wind forces, the importance of a continuous load path, and how good design and construction practices can improve the storm resistance of buildings. Learn more about APA’s above-code recommendations for wind-resistant building, as well as how attention to connection details, understanding lateral load concepts, and recognizing common failure modes seen in storm damage assessments can help builders cost effectively build a safer home and reduce future storm damage.
Note: Do not navigate to YouTube if you require a certificate. A downloadable certificate of completion is available only when webinars are viewed on this webpage in entirety, after completing a brief questionnaire.

 
Approximate length: 1 hour.
 

Webinar Participant Questions and Answers

Is it possible to receive hardcopies of the APA pubs that were shown in the presentation?

All publications can be downloaded from APA’s Resource Library.

Where can we find the details you showed in any of your publications?

All of the details can be seen in our Building for High Wind Resistance in Light-Frame Wood Construction Guide, Form M310.

There are published capacities for the portal frames with the hold-down straps. Is there any plan to publish portal frame values without the hold-down straps to the foundations?

The portal frame without hold-downs (CS-PF, Continuously Sheathed Portal Frame), in Section R602.10.6.4 of the 2015 IRC, is a prescriptive wall bracing method that relies on a fully sheathed wall line. It is used successfully at garage door openings in areas of the country that experience lower lateral loads. Given that it is a braced wall method, it does not have an assigned design value.

The published capacities are available in APA Technical Topics: A Portal Frame with Hold-Downs for Engineered Applications, Form TT-100.

What wind speed can these details commonly resist?

Many of these details will improve performance over houses built to the code-minimum requirements in the IRC. An engineer may be able to determine how they relate to the resistance of a specified load or wind speed, but that will depend on such things as the house design, including the layout, materials used, and building location.

Did you mention that staples should be installed parallel to the truss chord rather than perpendicular? Not sure I understand; could you clarify or elaborate?

The legs of a staple should run in line with the member they are attached to, not across. This minimizes the risk of one of the staple legs missing the 1-1/2-inch width of stud available for fastening. This is true for both wall studs and roof studs.

While overall overturning of the house may not be a significant risk, there are some instances where long span structural members (roof girder trusses) can develop large uplift loads due to wind that may be larger than can be resisted by continuous sheathing and sill plate washers alone.

Because the seminar focused on above-code prescriptive design recommendations, it did not go into a lot of detail on such situations that may require hold-downs or an engineered design as detailed in APA System Report 101, Design for Combined Shear and Uplift from Wind, Form SR-101.

Hold-downs should be installed as required by the building code and as dictated by the loads on your structure.

What about strength of foundation? If one "embeds" anchor bolts only into top of course of a block foundation, there is no resistance from foundation.

The foundation also needs to be properly designed. Foundation design is beyond the scope of the seminar.

Do you recommend strapping on raised rafter framing?

Raised rafters are an area where it is common to have a complicated load path. The specific detail would need to be evaluated by an engineer on a case-by-case basis.

APA does provide prescriptive construction guidance related to using wood structural panel wall sheathing to overlap raised heel roof trusses. See Figure 2 in Construction Guide: Raised-Heel Trusses, Form R330. Information is also available for engineered designs in APA System Report 103, Use of Wood Structural Panels for Energy-Heel Trusses, Form SR-103.

Note however, that these recommendations have applicability restrictions and apply to raised-heel roof trusses and not rafters.

Did you review design plans for any of these roof failures?

When we have the opportunity, we will try to look at the plans. Often, however, the plans are not available.

Are the sheathing methods achievable with tilt-up wall panels and building components?

Yes, many of these methods are incorporated in factory-built building components.

There are a lot of storms throughout the country; how do you select your observations?

We evaluate storm damage on a case-by-case basis. We believe that we can learn the most by looking at recently constructed homes built under newer building codes. Today’s homes are also bigger with more windows and fewer interior walls to help resist the loads caused by wind storms. With hurricanes, it’s important to clarify if most of the damage is due to storm surge, or if it’s due to the wind.

Can you provide us with some other examples of other “degrees of damage” that are used to assess the strength of a tornado?

Other damage indicators include small barns, farm outbuildings, big box retail buildings, warehouses, softwood trees, service station canopies, large shopping malls, and other structures.

Is there a single item that you can pinpoint as the most common failure?

Most failures happen at a connection point. We have seen a lot of connection failures lately at the sill and top plates. The connection of wood structural panel sheathing to the sill and top plates along the entire building perimeter, while above code, would likely have helped the performance of a number of the homes observed.

Do you think the brick veneer failures shown are due mostly to mis-installation?

While we have seen problems due to improperly installed brick ties, it does not comprise the entire extent of the brick veneer failures observed. All structures move during wind events, and tornados are a severe example of this. With observations that involve houses that incorporated both brick facades and flexible wall sheathing, we often see cracking in brick veneer due to the deflection of the structure behind the brick.

Plywood and OSB are 8 feet long. How do you fasten structural wall sheathing to a rim joist or lap all the plates when the distance is greater than 96 inches?

Many manufacturers make panels that come in sizes longer than 8 feet. They can be used to accommodate lapping the rim board. Panel joints can also be spliced when using 4x8 or longer panels, whether they are installed vertically or horizontally, provided that the panel joints are blocked at braced wall panel locations, or as required by the building code.

Does APA have any information about "ThermoPly" sheathing?

Product information must be obtained from the manufacturer. Please see Product Advisory: Laboratory Tests Evaluate Design Values of ThermoPly® Red, Form SP-1172, which identifies discrepancies in the manufacturer’s published design values and those determined by testing at an independent laboratory.

Using ASCE7-10, at what wind speed do you recommend increasing your nail spacings to 4" oc edge/6" oc field—120 mph, 130 mph?

Our above code recommendations are useful for homes in any wind speed. For a minimal cost, the performance of a home can be greatly increased and the safety risk of the occupants is reduced. The presenter’s family in Nebraska incorporated all of these details into their homes in a 115 mph design wind speed.

For enhanced nailing of wall sheathing (4” oc fastener spacing), should nails be staggered between adjacent panels?

When two structural panels are fastened at a common 2 x framing member with 4” oc 8d fasteners, the rows of nails generally do not need to be staggered. While not required, offset nail placement at common framing is a good practice when dealing with tighter fastening schedules. Note that special fastening and framing width requirements may apply in some Seismic Design Categories where shear wall loads exceed 350 pounds per lineal foot.

Please clarify the detail shown at the gable end. The load path is not intuitive and it seems as though the ceiling or attic floor would help in this situation. Are there other options in this location?

The gable end brace discussed in the webinar is an additional detail to help resist the hinge that is often seen at a gable end. This brace should be used in conjunction with a proper gable design, as well as additional truss bracing to distribute the load and allow the walls and gable end to properly distribute the loads to the roof and floor below. Assuming they are properly connected, drywall or attic floor sheathing do provide some bracing at this location, but the additional bracing at the hinge point can make a big difference.

Of course, as with any of the details shown, there may be many ways to resist the same loads. We only showed one option.

Is there a risk of cross-grain failure at the band joist when the sheathing is nailed to a common piece of lumber?

In general, sheathing attachment into an engineered rim joist will provide a better uplift connection than end nailing of plates and studs between stories, as permitted by the IRC. To minimize the risk of lumber or LVL rim joists splitting due to tensile stresses perpendicular to grain at this location, APA recommends the use of an engineered wood rim joist that has cross-laminated layers.

APA Rimboard® made from OSB and plywood has published tension perpendicular to grain design values. For engineered designs where wood structural panel wall sheathing is used to resist both shear and uplift forces, connections can also be designed at lumber and LVL rim joists. This is done by incorporating plywood and OSB splice plates as detailed in Figure 5 of APA System Report SR-101, Design for Combined Shear and Uplift from Wind.

In your opinion, does staggering the structural sheathing joints improve performance, or is there no noticeable effect?

While staggering sheathing joints improves diaphragm performance to some extent (see diaphragm load cases), tabulated shear wall values are the same with or without staggering sheathing joints.