Connection Design Solutions for Wood-Frame Structures

This course is approved by AIA (1 HSW/LU) and ICC (0.10 CEU).
Proper connection design and specification is important to the structural performance and serviceability of any wood-frame structure. APA Engineered Wood Specialist Robert Kuserk, PE, presents Connection Design Solutions for Wood-Frame Structures, a one-hour presentation covering wood connection design, including common fastener types and their design values. The webinar also explains the orthotropic nature of wood and its role in connection design, commodity and specialty connectors, the use of steel connectors in wood-frame construction, and techniques for designing efficient, durable and code-compliant connections. 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 wood really weakest in the radial direction? Can you clarify?

For connection design, only two directions are considered: parallel-to-grain and perpendicular-to-grain instead of parallel, tangential and radial. In general, connections are strongest when they are oriented parallel-to-grain. When connections are oriented other than parallel-to-grain, it is not practical to determine if the connection is loading the wood member in the tangential direction or the radial direction; therefore the NDS combines these two directions as being “perpendicular-to-grain.”

If possible, can you please provide information on design of timber-to-timber connection using wooden dowels?

Designing with timber pegs is outside the scope of this presentation.

If you are interested in designing such connections, see the Timber Framers Guild for more detailed information from the Timber Frame Engineer Council.

It was mentioned not to use glue in the wall or roof assembly. Does this apply to flat deck roofs as well?

APA does not recommend the use of adhesives for fastening wall or roof sheathing to framing, as this can create a situation where the dry wood structural panel sheathing is over-restrained and unable to expand as it takes on moisture. Over-restraining panels can increase the likelihood of them buckling out of plane. Check with adhesive manufacturer recommendations if this is attempted.

Also note that it is not recommended to glue shear walls in seismic areas, since mechanically fastened connections provide a level of ductility, whereas more rigid panel connections do not.

Looks like nailing of panels would inhibit a panel's elongation or shrinkage. Correct?

Correct, panel fastening does play a role in constraining wood structural panel movement. Panels that are marked “Sized for Spacing” are manufactured at a length and width slightly less than traditional nominal length and width. This facilitates proper panel spacing during construction to accommodate natural panel expansion that occurs as the panel acclimates to construction or in-service moisture conditions. Tighter nailing patterns can restrain panel expansion, increasing the possibility of panel buckling. Refer to Builder Tip: Prevent Buckling with Proper Spacing, Form M300, for additional information on panel buckling.

Should a connector have expansion space, and if so, how much?

Generally, for glulam beams, the connector seat is 1/8” wider than the width of the beam to allow for beam growth. For structural composite lumber, hanger seats typically vary from 1/8” to 1/4” wider than the beam width.

What about wood beams bearing directly on concrete columns not exposed to weather? Would a retrofit be required?

To clarify, the 2015 International Building Code Section 2304.12 requires preservative-treated wood or naturally durable wood for members supported by exterior foundation walls. This requirement exists because the porous nature of the concrete allows moisture to migrate through the foundation to the wood beam.

Beams resting on “dry” concrete columns, not exposed to weathering, are not subject to the same moisture movement. The operative word here is “dry.” It is up to the design professional to evaluate specific environment to determine if a retrofit is required.

What is the typical capacity reduction factor for face-mounted hangers over sheathing or drywall?

There is no reduction factor for face-mount hangers installed over wood sheathing, as you have a direct connection of the hanger to the wood. However, when installing a face-mount or top-mount hanger over drywall, the engineer should note that drywall does not have the same characteristics of wood. Hanger manufacturers have hangers specifically designed for this application.

Can glulam beams be notched? I'm concerned about delamination.

Yes, glulam beams can be notched. Please refer to Glulam Connection Details, Figure 1C, which details requirements for the bottom notching of glulam beams at bearing. Interior notches are not permitted, as glulam beams have different grade laminations at the edges of the beams, so NDS notching criteria is not appropriate. Additional information can also be found in Technical Note: Field Notching and Drilling of Glued Laminated Timber Beams, Form S560.

Can we use screws in tension mode under gravity loading?

Yes, Section 12 of NDS on dowel-type fasteners addresses the design of fasteners, including screws, in withdrawal that can be used for designing a connection with screws in tension.

Could you give me an example of the use of inverted hangers for cantilever joist or inverted truss hangers?

The concept of specifying an inverted hanger is not much different than installing a conventional hanger.

In designing a connection, the designer should consider all loads and load cases, including loads on alternate spans that can result in upward or downward reactions at the end of the member. In either case, the connection should be designed to take the load in each direction.

Hanger manufactures will typically publish downward and uplift capacities for their hangers. Note the associated load duration factors that may need to be considered when selecting the hanger.

Lag screws seem variable in capacity and correct installation. Do you think that lag screws should ever be used when thru-bolts can be installed (for laminate beams)?

Lag screws do require additional considerations when being installed, such as lead holes to avoid splitting, and are detailed in Section 12.1.4 of NDS.

Provided that the lag screw is installed correctly, there should not be an issue, and they are acceptable. However, whether to use lag screws or thru-bolts is an individual choice of the designer.

What’s the intent of the building code where it says posts or columns shall be naturally durable unless a) supported by concrete pier or metal pedestal projected at least 1” above the slab or deck? Is that indicating a built up 1" height either of steel or concrete, or between that and the wood an "impervious moisture barrier"?

This question relates to the International Building Code, Section 2304.12.2.2. This section indicates the post would be installed on a vapor barrier, on a steel or concrete pedestal that is at least 1” above an adjacent concrete slab or 8” above exposed earth. If the post meets these criteria, it does not need to be pressure-treated or naturally durable material. Design of Wood Frame Structure for Permanence – WCD 6 from AWC is a good reference document that has additional details on this requirement.

Will there be a webinar for the NDS 2018 with ASCE 7-16 and the new RSRS for roofing/sheathing?

We are currently reviewing the changes to NDS 2018 for inclusion in this presentation. In the interim, many good articles are available that discuss changes to NDS, including roof sheathing ring shank (RSRS) nails. An article that appeared in Structure Magazine, Changes to the 2018 National Design Specification (NDS) for Wood Construction, can serve as a good interim reference.

Could driving screws cause floor sheathing to lift away from floor framing?

Choosing the proper screw is important. First, note that drywall screws should not be used in a structural connection. APA recommends selecting a wood screw with a size equal to or greater than the recommended nail with an unthreaded shank under the head of the screw.

Additionally, there are many proprietary screws available intended specifically for this purpose. These fasteners should be installed in accordance with the manufacturer’s code reports and product literature.

What are the fastener requirements for borate-treated wood used for sill plates?

Wood products treated with borate are generally intended to be used in a dry-use <19% environment, so this in itself reduces the likelihood of fastener corrosion. Borate-treated lumber is compatible with the same fasteners used for untreated wood, carbon steel, galvanized steel, copper and silicon bronze.