Earlier this year, we hosted our first Virtual Training Summit, featuring 16 industry-related webinars covering a range of topics. Customers could register for up to four events and earn professional credits for each session. One session, “Designing Mass Timber Connections,” focused on mass timber and was led by Simpson Strong-Tie Senior Field Engineer Brian DeMeza and Senior Branch Engineer Kelly Sias. In the following post, they respond to some of the questions that they fielded regarding mass timber applications, advantages, and more following their presentation.
We received approximately 45 questions. Below are some of the most common, along with our responses.
General
Does SST provide connection design or are these details called out by the EOR?
Connection design is performed by the EOR or other registered design professional. In some cases, this can be the timber fabricator/erector, depending on how design responsibility has been delegated on a project. While Simpson does not provide connection design services, we have a field engineer in your area with the technical expertise to assist with product selection and technical information.
Fasteners
Could I use the SDHR Combo-Head screw to attach a regular nailer to a W-beam?
It is possible. However, the SDHR requires the steel to be predrilled and would install from below (steel-to-wood). Another option for attaching a nailer with a screw is our Strong-Drive® TB Wood-to-Steel screw or Strong-Drive TF Wood-to-Steel screw.
Do SST angled washers work with screws other than SST’s own?
Simpson’s MTW45-8 angled washer is designed to be used only with Strong-Drive SDCF Timber-CF screws. They can be used in our MTWS™ mass timber washer strap or any fabricated steel plate with a thickness of 10 gauge (0.134″) minimum and 1/2″ maximum, with minimum specified tensile properties of Fy = 33 ksi and Fu = 45 ksi. Refer to our Mass Timber catalog for more information.
Do you have end-grain withdrawal factor for SDWS or SDS?
Yes, we do! End-grain withdrawal factors for the SDS Heavy Duty Connector screw and SDWS Timber Screw are listed in footnote 3 of the reference allowable withdrawal load tables. Additionally, the SDCF Timber-CF and SDCFC Timber-CFC screws have been tested to establish an appropriate end-grain withdrawal factor, as published in our L-F-WDEND24 engineering letter.
Concealed Connectors
Do these connections address seismic/earthquake design?
We have done full-scale frame drift testing on all our concealed beam hangers in order to understand the effects of joint rotation on the connectors and assess their seismic deformation compatibility. The SCBH, CBH, and HSKP hangers are all approved for use in any seismic design category.
Why are the SDCF fasteners on the HSKP inclined upward rather than downward?
In the HSKP concealed beam hanger, the SDCF screws are installed inclined upward in order to resist downward shear forces through fastener withdrawal, rather than solely relying on dowel bearing, which greatly increases the strength and stiffness of the connection. While this might seem counterintuitive, it is also done in horizontal applications with our MTWS strap and 45-degree MTW washers. Inclined SDCF screws are also used for the ACBH and SCBH concealed beam hangers.
How do you find doubling up of CBH or ACBH affect installation and tolerances?
When using double CBH/ACBH/SCBH connectors side by side, additional care must be taken to ensure mating connectors fit together appropriately in the field. Installers typically create jigs to help ensure proper spacing and alignment. Confirmation that assemblies mate properly prior to field installation is strongly recommended. Refer to strongtie.com for installation recommendations.
What are the advantages of using the ACBH vs. the SCBH?
The main advantages of the SCBH are its high drift performance for use in any seismic design category and its ability to weld to a steel column or embed plate. If neither of these advantages is needed for your application, the ACBH presents a lower cost option with the similar download capacity.
Are the hangers presented good for roof locations where uplift needs to be considered?
The end grain concealed beam hangers (CBH, ACBH, SCBH) themselves do not have uplift resistance. However, these hangers can still be used in roof applications for uplift by installing a SDWC fastener through the two members from the top (per our detail) to provide uplift capacity. The HSKP installs with two screws through the bottom seat into the beam above, and the connector has been load rated for uplift resistance.
How would you protect these mass timber preinstalled connections from damage during shipping?
It’s common for the installer of the connectors to wrap the beam or column fully, covering the connection, and then deliver them to the jobsite where they remain wrapped until installed. It’s a great question for your project team when working out the details!
Can you provide guidance on reinforcement for beams using concealed hangers that do not meet the 0.75xDs or 0.75Dm geometry requirements? Tension perp. to grain.
Where connection geometry requirements of 0.75xDs or 0.75Dm cannot be met, it is recommended to reinforce the beam with fully threaded screws based on guidance in our forthcoming TEB-F-SDCFRINF24 technical engineering bulletin. The document outlines the technical background of the guidance, with multiple design examples, and provides installation guidelines for the reinforcement screws.
Does the use of SCBH allow the design of the CLT frame system as a moment frame?
The SCBH is not intended to be used in a moment frame application. Drift testing was performed to demonstrate the seismic deformation compatibility of the connection.
Straps and Angles
Does SST have any straps meant to be used with GLT, NLT, or DLT mass timber panels instead of using plywood sheathing? What about MPP?
GLT, NLT, and DLT floor systems require plywood sheathing to transfer diaphragm forces per the current 2021 American Wood Council Special Design Provisions for Wind and Seismic (SDPWS.) Simpson Strong-Tie has not evaluated any of our straps for use in this function on non-code-compliant diaphragm systems. Straps used to transfer diaphragm forces are intended to be used on CLT floor diaphragms, which include MPP because it qualifies under the same PRG -320 as CLT.
Can the straps be installed on the bottom of panels so they’re not in the way of flooring?
They sure can! However, your architect may not like the metal strap visible from below where most of the timber is exposed to view. It’s practical to install the straps on panel topsides and cover them with an acoustic mat and lightweight concrete or gypsum concrete topping to conceal the connections and provide necessary acoustic and fire performance.
How much of a bump would there be with the LDSS spline and WSV fastener?
The dimple in the LDSS™ light diaphragm spline strap increases the thickness of the strap to approximately 1/8″ thick. The Strong-Drive® WSV Subfloor screw head can also stick up a little bit above the dimple, depending on the drive depth. This ends up with an approximately 1/8″ to 3/16″ protrusion from the top of the CLT. However, this is typically not a concern with the use of concrete topping slabs.
