How to Connect doubled up TRIFORCE® joists

A question I often get these days is “How do I connect these doubled up TRIFORCE® Girders?”

TRIFORCE® joists are frequently doubled up for stair girders (see my recent post on this), and can be “sistered” in order to support vertical load as well.   Either way, you need some form of connection to properly transfer the load from the side (header) or from the top (to support a load bearing wall, for example).

The connection is important because it enables both TRIFORCE® joists to transfer load as one.

When a design calls for stair girders, the joist calculation will call out for the use of MJC clips or equivalent, to connect them.

Using MJC Clips

MJC clips are Simpson clips. They are applied on the top and bottom of the joists.


Simpson MJC clips for Doubled up TRIFORCE


I recently encountered a situation where the builder had forgotten to add the needed MJC clips. He discovered this after laying down the sheathing so it was basically too late to install this type of connector. That’s why there is an alternative interconnection method, namely using SDS or SDW screws. That’s the “or equivalent” I was referring to.

Using SDS or SDW screws

SDS screws for doubled up TriforceSDS screws (or the equivalent in Canada, SDW screws) are used a lot in the field when MJC clips can’t be applied or aren’t available from the distributor. These are heavy duty connectors at 180 Lbs per screw. The number of screws required depends on how many pounds of load transfer you need.

If you have a doubled up TRIFORCE® joists under a concentrated load, this is how to connect the two TRIFORCE® together for proper load transfer. Notice that the furthest screws can be no more than 12” on center, and not less than 4” on center from the load point.

Using SDS screws for doubled up TRIFORCE

Also, when you are using double TRIFORCE® girders connected with Simpson SDS screws, you have to calculate a 10% in reduction of load.

A note about connecting for side loads (stair girders)

If the header has already been installed and the ½” plywood reinforcement has been added on the sides of the TRIFORCE®, the screws will still be long enough to ensure proper connection. You just have to drill down into them.

Triforce videos

Connecting for top loads (load bearing wall)

Theoretically, you don’t need connections. If the two load supporting Joists are perfectly centered under the load bearing wall, with each joist bearing half the load.  In reality, though, this doesn’t happen very often. I would recommend connecting doubled up TRIFORCE® joists under these circumstances.

Even if the joists are a little off center, once connected, they will still be able to transfer load correctly.

One last thing

Screws go into the top and bottom chords for vertical load. For side-concentrated loads, screws are only necessary through the top chord.

Always connect doubled up TRIFORCE® joists

It’s always preferable to use MJC clips for an optimal connection. All the same, proper use of the SDS or SDW screws will do the job just fine. Either way, we recommend connecting doubled up TRIFORCE® joists. It is just good building practice.

Triforce Spec Guide

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