Bottom Chords and Gable End Frames

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Question of the Day
Issue #10210 - January 2017 | Page #34
By Stan Sias

Why do some still insist on placing flat bottom chord gable end frames adjacent to vaulted or scissor trusses when the codes clearly do not allow such framing without special engineering requirements? Can you point me to the code sections that say “No, thank you!”?

This is a great question and clearly a bad practice, yet we see these scenarios continue to play out regularly on both professionally and DIY framed projects across most, if not all, of the country.

Perhaps the best way to start this discussion is with the appropriate ICC code provisions. What do the Codes say and what are our industry best practices?

IRC 2009 & 2012 Chapter 6 (Wall Framing), Section R602.3 states in part:

Studs shall be continuous from support at the sole plate to a support at the top plate to resist loads perpendicular to the wall. The support shall be a foundation or floor, ceiling or roof diaphragm or shall be designed in accordance with accepted engineering practice.

IBC 2009 & 2012 Chapter 23, Section 2308.9.1 states in part:

Size, height and spacing. The size, height and spacing of studs shall be in accordance with Table 2308.9.1 except that utility-grade studs shall not be spaced more than 16 inches (406 mm) o.c., or support more than a roof and ceiling, or exceed 8 feet (2438 mm) in height for exterior walls and load-bearing walls or 10 feet (3048 mm) for interior nonload-bearing walls. Studs shall be continuous from a support at the sole plate to a support at the top plate to resist loads perpendicular to the wall. The support shall be a foundation or floor, ceiling or roof diaphragm or shall be designed in accordance with accepted engineering practice.

What does BCSI say about this scenario from a best practices perspective?

In service, Gable End Frames also experience lateral Loads parallel and perpendicular to their plane. The Gable End Frame shall be incorporated into the wall design by the Building Designer.

Gable End Frames rely on properly designed and installed Structural Sheathing, Bracing and Connections to the bearing wall, and roof and Ceiling Diaphragms to be able to adequately transfer lateral Loads acting parallel and perpendicular to its plane.

If the lateral Load is large enough, and the vertical studs are long enough, the Gable End Frame may require Bracing to prevent it from rotating at the Gable End Frame/end wall interface, along with Diagonal Bracing and/or Web Reinforcement to prevent vertical Webs from bending excessively. Serviceability failures often occur if the Gable End Frame is not properly braced.

Gable End Frame Bracing/reinforcement helps prevent these types of serviceability failures and safely transfers forces from the Gable End Frame into the associated Diaphragms.

Let’s look at some real life examples of what happens when we don’t follow the codes and industry best practices. [See PDF of View in Full Issue]

Let’s dwell for a few moments on the permanent bracing of the bottom chord plane. Much has been written on how the bottom chord plane is typically restrained and braced by the diaphragm created by the application of the drywall ceiling, and out in the middle, or field as we call it, that works great. What about at the intersection of the ceiling plane with the gable end wall? After all, this is the intersection we are worried about in this discussion.

BCSI B3 states in Section 2, Permanent Bracing of the Bottom Chord Plane:

Permanent Bracing for the Bottom Chords of Trusses is typically provided by attaching either gypsum board panels or Continuous lumber Lateral Restraint properly braced against lateral movement.

Bottom Chord Permanent Lateral Restraint shall be installed at the spacing indicated on the TDD and/or by the Building Designer with a maximum of 10’ on-center.

The TDD provides information on the assumed support for the Bottom Chord based on the Load conditions for which the Truss has been designed. This typically includes a directly applied rigid ceiling such as gypsum board panels or rows of Lateral Restraint at a specified maximum on-center spacing.

You’ll note that, in this passage, BCSI-B3 states that the bottom chord plane can be permanently braced by gypsum board panels or lateral restraints and diagonal bracing.

Let’s go back and look again at the connection between the ceiling plane diaphragm (gypsum wallboard) and the gable end wall/gable end frame. Looking again at BCSI B3-32, we see the gable end frame sitting on the gable end wall with a drywall backer installed on top of that wall. This acts to support the ceiling drywall when the gypsum panel applied to the wall is installed underneath it. Is the ceiling drywall fastened to this backer? Rarely! What kind of support does this really provide to the gable end wall/Gable end frame hinge connection?

A preferred solution is the installation of lateral restraints and diagonal bracing per BCSI B3-7 on flat and/or sloping ceilings!

Here are several more examples of disasters waiting to happen. [See PDF of View in Full Issue]

In this photo, the framer at least aligned the studs so that perhaps the drywall will be continuous…though not adding adequate stiffness or rigidity to prevent the unbraced top-of-wall from buckling and pretty much guaranteeing a failure under any kind of wind event.

All of these photos represent framing practices that do not comply with the IRC or any other best building practices. Gable end walls adjacent to scissor or vaulted ceilings shall be framed to be supported at their tops by either the ceiling or roof diaphragms (see BCSI for additional images).

An additional caution from BCSI is this:

CAUTION: A flat Bottom Chord Gable End Frame used with adjacent Trusses that have sloped Bottom Chords (see Figure B3-42B) creates a hinge in the wall/gable interface that is below the Bottom Chord Plane Diaphragm. This condition is prohibited by some Building codes because adequate Bracing of this condition is difficult and sometimes impossible. Special end wall Bracing design considerations are required by the Building Designer if the Gable End Frame profile does not match the adjacent Trusses.

Flat bottom chord gable end frames next to scissor, cathedral or vaulted ceilings rightfully get the proverbial warning triangle!

Thanks for reading, S.

You're reading an article from the January 2017 issue.

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