I was asked recently why Component Manufacturers (CMs) do not provide the bracing details and holdowns for gable end frames like they do for the common trusses. The question got me thinking because the details are readily available, as are any mechanical connectors that might be needed to complete the specification and/or installation. I wondered if the installers simply gave up reading once they saw how to restrain and brace the commons. It also got me thinking about how often we see post storm wind event news coverage where the gable end frames have been blown off, or sucked off, the supporting exterior wall below, often at recorded wind speeds well below design wind loads. Why is this happening and what can we do to prevent it? Let’s take a look at what is involved in specifying Gable End Frame connections and bracing and the resources readily available to assist you with this task. After all, it is the scene depicted in the two images below that we want to avoid before, during and after a wind event.
In designing a bracing system for a gable end wall application, we need to look at two separate and distinct functions. First, we need to brace the top of the gable wall itself as without it the gable wall is free to move in and out at will. In the completed structure, the gable end wall and gable end frame above it will be subjected to loads (wind and seismic) both parallel and perpendicular their direction (see Figure B8–6).
When wind or seismic forces act upon the structure, it attempts to resist the sliding and overturning forces via the sheathing, connections, bearing walls, and diaphragms.
The industry standard, Building Component Safety Information (BCSI), Guide to Good Practice for Handling, Installing, Restraining & Bracing of Metal Plate Connected Wood Trusses, provides guidance for bracing the joint between the wall and gable end frame. It is typically accomplished with lateral restraints and diagonal bracing in a means similar to the detail in BCSI–B3, Figure B3–35, shown below.
The restraint and bracing detail above work to eliminate the failure mode depicted in Figure B3–33 right. Be aware that the forces depicted in Figures 32 and 33 could be acting in either direction, positive as shown or negative as a sucking force pulling the connection outward.
Secondly, and certainly no less important at least from an aesthetic perspective, we often need to brace the gable end frame studs themselves. It is very common for gable end frames delivered with roof trusses to have their studs oriented flatwise in the plane of the gable. The flatwise studs (vertical webs) are designed to transfer vertical loads only to (or from) the bearing structure below. Loads acting perpendicular to these studs must be resisted by vertical and/or horizontal reinforcement along with diagonal bracing to the roof or ceiling structural diaphragms.
BCSI details individual web member reinforcement options in Section B3, Permanent Restraint/Bracing (in addition to the BCSI–B3 Summary Sheet). Gable end frame stud reinforcement specification is the responsibility of the Truss Designer. This may be done on the Gable End Frame Truss Design Drawing (TDD) or on a supplemental detail. These details will always have the Design Assumptions specified, including: wind speed, duration factor, Building Exposure, Category, and Maximum Mean roof Height, all as specified by the Building Designer. Truss Designers provide these standard details to the Building Designers to assist them in determining the bracing required to transfer the lateral loads into the targeted diaphragm(s).
The Standard Gable Bracing Details will have a table that shows the requirements for reinforcement or diagonal bracing based on the maximum stud length. These can vary from no requirements for relatively short studs to up to two sets of horizontal ‘L’ reinforcements with their associated diagonal braces per Section A–A above.
The Building Designer may, in addition to the lateral restraint and diagonal bracing details, specify a structural connector between the gable end frame and the supporting wall/structure below. These could include a variety of connectors based on the desired intent and anticipated flow of loads. Some options include:
Other details may include structural screws and/or metal straps.
In lieu of project specific details for attaching and bracing gable end frames being provided by the Building Designer, Component Manufacturers would be well served to consistently provide the standard industry details referenced above as part of their jobsite submittal packages.
Thanks for reading, SKS
From the Archives
Our “Question of the Day” series by Stan Sias, published from 2014 until 2017, continues to generate feedback today. Although all of the articles since December 2015 are posted in our online Library, the earlier pieces are now only available in full-issue PDFs. So, from time to time, we’ll bring back a few of these that deserve another look, From the Archives…