Fundamentals of Truss Bracing: What Have I Learned?
Like many truss designers who start in this industry, truss bracing can often be a very foreign subject to us and we feel like a deer in headlights when asked to consider this in our daily design routine. I was no different. So, what have I learned over the last 2.5 decades?
For truss bracing there are two types to consider, temporary and permanent.
This is required to:
- Resist winds loads or forces during construction
- Ensure truss spacing is correct
- Support temporary construction dead loads from roofing materials and sheathing
- Keep the trusses plumb
Failure to provide proper temporary bracing can lead to collapse during erection or overloading, buckling and possible permanent damage or worse, injury or death to workers. Click here for more details.
This is required to:
- Ensure the prevention of web braces buckling when loaded in compression
- Share loads between adjacent trusses
- Transfer loads to the diaphragms
- Restrain overall lateral displacement
Permanent top chord bracing is achieved when rigid roof sheathing is properly applied with staggered joints and adequate nailing. This creates a continuous diaphragm action and any additional bracing in the top chord plane is usually not required. Bottom chords must be braced with either a rigid ceiling or continuous lateral bracing (CLB). The most common form of permanent bracing is CLB where a 2x member is placed and connected to a chord or web at right angles to prevent it from buckling under loads. For webs it can be in the form of T-braces, L-braces and scabs.
As a truss designer I had to quickly learn what I was and was not responsible for with respect to bracing. I was not responsible for the temporary bracing as trusses were hoisted and put into place including if and how roofing material was stacked on the structure.
Neither was I responsible for the overall truss permanent bracing, that remains with the Building Designer. According to TPIC 2014, I am responsible for the design of individual metal plate connected wood truss components, including lateral bracing requirements to prevent buckling of individual truss members due to specified loads. These bracing requirements appear on the individual truss component design drawings and I have to ensure bracing is clearly specified. And if I ever did receive a call from the framer or site supervisor asking for clarification, it gave some peace of mind that they were giving this the attention it needed.
Some simple steps that I was taught to reduce or eliminate web bracing were:
- Flipping or reversing truss webs, a tool in MiTek’s Engineering program
- Increasing web lumber size
- Decreasing a panel size to reduce web length
- Aligning webs up with a minimum of three trusses
For girders trusses, extra attention had to be given with the goal to eliminate web bracing completely but if that was not possible:
- Change the bracing option from lateral to T-bracing
- Increase number of panels to add number of webs and effectively reducing web lengths
- Reduce the girder truss depth if possible to reduce web lengths (But if a supported top chord now becomes unsupported and needs bracing, what do you do? In this case, if trusses are supported off the girder bottom chord, adjust their span and have them top chord bear on top of the top chord. Problem solved, plus you’ve eliminated hangers, a time saver for the framer and a potential cost saving to the builder.)
Is bracing important? Absolutely! Can it be ignored? Certainly not. Lives have been lost when it was.
I know this only covers a tip of the iceberg on this subject but in the short time I have been exposed to GDI’s Professional Development program, it has been a privilege to see the resources made available to each team member whether new or has been designing for a number of years like myself to help us all excel and become the best that we can be.
What resources do you have access to?
Harold Isaac – Design Professional
Gould Design, Inc.