Timber Framed Trusses 6

[kschulte]

I am working on a project right now where we are leaving the desing of the heavy timber frame trusses to the manufacturer. One of the companies I am working with says that we are responsible for the lateral bracing of the members. Where do I find the requirements for bracing the trusses bottom chord?

I'd appreciate any guidance you can give me.
kfoley

 

[DJNENGINEER]

Only the truss engineer is going to know what THEY need. Member size is a function of the unbraced length, so you will at least need to be in contact with the truss engineer.

In other words, I believe that you are doing their work (IMHO).

 

[RARWOOD]

When you indicate you are working on Heavy Timber trusses I am assuming that you are designing trusses with large members such as 6 x 8's and larger. If you are dealing with press plate trusses using 2x4 or 2x6 I would probably agree with the truss manufacturer, unless your drawings or specifications require the manufacturer to provide that design. For press plate trusses you could contac The Wood Truss Council of America for more information.

If you are dealing with larger trusses using 6x8, 8x12, ect you are probably right. The bracing of larger timber or glulam trusses is a very controversial topic. Generally the bracing for this type of truss should be provided by the manufacture.

Generally in the trusses which I design which ussually are 8'-0" o.c. with spans between 30' to 100', I do not specify bottom chord bracing. The advice I received from two engineer with over 30 years of experience was, not to put in bottom chord bracing unless you have a specific reason to design it in. An example would be for a scissors truss with stress reversals in the bottom chord. The rule of thumb passed on to me was when in doubt about adding bottom chord bracing don't.

As I indicated above however there is a wide range of different opinions on this topic.

 

[SperlingPE]

Bottom chord bracing is the responsibility of the truss designer. That person is making assumptions of the unbraced length of the bottom chord. Lateral bracing for transferring lateral loads is the responsibility of the structural designer. This issue may have to be coordinated with the truss designer.

 

[JAE]

I agree with SperlilngPE - also, the Idaho PE board issued a statement recently where they propose a more "proper" format for wood trusses (whether gluelam or pressed gangnail:

Basis of Design

1. Premanufactured roof trusses are an engineered system - that is, each truss has a specified location in the overall structural system.

2. The premanufactured roof truss system is both a vertical and lateral load system.

3. The lateral load forces must be developed by the EOR and given to the truss engineer to design (drag truss load, collectors, etc.).

4. All truss-to-truss connections are the responsibility of the truss engineer.

5. All drag load (horizontal truss transfers) connections between the truss and the structure are the responsibility of the EOR.

6. All truss-to-structure (walls or beams) connections are the responsibility of the EOR.

7. It is the responsibility of the truss engineer to develop the loading required to implement the requirements of snow load Appendix A.

8. It is the responsibility of the truss engineer to review all of the EOR’s design specifications, roof truss support, and drag details and to incorporate these requirements into the engineering design of the roof truss system.


Required Information from Truss Engineer:

The truss engineer should develop a truss layout plan for the truss system that clearly indicates the truss vertical support conditions, truss-to-truss connections, drag trusses and collectors, and any other field-installed reinforcement, including field-installed top chord reinforcement at eaves necessary to execute the truss system design. The truss roof framing plan should be sealed by the truss engineer and be included with the individual truss cut sheets. The truss engineer should also provide proper supervision of any truss company technicians.

 

[AlohaBob]

See also Table 23-III - TT and SS
'99 UBC Chapter 2343.9

 

[rowe]

For residential roof trusses, the fabricator often provides the design as well. There have been many cases when a collapse occurred during erection of these type of truss systems. Some (maybe most) fabricators provide literature illustrating specific techniques for laterally bracing these trusses during erection (neccessary until sheathing is in place). Often, contractors do not install enough braces and the trusses will collapse due to lateral displacements during erection (like dominos). The fabricator usually states that bracing of the trusses during erection is not the fabricators responsibility.

With regard to the trusses - they are typically designed as a 2D frame (i.e. the loads are also in the plane). If a member is required to be laterally braced due to the in-plane forces for which the truss is designed for, then it should be the fabricator/designer's responsibility to provide that brace in the plans.

The Timber Plate Institute (TPI) has literature concerning bracing during erection as well as design manuals, etc.. However, they are not free - sorry.