Design Considerations – Special Loading

Many factors go into the design of a roof and/or floor structure. As component design professionals, we need to be aware of many situations that will affect the design of our trusses. Below are a few of these that you will come across.

Chimney/Steeple loads

These structures can have significant impact on the design of a roof truss system. When dealing with a chimney or a steeple, the most important thing to remember is:

  • Account for the load that will be generated by the wind overturning moment and not just the dead load. Wind acting on the steeple will want to push it over and the roof needs to be able to handle this.

These loads can be significant and they must be accounted for in each and every load case. For example, you may have a 1000# steeple on the roof. If you figure that it will be supported at each corner – you would have 250# of dead load at each corner which isn’t too bad.

Load from the wind overturning moment could add thousands more pounds depending on the size. Multiple load cases will need to be run to look at the reactions based the wind going parallel or perpendicular to the ridge. The EOR should provide this information on the plans, but that is not always the case. Be sure that you get these loads and incorporate them into your design.

Drag loads

Drag loads are lateral loads that are applied to a structure from wind and seismic loads. These unique loads need to be transferred from the roof diaphragm all the way down to the foundation. Common uses for this purpose are:

  • Drag trusses. These trusses will usually be located over a shear wall. The horizontal drag load will be applied across the top chord and transferred down to the truss bearing.
  • Shear panels. These are “box” type trusses that go between spacing over bearing walls.

Most truss design software has the ability to apply the loading. Connection of the truss to the shear wall and/or bearing is another important part of this condition.  The EOR is typically responsible for this design, as it has an impact on the design of the overall structure. The loading information should be found on the plans.

Drift

Wind can put snow in all kinds of places and make some impressive looking drifts.

dssl1

As far as roofs go – you will see drifting caused by a number of different situations:

  • Wind blowing across the ridge of a roof will cause an unbalanced load condition with snow accumulating on one side of the ridge.
  • Snow will slide off higher roofs down to lower roofs.
  • Parapets and raised structures will have snow accumulate against them.

dssl2

Calculating drift loads can be somewhat involved depending on the situation and are dependent on numerous factors. I have worked on many restaurant jobs with flat roofs and parapets around the perimeter of the building. Most of the time the architect/engineer indicates the drift load required. Drift loads are an important part of structures in snow regions and need to be incorporated into the truss system for the structure.

There were times in the past that this information was not provided and I had to manually calculate the loading to be verified during the shop drawing process. Now with GDI, there is an excel sheet available to me that will calculate the loading for me, reducing the time factor by tenfold.

Partition loads

When designing floor truss systems you may come across this load that needs to be incorporated into your floor system. This will be seen most frequently on commercial jobs.

In addition to the live and dead loads called out on a plan, there may be an additional partition load called out.  This comes from the IBC and it states:

  • ”In office buildings and in other buildings where partition locations are subject to change, provisions for partition weight shall be made, whether or not partitions are shown on the construction documents, unless the specified live load exceeds 80psf (3.83 kN/m2). The partition load shall not be less than a uniformly distributed live load of 15psf (0.74 kN/m2)”.

I recently came across this in an assisted living facility that called out for this load. The spans for some of the floor trusses were in the 28’ range and including this load made a big difference in the design of the trusses.

Keep an eye out for these loading conditions – they can have significant impact on the design of a truss system.  Check back with us for more discussion on these loading conditions.  Let us know of any experience you may have had with them. Stay tuned for more!

Bill Hoover – Design Professional

Gould Design, Inc