Curtain Wall Loading Requirements, Steve Kastner, P.E.
Curtain walls (CW) aka moveable partition walls can produce problems both in the field during construction (and possibly in the court room later) if the trusses aren’t loaded properly during truss design. The worst case loading condition is when the track for the wall is mounted parallel to the truss orientation where only one or two trusses must carry the entire load of the wall. The tracks for the walls are usually mounted after the drywall is installed. Then the weight of the wall (which can be very substantial and varies depending on the ceiling height) will cause either the truss to fail or have significantly more deflection than the adjacent trusses that under the right conditions could easily pop the ceiling board screw heads. Loading the trusses correctly during design and then comparing the deflection of the trusses in that area is very important. In general I always tried to keep differential deflection at less than ¾”.
Steps to loading the trusses properly:
1) The first thing to determine is how the CW is oriented in relation to the truss layout. If the CW is perpendicular to the trusses then a point load (this would be considered a live load because it changes) of two linear feet of the total wall weight should be applied to each of the trusses carrying the wall. Then a separate load case must be added and used to load the trusses involved when the wall is stored in the open position.
2) If the CW is oriented parallel to the trusses then it must be determined if one or two of the trusses are carrying the load. Then distribute the load accordingly. Again a separate load case must be added for when the CW is in the stored open position. One of the load cases will be a uniform load applied along the length of the truss(es) when the CW is extended. The other would be where the CW is stored in the open position. This would also be a uniform load but would be much higher because the entire weight of the wall would be spread over a much smaller area.
3) After the trusses are loaded then look closely at the deflection of the trusses in the area. If the difference in deflection exceeds ¾” then it is very probable that the ceiling deflection will be very noticeable.
In the worst case truss parallel orientation it would be a good idea to place 2×6 strong backs at 4’ on center, perpendicular to the trusses, centered over the load, and spanning to at least (2) trusses on each side. This will help spread the load and cause the heavier loaded truss(es) to deflect less and the adjacent trusses to deflect a little more mitigating the differential deflection.
If the weight of the CW is not already documented in the drawings or specifications and a professional of record is responsible for the overall design of the project, recommend sending a request to them via email regarding the weight of the wall to be used. This way the response is in writing and should be kept in the project file for the job.
Steve E. Kastner