I am looking to design a drop beam for a roof. Does anyone know of a helpful calculation example or design guide? Any suggestions/comments are appreciated.
I was trying to obtain an example of the calculation in the drop beam that takes into consideration the contraflexure points on the moment diagram. If you could elaborate of the concept behind the design that would help. I am new to the use of drop beams.
I don't think the term "drop beam" means the same thing to everyone. I thought you meant a beam dropped below the joists as opposed to a "flush beam" where the ceiling is uninterrupted by the beam.
If you mean the cantilever or Gerber system of framing as suggested by CANPRO, the simple spans supported at each end by cantilevered beams are just that, simple spans. They are sometimes referred to as "drop-in spans". It is usual to consider the ends of the drop-in spans as hinged. The cantilevered beams are not termed "drop beams".
Because the drop-in spans are hinged, the moment of each cantilevered beam is easily calculated but snow can blow around, so it is necessary to consider unbalanced snow load on a roof.
The reference given by CANPRO was prepared by CISC following the partial collapse of a grocery store in Burnaby, B.C. which employed the Gerber system on the roof. The roof was designed as a parking level and was largely occupied by cars at the time of collapse on opening day in April, 1988 but there was no snow. Lack of bracing in negative moment regions of the cantilevered beams was found to be one of the main factors leading to collapse of a roughly square area of the roof.
The reference given by CANPRO is one of the most comprehensive I have seen. I do not know of a better reference. The attachment shows a plan view of a typical Gerber system.
The unique attribute of the Gerber system is that all beams are statically determinate under any given distribution of load. Conversely, a continuous beam is statically indeterminate.
