Continuous Beam Capacity - 1960's

[pioneer09]

Currently working on a project that incorporates purlins (light beams- 12B16.5) at 9' oc for a building that was constructed in 1962. Looking at replacing RTU's that are heavier and am wondering if there was different design procedures +/-60 years ago that would allow these purlins to have the required capacity in their existing state. I have attached a sketch of how the purlins are framed. Since the members are continuous over supports, pos/neg moments develop. The metal deck braces the top flange at the positive moments, however there is no additional bracing for the negative moments that occur. Based on the moment diagram during analysis, the inflection points occur +/-6' on each side of the support locations. I could say that the flange is braced at the support locations, however there is no additional bracing located at the inflection points. Was it standard design procedure 60 years ago to assume that the bottom flange was braced at 6' oc in this scenario even though no additional bracing was provided as the inflection points?

Thanks

 

[jayrod12]

In those days, they used the inflection points as bracing. Today that isn't allowed. However you could always go and add bracing at the locations required to make them work.

 

[dik]

If Canada, it was likely 44 Grade, in the US possibly A36. The section is a Class 1 section and you can use plastic design to squeeze every pound of resistance out... Judging from the splice locations, plastic design may have been used, but no moment at connections... not a regular 'Gerber' type of framing. New design may require designing connections for shear and moment. With plastic design, other than splices, alternate loading is not an issue. Biggest issue with RTUs is not the weight usually, it is the snow accumulation if this is present. Good luck. You have to confirm the material... maybe from rolling mill marks.

Dik