I'm posting this in the API-650 forum, as the question pertains to steel storage tank design. I may post under AISC (Steel Construction) if others feel that is a more appropriate forum.
Consider a large steel storage tank with a conical roof that is self-supported with the aid of externally welded-on rafters and an external center compression ring. For the consideration of lateral bracing for the top (compression) flange, I have seen a design resource that simply uses web stiffeners spaced along the length of the rafter, so that the un-braced length is the maximum spacing between stiffeners. In this same resource, no mention is made of stiffeners being required at the outside end of the rafter. I normally use angle members attached to the top side of the compression flange (rather than web stiffeners) of the rafters to serve as the lateral bracing. Does the web stiffener approach for lateral bracing of compression flange seem reasonable?
Also, the AISC Ninth Edition requires the support points of beams to be restrained against rotation. At the inner end of the beam, the entire depth of the beam is welded into the inner compression ring, and thus, rotation is restrained. At the outer end, only the bottom flange of the rafter is welded down to the top angle at the roof-shell juncture. Is this considered sufficient rotational restraint? The cross-section could not totally rotate, but it could distort. However, adding stiffeners at the outer end may be overkill considering the fact that the roof plate offers some amount of rotational restraint all along the length of the rafter. I would appreciate your thoughts.
Consider a large steel storage tank with a conical roof that is self-supported with the aid of externally welded-on rafters and an external center compression ring. For the consideration of lateral bracing for the top (compression) flange, I have seen a design resource that simply uses web stiffeners spaced along the length of the rafter, so that the un-braced length is the maximum spacing between stiffeners. In this same resource, no mention is made of stiffeners being required at the outside end of the rafter. I normally use angle members attached to the top side of the compression flange (rather than web stiffeners) of the rafters to serve as the lateral bracing. Does the web stiffener approach for lateral bracing of compression flange seem reasonable?
Also, the AISC Ninth Edition requires the support points of beams to be restrained against rotation. At the inner end of the beam, the entire depth of the beam is welded into the inner compression ring, and thus, rotation is restrained. At the outer end, only the bottom flange of the rafter is welded down to the top angle at the roof-shell juncture. Is this considered sufficient rotational restraint? The cross-section could not totally rotate, but it could distort. However, adding stiffeners at the outer end may be overkill considering the fact that the roof plate offers some amount of rotational restraint all along the length of the rafter. I would appreciate your thoughts.
