Why do some roofs have more purlin braces than others? What are the requirements for bracing purlins, especially with standing seam roofs?
For more information on the "Base Test" and purlin bracing with standing seam roofs, see AISI Design Guide D111-09.
What are the limitations on allowable drift (sidesway) in metal buildings?
This topic is addressed in the AISC's Steel Design Guide Series No. 3: Serviceability Design Considerations for Low-Rise Buildings, available from the AISC website (www.aisc.org), as well as in Chapter 3 of the MBMA Metal Building Systems Manual. The limitations for drift vary, depending on the type of material attached to the framing and the tolerance for movement. For example, for metal panels attached to framing, the recommended maximum drift is between H/60 and H/100 where H is the building height. It is also recommended that the wind load used to compute the drift when checking against these allowable limits should be based on a 10 year recurrence interval, which is approximately equal to 75% of the normal 50 year design wind pressure. For other types of cladding, refer to the references cited for more discussion.
Does MBMA establish erection tolerances for metal buildings? If so, what are they?
Erection tolerances are addressed in the Common Industry Practices Section of the MBMA Metal Building Systems Manual. Erection tolerances are those set forth in the AISC Code of Standard Practice, available as a free download from the AISC website, www.aisc.org. When crane support systems are part of the metal building system, additional erection tolerances are specified in Section 6 of the Common Industry Practices, found in the MBMA Metal Building Systems Manual.