Glass Rail 3-Panel Rule

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Author Message George A. Everding, AIA, CSI, CCS, CCCA Senior Member Username: geverding Post Number: 513 Registered: 11-2004 Posted on Wednesday, March 03, 2010 - 10:53 am:    Okay, here is today's riddle. Code tells us that the hand or guard rail must be supported by three glass ballusters and must remain in place if one of the three ballusters breaks. (2407.1.2 in the 2003 IBC I have at hand, and I think it remains in 2006 and 2009). So, does this mean that the required loading calculations on the rail are done in the "as installed condition" or in the "as failed" condition? In other words, must the rail be designed to resist the point load/uniform load requirements for the greater span of a missing glass balluster, or does it just literally need to "remain in place" with no additional load applied. Code interpretations desired, but professional opinions also welcomed. George A. Everding AIA CSI CCS CCCA Cannon Design - St. Louis, MO John Bunzick, CCS, CCCA, LEED AP Senior Member Username: bunzick Post Number: 1175 Registered: 03-2002 Posted on Wednesday, March 03, 2010 - 11:02 am:    My opinion is that the load applies to the as installed condition, not the partially failed condition. After such breakage, it would be immediately recognized as a hazard and repaired; but at the moment of breaking it gives an extra edge of safety if someone was leaning on the railing. I think the code would say so if the loading requirements applied to the failed condition. There are other situations where the code is designed to anticipate partial failure. One is hurricane glazing. The code anticipates glass breakage under specific conditions and is explicit about performance after this failure. However, it certainly doesn't mean that the window must continue to keep water out per code, or to continue to insulate. Ronald L. Geren, AIA, CSI, CCS, CCCA, SCIP Senior Member Username: specman Post Number: 838 Registered: 03-2003 Posted on Wednesday, March 03, 2010 - 11:11 am:    I agree with John. Additionally, in 2407.1.1, it states that a safety factor of 4 is to be used when applying the loads indicated in Chapter 16, which should provide any temporary support should a balluster fail. Ron Geren, AIA, CSI, CCS, CCCA, SCIP www.specsandcodes.com (Unregistered Guest) Unregistered guest Posted on Wednesday, March 03, 2010 - 11:21 am:    From old code guy, I'd say the idea is for protection in the failed condition. Simply, it has to hold up and cannot be predicated on immediate recognition of the problem and prompt repair [the material may not be at hand so temporary barricade may be required to allow continued use]. And of course, you can never predict what the public will do, even when on notice. Failure on even tenuous grounds [like obvious shortcomings] leads to lawsuits and awards. Tim Werbstein, AIA, CSI, CCS Senior Member Username: tim_werbstein Post Number: 21 Registered: 09-2006 Posted on Thursday, March 04, 2010 - 10:50 am:    I tend to agree with the masked man above. In his revealing book, "Why Buildings Fall Down: How Structures Fail," engineer Mario Salvadori investigated building failures and found that the one common condition for failure was a LACK OF REDUNDANCY in the design. When an element failed, the failure cascaded like dominoes. Safety factors accommodate construction and material inadequacies of single elements being insufficient, but do not necessarily protect against sequential failure of a system of elements. John Bunzick, CCS, CCCA, LEED AP Senior Member Username: bunzick Post Number: 1176 Registered: 03-2002 Posted on Thursday, March 04, 2010 - 04:38 pm:    I would argue that the three-panel rule IS the redundancy. First level is that the installed railing meets the loads; second level is that the railing stays in place after a partial failure. In redundant structural systems, a structural element can fail without the building falling down. But there is no expectation that the building will continue to have the same structural capacity after, say, a column fails. On the contrary, the building would be considered unsafe, but at least everyone can get out and repairs could (possibly) be made. Mark Gilligan SE, CSI Senior Member Username: mark_gilligan Post Number: 261 Registered: 10-2007 Posted on Thursday, March 04, 2010 - 05:28 pm:    My bias is to adopt the approach John suggested. Tim Werbstein, AIA, CSI, CCS Senior Member Username: tim_werbstein Post Number: 22 Registered: 09-2006 Posted on Friday, March 05, 2010 - 08:25 am:    I agree also. The second level John cites requires that the remaining two ballusters are designed to hold more than just their own share of railing load. They must be designed for the condition where one panel fails; otherwise the railing might not "remain in place." An overload (unless it is of very short duration) might cause another, overloaded balluster to fail before the load is removed.