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February 2000: Preventing Roof Collapse - Metal Roofing Systems

Did you ever hear the famous saying, "It meets the code." More and more we hear this in the design of metal buildings. In most cases buildings are designed to meet the local building or jurisdictional code. In the case of metal buildings, meeting the code can be disastrous.

In March of 1993 a heavy snow blanketed the southeastern region of the United States. The heavy snow contributed directly to a large number of metal building collapses. The following winter in 1994 saw a repeat of collapses, this time in the areas of Maryland and Pennsylvania. It became clear that the method of construction of metal buildings needed to be revisited.

Background

There are two types of metal panel roof systems:

1. Standing seam roof systems, which use concealed clip fasteners. The panels are normally attached to the supporting purlins with fixed one piece or two piece sliding clips that are concealed in the seam. The purlin is thin and of high strength steel formed into letters resembling "C" or "Z". The purlins will carry loads adequately if kept strictly in their intended position. If allowed to twist or roll, the purlin load capacity will be reduced below design. Steel purlins are designed according to the American Iron and Steel Institute (AISI) Specification. The model building codes in the United States stipulate the use of the AISI Specification for design of steel structural members.

2. Lap seam roof panels, also known as through-fastened roofs. This roof features fasteners that penetrate the deck and attach directly to the purlins. This type of fastening features a more positive mechanical connection but are less desirable for building owners as they are more prone to leaks and problems with thermal expansion.

Response

After detailed research by a renowned expert in the field from Virginia Tech University and Factory Mutual, the same conclusion was reached. Where excessive lengths between purlin braces are found, an additional line(s) of purlin braces should be provided. In some cases, one additional row of bracing per bay can significantly increase the strength of the roof.

This is an economical and simple enhancement. In most cases the purlins already have holes that are pre-punched to accept the additional braces.

The Concern

Based on the industry findings, in 1996 the AISI Specification was changed taking into consideration the need for additional purlin bracing for standing seam roofs. However the BOCA code did not change. In fact the BOCA code references the 1986 version of the AISI Specification.

Why?

When will they update the BOCA code to the current and updated version of the 1996 AISI Specification? Contractors, architects and metal building owners design to the law or building code. They are probably all aware of the enhanced version of the 1996 AISI Specification. Yet all they want is to meet the BOCA code. The sad part is the client or building owner is not aware the AISI Specification was changed for a reason, yet the current BOCA code still adopts a version of the AISI Specification that is 14 years old.

Today we constantly hear, "It meet's the code." The BOCA code, in this case, is not enough.

Our goal is to help educate our clients into the deficient designs of standing seam roofs. In certain cases bracing reinforcement or other modifications are necessary to provide adequate protection against a potential roof collapse.