Simplified Wind Load Assumptions – MWFRS Method 1

MWFRS Method 1 Explanation

Multi-Lat™ 2008 is intended for use on light-framed Bearing Wall or Building Frame Systems that assume the limitations of 3-story wood framed structures with shear transferred though the diaphragms to a lateral load system primarily based on conventional plywood shear walls or proprietary shear wall systems. In most cases buildings under 30-feet in height (40-feet as defined in ASCE 7-05 Section 12.14.1 for certain conditions) and defined as a closed system may fall into the category of ASCE 7-05 Main Wind Force Resisting Systems using Method 1 – the Simplfied design methods.

An excellent resource on this is Donald Breyer’s book published by McGraw Hill “Design of wood Structures ASD/LFRD” for the 2006 IBC and ASCE 7-05 references. However, there are a number of inaccuracies that the user of our software should be aware of:

  1. Breyer defines compliance with Method 1 – the Simplified Design Method as limited to 2-story light framed structures not exceeding 30-feet in height. And;
  2. A building with virtually no irregularities in the design.

The current code clarifies this definition to a maximum of 60-feet overall height but provides no restriction as to the number of stories. Although a structure that contains irregularities in the design is fairly common in residential structures, the main purpose of the code is to define the entire structure rather than its components. Breyer explains that the main portion of the design used for occupancy (other than covered open patios considers how the main structure can be sealed from debris occurring during a high wind event. In other words, the ability to close windows, doors and in some cases the use of tempered glass panels to defelct debree will qualify most buildings for compliance with the MWFRS Method 1.

Breyer makes some errors in this section ( 2.23-2.28 ) that I have not found an errata which corrects the design assumption, but the ultimate voice on this issue is the code itself. Garages with doors that can be closed to seal the structure against interior wind gust and flying debree may be considered under Method 1. Building normally left open (parking structures, airline hangers etc. ) are the exception to the rule and best left to the use of Method 2 which is included in Multi-Lat™ 2008.

The user is refered to local municpalities for revisions to ASCE 7-05 and the 2006 IBC which have been modified and made into law are an exception to the rule or Breyer’s text. It is, therefore, prudent to check with your local buildng department where the project is to be constructed for the final word.

While Multi-Lat™ 2008 references Breyer’s book on wood design, it does define compliance based on the definition of the entire structure as a preliminary. The software allows the user to break to structure into blocks, irregularities in plan is based not on exterior wall geometry but on the general shape of the entire shear transfer ability of the roof. In other words, if the exterior wall geometry would normally define a “U” shaped building, if the roof spans the courtyard to creating a continuous rectangular plan geometry, then the structure would be defined as a regular plan and the shear would be expected to be dragged across the courtyard.

The discussions that help to understand the discrepancy in the texts and in some cases, the code, are often discussed and archived on the Structural Engineers Association International (SEAINT) List Service in addition to this weblog. The user is recommended to search through the List archives located at http://www.seaint.org . Follow the links to the List archives and use this to search for all discussion threads. It is highly recommended that the user seek the discussions held within the community to become familiar with the code (especially those of us located in the Pacific Ring of Fire that have been using the 1997 Uniform Building Code for the last ten years.

Donald Breyer’s PE book “Design of wood structures – ASD/LFRD” has been the “bible” of the educational institutes and a practical guide – more so than the Seismic Design Manuals – that this author has found in light-framing residential structures.

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~ by structuralist on August 2, 2008.

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