Friday, May 27, 2011

Repetitive Member Factor: An Introduction

An increase in the reliability of wood members when placed into assemblies with repetitive use components has been recognized for many years by the wood industry.  The term assembly is typically defined as 3 or more members spaced at 24” on center or less and attached with load distributing elements capable of supporting the design loads.  Examples include floors, walls, roofs and three-ply beams with load distributing elements of wood structural sheathing or transverse mechanical fasteners. 

Floors and walls are typically solid-sawn members attached with plywood or OSB sheathing.  Roofs, however, are solid-sawn members or prefabricated trusses that have the top chord and/or bottom chord connected with structural wood sheathing.  The sheathing creates load sharing between the members of the assembly.  Three-ply beams share load through the mechanical fastening system used to build the member. 

The capacity of the assembly benefits from an increase in lateral stability of the members, partial composite action where the load is distributed lengthwise along the member, and load sharing where the load is distributed across parallel members that have different stiffnesses.  The parallel members having different stiffnesses result in varying amounts of deflection under uniform loads.  The load distributing element reduces the effects of various deflections by distributing the load away from more flexible members to stiffer members.  Members that are more flexible tend to be weaker than stiffer members, resulting in load being distributed away from weaker members and into stronger members.

Load sharing is accounted for through the use of repetitive member factors.  Since 1968, the National Design Specification for Wood Construction (NDS) has specified a repetitive member factor of 1.15 for bending design values of dimension lumber.  Also, since 1970, ASTM D245 has recommended a load sharing factor of 1.15 for bending stresses in multiple member systems.

Tuesday, May 17, 2011

Lessons from a Tornado

The April 27 tornadoes left many opportunities to study structural performance of both commercial and residential buildings.  One area that I have observed was a residential development located just out of the path of an EF3 tornado that made its trail not far from my home.  With the exception of one building's collapse, damage to each home generally consisted of a portion of roof missing, as seen in the photo below. 

A close up inspection of the rafter heel condition at the top plate highlighted the failure of the roof system.  A birds-mouthed rafter with (3) toe-nails into the wall top plate is a common framing detail used in residential structures.  This detail was used throughout this development.  There appear to be two realistic modes of failure for this detail as highlighted in the pictures below.  The first mode is failure of the 2x rafter at the birds-mouth.  This failure mode occurred in a small portion of rafters witnessed. 

The second mode of failure is at the toe-nail connection.  Although this connection works reasonably well for lateral shear, the typical limit state for uplift loading is nail withdrawal from the wall top plate.  The nails in the picture below appear to have seen very little stress during failure.     

Athough this is a common detail, I avoid detailing a rafter/truss to top plate connection with toe-nails. A Simpson H2.5A anchor, available at local hardware stores, is inexpensive and provides a significantly improved connection capacity. This syle of clip results in a more consistent installation and brings a different connection failure mode that does not include nail withdrawal. For a small home with a simple framed roof, such as homes in this development, a small investment in H2.5A clips can have a major impact on structural performance.

Tuesday, May 3, 2011

Tuscaloosa, Alabama Tornado Meets Wood Truss

I pulled the picture below from The Birmingham Business Journal's coverage of the April 27, 2011 tornado damage.  I have been slow to look at online pictures and videos of the event for a myriad of reasons.  I lived in Tuscaloosa for several years while studying for my BS and MS.  Even though many people have seen the devestation via TV, Web, or first hand, I could not resist sharing this picture here on the blog. 

A close look at the stick of wood embedded in the bumper of this car, and the wood lying on the ground in front of the car, reveals that this "stick" is actually from a metal plate connected wood truss.   I do not know the context of this picture to understand how this may have happened or the location of adjacent buildings from where this truss may have lived.  Whatever the process, the end result is amazing.