why LRFD in wood?

Post by JH » Wed Oct 04, 2006 7:55 pm

why is LRFD in the new NDS? deflection usually controls in wood.....

LRFD:
~ takes longer to design, & time is money


In what case is LRFD useful? Very short beams/joists like a window header?


cheers,
JH =)

by ken » Thu Oct 05, 2006 10:12 am

This is a very good-and-important question.

In engineering design process, engineer is always busy learning many methods from working stress design vs. ultimate strength design (concrete design), plastic design, limited-state design (steel design), to ASD (allowable stress design) vs LRFD (load and resistance factor design) (steel design).

There is a single purpose exists that through different design methods we learn how to design structural memberss and systems properly knowing the material strength and the behavior of the members in systems.

LRFD is the merging trend of the design method acrossing different uses of the material under different possible loading conditions.
For example, steel design is using a new "combined" ASD & LRFD design manual starting from 2006.

In wood design, the reason why we also use LRFD is as the following viewpoints:

1. ASD is using the factor-of-safety approach by setting the limit of the "allowable" design value for the members under service load conditions.
(i.e. Checking the strength of the materials when service loads are applied at the materials. You memorize these factors for different loading conditions.)

2. LRFD is using a two-way calibration approach to consider the materila's "resistance" under the consequence of the possible failure when a combination of the "uncertainty" may occur during possible loading conditions.
(i.e. Examining the material being used more efficiently under working loads. Use two types of factors to calibrate the useful strength of material without comprosing the safety concern from the viewpoint of the structural behavior and the more consistent analytic procedure.)

For wood used as a building material, it is merging into the trend of other materials (steel, concrete, etc.) used in the construction process.
When many pieces of wood members are used and formed to behave like a structural system, the performance of the final structural system will be well and known better if we use the LRFD design approach verus using the ASD approach.

Even the LRFD seems to consume more time and steps to perform a single design task comparing with using ASD approach, I think it will show you a better approach to learn the structural behavior from a single member to a complete system more efficiently and consistently.

by Bruce Pooley » Wed Oct 11, 2006 5:28 pm

When I was on the technical committee for the NDS, several reasons were given for developing and publishing LRFD:

1) statistical approach to make the designs safer,
2) other materials use these design methods,
3) graduating college engineering students are more familiar with this approach, and
4) historically, LRFD design methods have been safely used in the past.

It was generally perceived that if LRFD design method was available, that graduating engineers would not have such a steep learning curve and would not be reluctant to design with wood.

Post by ken » Fri Oct 13, 2006 12:10 am

Thank you Bruce.
It is always good to hear the direct reasons from the member of the committee regarding the development of LRFD method.

by JH » Fri Nov 17, 2006 4:01 am

Thank you!!!!

Some very good points.