LSD, What is it?

LSD, What is it? No, it's not a mind-altering drug, it actually stands for Limit State Design and it has replaced the existing Working Stress Design (WSD) as the accepted method of timber engineering.  Binging timber in line with the other major construction materials, such as steel and concrete and conforming to international standards.

Now there is no need to panic. The change from Working Stress Design to Limit State Design actually took place back in 1997. Few apart from those in the engineering sector were aware of such a change. Even though this change took place, altering the way in which engineers approached timber design, the TRADAC was still the accepted timber design manual.

The Australian Standard, AS 1684 - the National Timber Framing Code - was released in December 1999. This framing code was written with input from various sectors of the timber industry including the Timber Research and Development Advisory Council (TRADAC).

Revision of the code took place in January 2000 and will fully replace the existing TRADAC manual as of the end of this year. This means that as of January 2001 the TRADAC manual will no longer be accepted.  So what does this mean to the average builder? The most notable change is the affect Limit State Design has on wind ratings.

As an example, LSD design wind speeds are 22% more than the WSD values. What was W33 is 40 rn/sec. and W41 becomes 50 m/sec. To avoid the confusion and the possibility of using the incorrect wind speeds, AS1684 has adopted AS4055 terminology for wind areas such as N1, N2 etc. for example N2 will represent the old W33NC and the new 40rn/sec non-cyclonic. (Refer to Table 1) 

Table 1
OLD WSD
NEW ZONE
LSD SPEED
W28N
N1
34m/s
W33N
N2
40m/s
W41N
N3
50m/s
W50N
N4
61m/s
W41C
C1
50m/s
W50C
C2
61m/s
W60C
C3
74m/s

 

We as truss manufactures, are still detailing trusses, using the old Working Stress Design method, so the full impact of the change to LSD is not fully known.

According to Robert Tan, Senior Engineer for Mitek Australia Limited, an "impact that AS1684 part (1) has on truss and frame design is in the tightening of allowable deflections. In particular, conventionally framed hip end rafter spans have been trimmed. Truss tie down also shows an increase in requirements mainly in the low to moderate wind zones, however roof bracing remains unaffected."

One impact on frames alone which builders need to become aware of, is the tie down requirements for top plates. The once standard 1800mm tie down centres is no longer the case, with each frame required to be looked at individually to determine the required tie down centres for either plate ties or cyclone rods.

In conclusion AS 1684 is replacing the existing TRADAC manual. Builders need to obtain a copy of the new code and familiarise themselves with it. In all I believe you will find AS1684 to be a better manual, with more accurate and reliable information. Basically the impact AS 1684 has on truss and frame design is only minimal and should remain close to those obtained from using WSD.

Links to more information: 

  • Australian Standards site includes standards for the timber industry and timber span tables
  • Mitek Australia
  • TRADAC

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