CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL ...

esistances for small dimension lumber members. However, as the
calibration point is always achieving safety indexes under partial coefficient
design that maintain parity with ASD within selected calibration
problems, it is simply a very fancy way to do so-called 'soft code
conversion'. For genuine progress under item II it is essential to deal
with real reliability levels, i.e. not bother about parity with ASD solutions.
IV. Modernise the connection design provisions many of which have their
origins in unrecorded committee decisions made in the order of 50
years ago, based on US studies for military purposes at around the
time of the Second World War.
V. Rapid integration of new products, typically proprietary, into the market
place. This requires the establishment of a framework and methods
that ensures consistency in how design properties are assigned to
old and new products.
There have and will continue to be also major changes in provisions of the
National Building Code of Canada (NRC 2005), i.e. the model document
that lays down general - construction type independent - design requirement,
and specifies the magnitudes of environmental loads and load factors.
The 2005 edition (to be published in September) provides, for the
first time, explicit statements of mandatory 'design objectives' and functional
statements. The role of the timber code, and other material specific
codes, is to provide a basis from which designers can produce solutions
that satisfy various objectives listed in the building code. Implicit in items
I to V listed above is recognition the potential that changes in the building
code releases will only be fully harnessed by the timber construction sector
if accompanied by major change in the timber design code.
The last major overhaul of code provisions in Canada that relate to timber
connections occurred in the early 1980's. But what was done then was
mostly concerned with reformatting, information as the code as a whole
was transformed from an ASD to a partial coefficients format. Here the
need to revise connection design methods is employed as the tool for tying
the sections of the national timber design code together, just as actual connections
tie timber structures together. This touches on all of the issues I to
V.
Extensive use is made below of working documents produced by the
University of New Brunswick's Timber Engineering Group under the
leadership of Professor Dr. Ian Smith. Specific consideration is given in
those documents, and this one, to promoting design practices and provisions
that:
• Make it transparent to designers what mode of failure governs the
strength of particular connections, and how that relates to system behaviour.
• Embody probabilistic Load and Resistance Factor Design (LRFD) concepts.
• Guide designers toward an appropriate choice of structural systems to
resist given sets of loading combinations.
• Guide designers toward appropriate selection of wood and other structural
materials.
• Integrate design provisions specific to connections with those pertaining
to the overall system design. In the context of CSA Standard 086-01
this amounts to integration of connection and general design requirements.
• Maximize possibilities for technical harmonization of Canadian and international
practice.
What is outlined here is intended to be consistent with activities by other
Canadian experts, especially those working on issues related to system behaviour.
Although as yet so inconsistencies remain, work reported in the
paper "Framework for lateral load design provisions for engineered wood
structures in Canada" (Popovski and Karacabeyli 2005) is convergent with
what is discussed here. The next three sections discuss the logic for new
connection code provisions, general design code provisions that interrelate
with design of connections, and current activities.
41-1-1 J Köhler, A Frangi, R Steiger
On the role of stiffness properties for ultimate limit state design of
slender columns
Introduction
In the daily practice the engineering codes and regulations form the premises
for the use of timber as a structural material. Code regulations in
North America, Australia and Europe are based on the limit states design
(LSD) approach which is put into practice as load and resistance factor design
(LRFD) formats. Initially, LRFD methods where converted as so
called "soft conversions" of allowable stress design (ASD), the design
CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL PROPERTIES page 2.54