Characteristic and Design Capacities
June 5, 2020
Suppliers of Fasteners (screws) and Connectors (brackets and similar) provide load capacities for designers to select appropriate products to suit a given application. Two load types are often referred to: Characteristic and Design (or Ultimate) Capacities. These two are distinctly different and it is important to understand how they differ and which one a designer should refer to.
Characteristic Capacities
The NZ Timber Structures Standard (NZS 3603) used by engineers to design timber buildings, includes methodology to determine fastener and connector Characteristic strength capacities. It is based on Australian standard AS1649 and provides an estimate of the 5th percentile strength value; that is, 95% of the time the published Characteristic capacity will be exceeded.
Design Strength
For specific use applications the Characteristic capacity is then adjusted by various modification factors, including but not limited to:
- The material strength reduction factor (ɸ) to account for the variability of performance in timber (being a natural and hence variable material). The ɸ factor is 0.7 or 0.8 depending on fastener or connector type and the load orientation in timber.
- The load duration factor (k1) depending on whether the applied load is long term or permanent such as materials the building is made of (k1 = 0.6), medium duration such as people (k1 = 0.8), or short duration such as wind (k1 = 1.0).
Design vs Characteristic
As the Design Strength is calculated by applying the ɸ and k1, plus possible other factors, to the Characteristic capacity, in essence the Characteristic Capacity is ‘generic’ whereas the Design Capacity is factored for a given unique product application.
It is important to refer to the correct load or capacity type, considering that with the timber ɸ factor of 0.7 or 0.8 and k1 between 0.6 to 1.0, if these factors are not applied and Characteristic capacities inadvertently compared to Design Strengths, these could be as much as half of their required capacity in worst case scenario, and still only ~ 2/3 of the required capacity in most other instances. Different scenarios of these potential differences are illustrated by the calculations shown in the table.
|
SCENARIO |
Characteristic |
Design |
% Difference |
|---|---|---|---|
|
Worst case scenario (screws) |
1.0 |
0.7 x 0.6 x 1.0 |
= 0.42 (42% of Characteristic) |
|
Long term load (nails) |
1.0 |
0.8 x 0.6 x 1.0 |
= 0.48 (48% of Characteristic) |
|
Medium term load (nails) |
1.0 |
0.8 x 0.8 x 1.0 |
= 0.64 (64% of Characteristic) |
|
Short term load (nails) |
1.0 |
0.8 x 1.0 x 1.0 |
= 0.80 (80% of Characteristic) |
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