Since it is on the resistance side of the equation, it varies only with the type of force axial, shear, bending moment. In equation form, again borrowing terminology from steel design, it is expressed as:. Comparing the two equations, we see that there is not an apparent factor of safety term as there is with ASD. The resistance factor may also account for known variations in design assumptions and fabrication or installation factors known to have a potential negative impact on the available strength.
The second major difference with LRFD is that the required strength on the left side is determined by increasing the applied loads based on how they are combined.
All of these load factors are given in Section The increase in loads with LRFD serves two purposes. First, they increase the expected applied loads and their effects to a level that more closely balances the LRFD available strength.
Second, the increase more directly acknowledges uncertainty about dynamic, time-varying transient loads like wind, snow and earthquake and uses individual load factors to increase them when combined with other loads. The graph below illustrates the difference between the ASD and LRFD available strength values right-hand side of the equation when using a simplified load-deflection graph for steel but the concepts apply to other materials too. This will become evident as the limit states are explained and demonstrated throughout this text.
The other two forms are useful when analyzing the capacity of a particular member. Another approach to comparing the two methods is to compute an effective factor of safety for the LRFD method that can be compared with the ASD factors of safety. This involves combining the load and resistance factors. You can divide through by the load factors to get an equivalent factor of safety:. The result is a variable factor of safety for LRFD. In ASD this factor of safety is taken as a constant. It can be argued that the variable LRFD W eff is more consistent with the probabilities associated with design.
The result is that structures with highly predictable loadings i. For structures subjected to highly unpredictable loads live, wind, and seismic loads for example the LRFD W eff is higher than the ASD W which results in stronger structures.
The LRFD argument is that ASD is overly conservative for structures with predicable loads and non conservative for those subject to less predictable loads.
Finally, you should be aware that you must select one or the other of the design philosophies when you design a structure. You cannot switch between the two philosophies in a given project! Introduction to Design Theory. Limit State Concepts. Loads and Their Combinations. Homework Problems. Report Errors or Make Suggestions. Make Donation. Section DC. Timber LRFD is relatively new to timber. Concrete Because of the complexities of analyzing composite sections using working stress method, the much simpler strength approach was easily adopted with it was first introduced.
That's the problem with ASD—it's really based on experience. Loads are usually estimations. Are you dealing with exactly pounds or is there some slight variations in the equipment you used to test. Maybe one test was You have different probabilities of different load cases. How much wind is there?
Is the wind exactly the same all the time? You get the point. Material quality. When you have a certain material what is the quality like? Is that wood chair the exact same as the next one? Are there defects in this piece that aren't in the next? You can see that the material quality can have a big effect on the resistances calculated. Fabrication tolerances. What kind of tolerances are acceptable? Because these change, it can change how you calculate things. Is everything constructed the same?
Of course not. Construction issues need to be accounted for in the design, too. It is also done through the use of reliability based methods.
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