Dead Loads and Live Loads

Structural loads are placed into two main categories, dead loads and live loads.

Dead loads are loads on a structure due to the structure's own weight.

Live loads are loads on the structure due to objects that are not permanent like people, furniture, and vehicles.

The goal of structural design is to accurately tally up the projected loads in a structure, convert them to stress, and compare that stress to what the structure can handle (the designed strength). There are two camps on how this is to be done:

Allowable Stress Design (ASD)

ASD has been around a long time and is considered the "older" method.

1. Calculate the designed strength of the material from the material properties and shape,  \sigma.

2. Apply a factor of safety to obtain an allowable stress \sigma_a = \dfrac{\sigma}{FS} (FS is the same as \Omega)

3. Tally up the actual load (using an ASD load combination) as \sigma_{actual}

4. If \sigma_{actual} \leq \sigma_a then the structure is acceptable.

Remember with allowable stress, the factor is applied to the designed stress to determine what is allowable (for some reason I think of a strict teacher or mentor dictating what is allowed). ASD is still used in some cases in steel and a lot in wood design.

Load Reduction Factor Design (LRFD)

LRFD has all-but replaced ASD, it is the only method used in concrete design and is often used in steel and wood design. The big difference here is that the loads are factored and come to represent the ultimate strength.

1.  Factor the loads using the governing LRFD Load Combination

2. Use the factored loads to determine the ultimate strength required, M_u or V_u (Moment or Shear stress)

3. Factor the ultimate strength by the capacity reduction factor (\phi) which usually increases the required strength.

4. Calculate the design strength (nominal strength) from the shape and material properties, M_n

5. If M_n \geq \dfrac{M_u}{\phi} then the design is acceptable for the loading.

Both methods are similar and do not really have different steps. Both have load combinations and both have factors to be applied.  ASD applies the factor to the DESIGN and LRFD applies the factor to the load.

In the test (and also in practice) remember that concrete will always use LRFD and for steel you need to make a decision based on the wording. If nominal strength or ultimate strength are asked for use LRFD. If allowable or actual stress are asked for use ASD.

About Conrad

I am a Civil Engineer. I work in San Diego and am preparing to take the PE Exam. I am interested in surfing, business, travelling, and spending time with my wife. Thanks!

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