Earthquake Load Combinations

Sizer implements earthquake load combinations according to the following design code references.

NBC

• 4.1.2.1 1) p 4-2 defines E loads as a “rare” load.
• 4.1.3.2 8) p 4-5 mentions lateral earth pressure loads due to earthquakes
• Table 4.1.3.2 p 4-5 includes earthquake load combination
• 4.1.8 p4-18- 4- 35 describes seismic load generation procedures.
• 4.1.8.5 – gives Importance factors – For SLS Note 1 refers to 4.1.8.13, and note 2 to the appendix A4.1.8.5, which refers to Commentary J
• 4.1.8.13 – Refers only to lateral deflection only.

NBC Structural Commentaries

• Commentary A-8, table A-1, gives earthquake return period
• Commentary A-9 explains that earthquake importance factors
• Commentary A-12 gives load combinations for rare loads in Eqn. 3
• Table A-3, commentary A-12, gives load factors for ULS of 1.0
• Note 3 of Table A-3 indicates 1.0 is to be used as D or H factor, 0.25 for snow companion factor, and 0 for wind companion. .
• Commentary A-14 explains the performance standards for rare loads
• Commentary A-19 indicates why D= 1.0 is used for stress reversal combinations for E
• Commentary 27, Table A4 includes E loads for serviceability, damage to components.
• Commentary 28, Table A5 includes E loads for serviceability, short term deflection
• Commentary J addresses Earthquake load generation and design.
• Commentary J5 discusses performance standards
• Commentary J8 discusses minimising deflections to avoid damage to detailing.

CSA O86-14

• 5.2.3.1 p 16 defines load type E
• Table 5.2.3.1 gives Importance factors for E
• Table 5.2.4.1 gives ultimate limit state load combinations for E
• Table 5.1 Includes earthquake loads among short-term loads
• Table 5.2.4.2 does not give serviceability load combinations for E.

The table below gives the load combinations that include E along with load types D, L, Ls, S and W, and a comparison between the old and new design codes in this regard. Refer to the comments at the end of this section to extend this for load combinations with Ds, Lt or H loads.

Note that in this table, we do not list all the sub-combinations of combinations – if the combination says “plus D, S, W”, then one would go to the ultimate limit states table and see that D + S; D+ W, and D are also generated.

³D, L or D,S not strictly necessary but created for technical reasons

⁴Design code refers to storage “occupancies”, indicating that all live loads are increased to 1.0

Earthquake and Wind Loads

When E, Ls and W are on the same member, the combinations containing Ls and W, but not E are not listed. Refer to the tables in Live Storage Load Combinations for these combinations.

Liquids in Tanks Loads Lt

The combinations containing Lt can be derived by replacing L by L + Lt.

Hydrostatic Loads

Adding H arrives at the load combinations for hydrostatic loads H to each of these combinations, while also retaining the combinations without H.

Dead loads due to soil

The dead load due to soil factor of 1.25 does not apply to earthquake load combinations. However, entry of the Ds load is still possible with earthquakes to examine the non-E combinations. Therefore the program will continue to make Ds part of the E combinations. These combinations are arrived at by replacing D with D + Ds in this table, without repeating the load combinations for D only.

Dead-load-resisting-failure load combinations

Notes 2 and 3 of NBC Commentaries Table A-3, indicate that 1.0 is to be used as the stress reversal factor for Dead loads combined with Earthquake, and neither the CSA O86 or NBC include a 0.9 factor for earthquake load combinations, therefore SIzer does not include dead-load-resisting-failure combinations for earthquake loading.

Serviceability load combinations

Neither the NBC nor the CSA O86 includes earthquake serviceability factors. The NBC structural commentaries, tables A4 and A5, do include earthquake load combinations for tables for damage to components and short term structural deflection, but further commentaries discuss lateral deflection (storey drift), and do not consider deflection of bending members.

Therefore, and there are no serviceability load combinations for Earthquake loads in Sizer, and none of the combinations containing E in the above table will be checked for deflection.

Patterns

Earthquake loads are not patterned, and live and snow loads are not patterned in earthquake combinations.

The load types referred to here are D=dead W=wind S=snow H=earth,groundwater E=earthquake L=live(use,occupancy) Ls=live(storage,equipment) Lt =live(liquids in tanks) Ds=dead load due to soil.