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Service Load Combinations

Service Load Combinations

Service Load Combinations


I'm struggling quite a bit with something that should be really simple and Im hoping someone can break it down for me.

Im using Risa to design a 2-story residential house with a 9-foot retaining wall in the basement. The model has DL, LL, RLL, EL and SL in it.

When I'm specifying the load combinations in Risa Foundation, using the LC Generator, it's giving me a bunch of them. When you run the model, it requires that one of the load combinations be a service load combination. This is where I get confused:

If I specify 1.4*DL as the service load combination, the model fails in soil bearing.
If I specify 1.2*DL + 1.6LL + 0.5*SL as the service load combination, the model passes.

As I understand service loads, they are just the actual load without the load factors (DL vs 1.4*DL). Am I wrong with that? It seems to me like using 1.4*DL as the service load should be the one to pass since it only has DL in it, but I'm clearly missing something.

Thanks in advance!
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RE: Service Load Combinations


For your specific situation, when checking ground bearing pressure, you want to only use service load combinations (those equations found in ASCE 7 Chapter 2.4.1, or IBC* Chapter 1605.3). Note the title of these sections: "Combining Nominal Loads Using Allowable Stress Design" and "Load combinations using allowable stress design". Your ground bearing pressure is likely an allowable ground bearing pressure. Presumably, the geotechnical engineer who did your soil report went ahead and calculated a value for the ground bearing pressure at failure and applied a Factor of Safety to it --> that is, they divided the pressure at failure by 3 (or some other safety factor based on soil type, experience, etc.).

For doing the design of the concrete, you need to use load combinations that are specifically for Load-Resistance Factor Design (LRFD). These are found in Chapter 2.3 of ASCE 7, and Chapter 1605.2 of the IBC*. The global factor of safety is now gone, and you apply load factors that increase your loading and resistance factors that reduce your capacity (statistics, reliability, check a textbook for more info). At any rate, this is how concrete design is now done.

As for RISA Foundation, I would delete all the load combinations you currently have. THEN, up at the top of the screen, under the help menu (ish... under it and a little to the right) is the LC generator. Go ahead and pick 2018 IBC ASD from the "code" drop down menu. Apply the correct wind, seismic and over-strength options. Click generate. Now you have actual service level load combos. Now, go back to the "code" drop down menu and pick 2018 IBC Strength. Your wind/seismic/over-strength options should remain the same as before. Click generate now you have strength level load combinations. Click the x on the LC generator.

Take a look at your load combinations. RISA Foundation will have already tagged the IBC ASD combinations as "service" - thus, Foundation will check bearing pressures with these combinations; note that all of the IBC ASD lc's also have a value in the SF column - this is the factor of safety against overturning that Foundation will check (there is an exception to the SF column on the ASD lc's - for the 0.6 Dead + 0.6 Wind combination, the SF is already built into the combination, so RISA Foundation leaves that blank). Please note that Foundation will NOT design any of the concrete reinforcement using these combinations.

All of the lc's generated using IBC Strength no longer have the check mark in the "Service" column. RISA Foundation will not check bearing pressure using these load combinations. It WILL design your concrete reinforcement using these lc's.

In order to use RISA Foundation effectively, you need to create both service/ASD load combinations and strength/LRFD load combinations. Hope this helps!

*I was looking in the 2015 IBC as that's what I had nearby. Actual section may be different depending on what IBC year you are using.

Please note that is a "v" (as in Violin) not a "y".

RE: Service Load Combinations

Thanks winelanv!

That is very helpful and gets my model to pass. Im curious as to why the foundation bearing, sliding, and over turning are checked using service combinations and not strength combinations? Is that just standard engineering practice or does that come from somewhere specific?

RE: Service Load Combinations

Because most soil report gives you allowable bearing pressure as opposed to ultimate bearing pressure.

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