NBCC Foundations Allowable Stress Wind load combo

[palk7 EIT]

Hi,

For the foundation bearing pressure checks in NBCC for Wind loads I see the SLS importance factor as 0.75. So when checking the footings can I apply this reduction factor of 0.75 For Wind. So one of the load combo will be D+0.75W.

In ASCE it makes sense with ASD load combinations as they reduce the wind by 0.6 D+0.6W combo.

Any advice on this.

Thank you

 

[jayrod12]

For SLS loads, yes you can reduce the Iw factor. Often I don't, so I don't need to be carrying two separate loads, I usually just run with Iw=1.0 (or greater where necessary) even for SLS. It doesn't affect the designs that much, except where the members are deflection controlled. To be honest, I don't mind my structure being a bit stiffer than the code minimums.

 

[palk7 EIT]

Thank you, it makes sense, for the foundation design the wind load to be considered only 70% as the effective load for foundation design as per NBCC, so then if I apply my importance factor of 0.75*0.7, then the wind load will only be 0.525 in the combinations does that sound reasonable as per the code?

 

[jayrod12]

For SLS conditions maybe. But that's only one part of the equation. For the ULS factored loads (strength design) you'd still have a situation where 1.4W governs the design right?

Also, what's this 0.7 factor you're indicating on wind load for foundations? What year of the code are you using? I want to go read it because where I practice I don't do a 0.7 for wind load for foundations. And now I want to know if my entire office is missing something.

 

[palk7 EIT]

Attached the snip its NBCC 2015 Vol.1 point 4

 

[palk7 EIT]

Jayrod12,

Agree with 1.4 factor for strength design (LRFD), but since I am dealing with Allowable bearing pressure of soil instead of the Ultimate factored bearing pressure, I tend to consider the Wind Load factors as a SLS for ASD methodology rather than a ULS for the footings.

 

[EireChch]

how has the allowable bearing pressure been calculated?

 

[palk7 EIT]

EireChch

From the Geotech report. for the allowable bearing pressure and the ultimate bearing pressure

 

[jayrod12]

If you were comparing allowable pressures, I would not be doubling up on the reductions. They are not the same thing.

In fact, geotech's where I am (central prairies) are not allowed to provide allowable bearing pressures for part 4 buildings, only for part 9 buildings.

You can't apply the 0.75 Iw factor for SLS and then go into an allowable stress/strength type design, that's not how that is intended to work.

 

[palk7 EIT]

Jayrod12,

If we are to use the full Wind load for allowable bearing pressure, then those combinations vary from the ASCE7-16 ASD combos for foundation approach right, it becomes more coservative

 

[jayrod12]

I honestly don't think you should be mixing codes or allowable/LSD. I think you should be telling the geotechnical engineer to do their job as per the code requirements and provide you the LSD bearing pressures.

 

[ryaneng]

I'm getting the sense that there is a bit of a misunderstanding here. In the geotech reports I receive, we often are given a SLS and ULS bearing capacity. Both of these have factors applied to them as far as I am aware, but this is done on the geo side and we are just presented with the final values.

@palk7, when you say allowable pressure, do you really mean allowable or SLS provided by the geotech?

In general, I think you would be allowed to check the SLS bearing pressure using the 0.75 importance factor in addition to the 0.7 factor from the tables. Keep in mind that the 0.7 from Note 4 of Figure 4.1.7.6.-A only applies to that figure, you wouldn't be able to apply that factor if you were not using that figure.

 

[TLHS]

This is not a good thing to do. The NBCC no longer has any allowable stress load combinations. The factor for serviceability is literally just for serviceability. It's saying that you can reduce the load for things like deflection checks.

It is not saying that you can use that for ASD checks.

In theory, all design should be limit states, including foundation work. The geotech and foundation stuff has been an adventure to transition over, though, and it's still a weird hodgepodge 20 years later.

You can go back to earlier codes if you want to roughly understand what equivalent ASD combinations would be:

https://nrc-publications.canada.ca/eng/view/ft/?id=a6edb19d-6a71-44f7-84a9-bd9ed46a065d&dsl=en

It's not current, though, doesn't prove code compliance, and doesn't include any of the various load combination changes in the last twenty five years.

On the geotechnical side of things, I feel bad for the guys. The guidance they've gotten has been pretty poor. They got thrown some resistance factors and got told to use those for ultimate loads and then also figure out serviceability loads. But all of their day-to-day stuff tends to have implicit factors of safety in them and no real separation between serviceability and ultimate capacity. The numbers vary wildly between practitioners. If I were them, I'd just be baselining my ultimate capacities for shall foundations at 1.3 times whatever they'd normally do for allowable and service level and then doing a quick check to make sure I can prove at least that.

Also, every structural engineer learned how to do foundations with service level loads, even the recent grads. All the stuff about kern location doesn't hold up when you start using factored loads, and the whole concept of having a rectangular stress block in certain types of moment cases is always a fun discussion.