LRFD Shear Wall design Phi factor for uplift

[MILRAD]

I am designing a shear wall and want the weight of concrete foundation to resist the uplift. I am thinking LRFD might work better than ASD - 0.9D+1.0W vs. 0.6D+0.6W.

But if I use LRFD what would the phi factor for uplift be? (The equivalent to phi = 0.9 for a beam using 1.2D+1.6L). For ASD, is it 0.6D (down) > 0.6W (up) or 0.6W/0.6D < 1.0. Is it the same for LRDF - 1.0W/0.9D < 1.0? Should the 1.0 be 1.2 or 1.5 in either ASD or LRFD?

ACI changes the load factor for F and H depending if F or H are driving or resisting forces but says nothing about phi.

Thanks

 

[phamENG]

There is no phi for uplift. Uplift is going to be a load (or, perhaps more accurately, a load effect), and phi is a strength reduction factor for the material. Loads get factored, materials get strength reduction. So I'm not really sure what you're asking?

 

[MILRAD]

Assume for the shear wall the uplift is W=1000# and the weight of the concrete footing at the end of the shear wall is D=1000#.

Using 0.9D+1.0W the net uplift is 100#. That is the factored load. The resisting force is the dead load of 1000# or is it 0?

Should I think of the equation as 1.0W is driving and 0.9D is the resisting force so the phi factor is 0.9? For a 1000# uplift the needed D is 1110#. Or should there be a different phi factor.

Using ASD and 0.6D+0.6W the net uplift is 0. ASD requires less D? In this example the unity (D/W) is 1.0. Should it be higher? 1.1 to match LRFD? 1.5 to match the overturning and sliding factor of safety for a retaining wall?

Maybe another way to ask is if you have uplift of W=1000# what is the minimum amount of D do you need? 1000# seems low but 1500# seems high.

 

[phamENG]

If D=1000lbs and W=-1000lbs (remember, signs are actually important when adding vectors), then

0.9D+1.0W=-100lbs. You need an additional 100lbs to resist the uplift.

0.6D+0.6W=0lbs. You're balanced.

There is not phi. phi is a strength reduction factor and has absolutely no place in a load combination.

So it's going to depend on what design philosophy you're using for the building. If you designed it using LRFD, add a bit of concrete. If you designed it with ASD, you're alright. You can't look at these things on the micro or individual component level. They work because the whole building is working together under the same design philosophy. If you pick which one gives you the lighter element in each member, you're going to end up with a building that doesn't meet minimum reliability requirements.

 

[driftLimiter]

I just use 0.6D + 0.6W and get it to unity. Essentially 0.6D + 0.6W is the same as 1.0D + 1.0W. Since the 1.0W is an ultimate level deal I've felt okay about this.

I do not use 1.5x FOS requirement for overturning checks when using ASD Load combinations unless its on retaining walls.

If you designed it using LRFD, add a bit of concrete. If you designed it with ASD you're alright.

This part either I don't understand or don't agree with. You don't design a whole building by one methodology. You really can't "Design a Building" by ASD anymore so I don't get it... The lateral loads you determine unfactored are not a function of design methodology, neither should the overturning of the stability of the structure. The bearing pressure doesn't care what method you selected above, why should overturning??

I figure if you are designing the bearing pressure based on ASD than it makes sense to check overturning at ASD level.

 

[MILRAD]

phamENG

The LRFD equation is (Load Factors)x(Driving Forces) < (Capacity Reduction Factor)x(Resistance). The left hand side comes from the combos - 1.2D+1.6L etc. The capacity reduction factor or phi factor comes from a code - AISC has 0.9 for shear unless a listed I-shaped member with a compact web then 1.0, ACI has 0.65 to 0.90 for beam-columns depending on steel strain.

In my example I have a factored uplift of 1000# from 1.0W. So (Load Factors) equals 1.0 and (Driving Forces) equals 1000#. For (Resistance) I am using a self-weight of 1000#. What capacity reduction factor or phi factor do I apply to the self-weight?

driftLimiter

I have a wood shear wall anchored to a concrete foundation. The concrete and (ACI) anchorage design are LRFD and wood can be LRFD. I have to size the footing to counter the uplift and I thought I would stay in LRFD (not thinking that so much anymore) and then flip to ASD for bearing. (Usually it is ASD for the wood shear wall, uplift, and bearing, then switching to LRFD for anchorage.)

I usually use ASD and the 0.6D+0.6W case too. (Does the D+0.6W case ever control anything?) But making it equal to unity means you have exactly the right weight to counteract the extreme wind force (1.0W) and it feels like there is no factor of safety during the hurricane or whatever. Another way to ask my question is: What code tells me the factor of safety needed for uplift? ASCE? Or do I just get to pick?

 

[driftLimiter]

Two things.

D+06W Can easily control on the compression side of the wall. Bearing Pressure chords etc.

means you have exactly the right weight to counteract the extreme wind force (1.0W) and it feels like there is no factor of safety during the hurricane or whatever.

No you have exactly the weight of the probabilistic wind load. We don't know what the wind load is really.

My take is that if you follow the design procedures and develop the loading correctly, then you have whatever level of Factor of safety the code writers intended. If you're concerned that you don't have enough (I don't think you should be) then design to a value less than unity. But essentially that means you are just picking.

 

[MotorCity]

Uplift is a serviceability issue, not a strength issue so you should be using ASD load combinations.