asd to strength conversion

[shacked]

I have a question concerning the conversion of loads obtained through asd method to strength level.

Specifically uplift forces from a shearwall. I use asd load combinations to obtain my uplift forces at shearwalls now I want to design anchorbolts per aci 318-05 appendix D which uses loads that are at the strength level. In the past I have been multiplying the load by 1.4 to bring it to the strength level, then after the result is obtained, divide that by 1.4 to bring it back to allowable stress level. Is this correct? I was always taught that to bring a load to strength level multiply it by 1.4, was I taught wrong?

 

[Lion06]

I wouldn't multiply by 1.4, I would use a higher value. The load factors are 1.2DL and 1.6(Lateral Load). An extremely large portion (if not all) of overturning is attributable to Lateral Loads, so I would use 1.6 if I used your methodology. That being said, why not just run the analysis with LRFD combinations and then pull the actual, factored loads right out of the analysis?

Finally, once you multiply by your factor to get up to factored loads, why would you divide again to get it back to service loads? You already have the service load from the analysis.

 

[shacked]

We are trying to make an anchorbolt schedule to reflect the changes in the code, ACI-318-05 appendix D.

So I want to know the allowable shear value that the anchorbolt can withstand to determine the spacing for shearwalls.

The way our spreadsheets are set up in our office would require a lot of time to convert to strength combinations, and since time is money, especially now in thie economy, we are sticking with allowable stress design.

 

[Lion06]

Are you not using an analysis program? If you are, just put the LRFD combinations in that and let it spit out a factored load.

Why would you need to get back to ASD once you design them? You don't publish capacities on your drawings, do you?

 

[shacked]

For example:

Wood shearwall 'A' 1/2" struct 1 plywood has a capacity of 340 plf which is the ASD allowable shear capacity. Now I need to compare the allowable shear for a 5/8" dia anchor bolt to the respective shearwall using various anchorbolt spacing.

If a 5/8" dia AB with 1-3/4" edge distance has an allowable shear load of 700#(at ASD capacity) then these AB's would be spaced at 24" O.C. for SW 'A'

 

[JAE]

If you don't want to go back and directly calculate the "real" factored load on the bolts, use 1.6 and you will be safe.

 

[WillisV]

1.6 is definitely safe. The ASD to LRFD conversion for steel design is targeted at 1.5.

 

[shacked]

So I am right about dividing the strength shear by a (1.5 factor now) to bring it to ASD value.

 

[Lion06]

I wouldn't use 1.5. I would use 1.6. Unless you can show that any portion of the shear is from something OTHER than a load that gets a 1.6 Load Factor, then you need to use 1.6.

As an alternative, since it looks like you are using the capacity of the shearwall as the basis for anchor bolt design, consider this:
Multiply your allowable shear wall load by 1.6(making the assumption that it is all from wind, again unless you can demonstrate that any of this is from something OTHER than a load requiring a 1.6 LF). Then just do the same thing that you've been doing with ASD, but use the capacity from App. D and the "Factored" load from your shearwall capacity.

If you don't want to do that, then you should really use 1.6. The 1.5 that willis mentions is the "break even" point where ASD and LRFD will give you the same answer. I don't think that's necessarily applicable to your condition.

 

[shacked]

Ok, thanks guys.

 

[sandman21]

If the lateral load is seismic than multiplying it by 1.4 is correct.

 

[shacked]

Yes the lateral load is seismic. Is the 1.4 just the factor for the dead load?

 

[OldPaperMaker]

I think that we are mixing seismic load with the Strength level capacity.
When one calculates a seismic load, E it is always a Strength level. If we are using ASD then we divide E by 1.4 (or multiply by 0.7) to get our load to the wood shearwalls.

The complication is that the 2006 IBC, in section 1911.1 directs us to use Strength design to determine anchor bolt capacity when earthquake loads are involved which directs us to ACI 318-05 Appendix D. Then, if you are in Seismic Design Category C,D,E or F the anchor bolt shall be designed to be governed by the strength of a ductile steel element or the strength of the anchors shall be at least 2.5 times the factored forces. This is a Strength level capacity. Therefore, I think that one must divide this value by 1.4 to use to calculate the anchor bolt spacing from your ASD loads from the wood shearwalls.

Of course, this should be compared with the NDS values in Table 11E "Bolts for Lumber to Concrete".

 

[sandman21]

Yes seismic loads are at strength level from ASCE 7, they are then reduced to ASD level by dividing by 1.4 or multiplying .7. The 1.4 factor applies to the alterative load case which appears in the IBC/CBC. The 1.4 is applied to the seismic load not the dead load; the dead factor can range from ~.4-1.2, this factor depends on what you are using and the worst case should be chosen.