Simpson Hold-down Nominal Capcities

[sgs114]

Hello,

We are doing an ASCE 41-13 upgrade to a wood building and need to add hold-downs to the ends of some shear walls to resolve the overturning forces. All Simpson holdown values published are allowable loads and we need to convert them to the nominal value. Does anyone have any references on how to handle this? Simpson says to multiply the values by 1.4 to convert they to LRFD, but that isn't the nominal value and provides a solution that intuitively looks over-designed. I looked through the Simpson ESR and cannot find a factor of safety that I could simply factor out. Has anyone encountered this in the past? How did they handle it?

SGS

 

[Deadblow]

SGS,

I'm not sure I'm fully understanding the problem. Are you designing using allowable strength design (ASD) or load and resistance factor design (LRFD)?

If you list the loads in ASD and the allowable capacity as listed by Simpson, the solution should not look "over-designed". Use the same process if you designed per LRFD.

EIT

 

[BAretired]

You have the allowable value. Multiplying by 1.4 gives the factored value. What do you mean by nominal value? It is not a term I am familiar with.

BA

 

[structSU10]

ASCE 41 utilizes the 'full' capacity of a component, so he needs the full capacity, with no safety factors applied.
In their cold form hold down catalog they note the nominal tension load for hold downs based on testing. I believe these hold downs are generally the same as the wood hold downs.

 

[MrHershey]

Would recommend contacting Simpson directly for this. If it's not published, they usually have test data that they can send you. Sometimes they just want to doublecheck what you're using it for.

Some companies still do it, but the problem I've seen with publishing tables of the ultimate/tested values is you'll get people just pulling values from the tables for day-to-day stuff with no safety factor. See it all the time with ITW's Tapcons and contractors. They'll pull the number from the ultimate table and assume they're good while missing/ignoring the 'safe working loads...should not exceed 25% of the ultimate load capacity' footnote and get upset with me when I tell them they need 2-4 times as many anchors as they thought they needed.

 

[moments]

Look at the notes to the designer in the front of the catalog, and they tell you the factor of safety applied to the nominal capacity is 3.

 

[EngineeringEric]

It may be me, but i would not multply the allowable by 3 to get the ultimate and use that as part of an LRFD load combination. I'd take the published value (500#) and mulitply by 1.4 to get it into LRFD Resistance (700#). Then P_factored < 700#

 

[cnorvell]

I've done a fair number of wood buildings with ASCE 41 and this has been my approach to hold downs:

ASCE 41-13 C12.2.2.5.1 gives overall guidance and recommends a procedure in the 1996 LRFD Manual for Engineered Wood Construction for converting the tabulated ASD capacity for pre-engineered metal connectors to an LRFD capacity. Usually that entails dividing the tabulated seismic capacity by 1.6 (to bring it from the tabulated 160% value to 100% value, in other words removing Cd = 1.6), then multiplying by the format conversion factor (Kf), the typical NDS way of converting from ASD to LRFD. Then you apply your time effect factor for LRFD (lambda) which is 1.0 for seismic loads. Now normally you would multiply by a phi-z factor to get an LRFD factored strength, but since we're in ASCE 41 world we actually want an expected strength, which we get by using phi-z = 1.0.

Per ASCE 41-13 Table 12-3 footnote "e", connectors not listed are assumed to be force controlled, meaning that the lower-bound strength must be used, which per ASCE 41-13 12.2.2.5 is obtained by multiplying the expected strength by 0.85. So just multiply the expected strength calculated above by 0.85 to get the lower bound strength you can use on an apples-to-apples basis with your ASCE 41 loads.

 

[AaronMcD]

May I revive this to piggy back on it?

cnorvell, I thought unfactored LRFD loads were "lower bound" capacities as defined in ASCE 41, so it looks like you are using 85% of the lower bound.

Here is what I understand (which isn't much, just a few days of studying this ASCE 41):

For force controlled actions, lower bound capacity is matched up against a slightly lower demand (divide out C1, C2, and a J factor) 1:1.
Deformation controlled actions use expected strengths instead of lower bound strengths, use the full seismic load with the C factors, and the DCR can be as high as kappa*m (or just m for new components).

Now, what I am wondering is - can I consider a holdown as deformation controlled by considering it to be nails or screws from metal to wood, and use that m-factor? I would then essentially be assuming the holdown itself would not fracture. I would consider expected strength to be

Unfortunately I think it may not be possible to get enough capacity out of force-controlled anchors to fully comply with ASCE 41

 

[MrHershey]

I'd be fairly skeptical at treating a hold-down as deformation-controlled. Perhaps the anchor rod to foundation could be, but the hold-down hardware itself likely won't undergo significant inelastic deformation like deformation-controlled actions are typically intended to.

That said, my feel is published capacities are likely a fair bit lower than what ASCE 41 would consider a lower bound. Simpson, for instance, says their strength-level hold-down capacities are the lowest ultimate load of three tests (or average of six) divided by 3 (Link). I don't know what ASCE 41 would do for hold-downs, but I know for the steel testing I've done the 'lower bound' has been the average yield minus a standard deviation which is a lot different than what Simpson is doing.

Would recommend getting in touch with the hold-down manufacturer, they can probably provide more help than this forum can.