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Simpson Hold-down Nominal Capcities

Simpson Hold-down Nominal Capcities

Simpson Hold-down Nominal Capcities

(OP)
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

RE: Simpson Hold-down Nominal Capcities

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

RE: Simpson Hold-down Nominal Capcities

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

RE: Simpson Hold-down Nominal Capcities

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.

RE: Simpson Hold-down Nominal Capcities

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.

RE: Simpson Hold-down Nominal Capcities

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.

RE: Simpson Hold-down Nominal Capcities

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#

RE: Simpson Hold-down Nominal Capcities

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.

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