Cracked or uncracked capacities of Simpson STHD hold downs?

[AELLC]

In the 2011 Simpson catalog, where it lists the capacities of LSTHD/STHD hold downs, there are values under cracked that are in most cases lower than the values under uncracked.

Is this related to seismic governing, or what? Exactly what determines cracked or uncracked?

 

[dcarr82775]

Without actually looking at the catalog I suspect it the same as ACI318 App D. Basically if the region where the anchor is located would be expected to have cracking at service loads (top of a beam over a column, or bottom of a beam at mid-span for example) then you use cracked. If it is no expected to have cracks that you use uncracked. Often a judgement call IMO.

 

[AELLC]

I have no idea how to determine cracked or uncracked in a PT slab on grade, nor a conventional concrete wall footing.

Does anyone ever do such an analysis?

I am also assuming the above applies to both wind and seismic (I don't have recent AC318)

 

[CANPRO]

For conventional reinforced concrete, I think you ignore the reinforcing steel, then check your cracking moment based the modulus of rupture...if Mcrack < Mapplied, then you have a cracked section.

For a PT slab, maybe you add your PT stress to the modulus of rupture? Not sure though...

Would you count hairline cracks from shrinkage as 'cracked concrete'? If that was the case wouldn't you always have to assume a cracked section?

 

[AELLC]

CANEIT,

I still don't know because what would be a rational analysis of actual bending moment in a continuous concrete wall footing and stem?

The footing is bearing on soil so that in the case of uniform load, then there would be no moment in the stem?

Where beams or girder trusses are putting a heavy concentrated load on the stem, how could it be practical to analyze all that, plus it is bearing on soil, which is to me a difficult support condition to analyze.

 

[AELLC]

OK, I will use the cracked values to avoid all the complications.

The highest-rated hold down (STHD14) has the same values for both cracked and uncracked, so it won't be a bother to downgrade the lesser hold downs.

 

[SLTA]

Easiest answer - call Simpson at 800-999-5099 and ask to speak to their technical department. Those are a bunch of smart folks there, and if they don't immediately know the answer, they'll get you to an engineer that does.

 

[MiketheEngineer]

Like Slta said - pick up that 2 ton phone and call Simpson. They are very helpful!!

 

[AELLC]

OK I just called Simpson. They said the cracked/uncracked is a Code requirement for high seismic areas, otherwise it is OK to use uncracked values.

 

[msquared48]

My guess - go back to the 50% downgrade of sheetrock shear walls for seismic events - and the idea that after the first event, the wall is likely to be weakened.

I suggest that the cracked values are values that can be relied on after a seismic event of a certain level. But as to what that level is, 6 or 7, you would have to call Simpson as suggested.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com