Calculating Hold Downs at the end of shearwalls 1

[trussdude]

Can anyone clarify whether I am on the right track? When you calculate shear due to wind, this is the method I used to calculate the demand V. Wind pressure according to MWFRS with all appropriate factors X width of the face of the building X total height of the wall+any roof. Then I used the same method of the lower stories until I reached the foundation. The Hold down force at the end of the shearwall is a cumulative of the forces for the upper walls? For a two storey building on the Texas coast with about 40 psf wind pressure, I am getting hold down forces of the order of 30,000 lbs, for a 27' mean roof height x 30 x 30 building. Is that in the ball park? What sort of reduction factors I can use? Can I use 1/2 height of the building in V.h/Co.Li = T = C? This may be pretty basic stuff, but it is always good to check one's basics.

 

[Lion06]

The shear (V) is not the required hold down force at the shearwalls. You have to calc the net overturning moment and then divide that by roughly the length of the shearwall to get the uplift force acting on the end of the shearwall.

 

[JAE]

Yes, - don't forget to include your dead load as a reducing force in your holddowns. Per the IBC, the applicable load combination would be (for ASD): 0.6D +/- W

 

[msquared48]

Depending on the length and amount of shearwall in any individual line, 30K would not be unreasonable for a 40 psf wind load. I would suggest that you should ask for more wall though, as this puts the holddowns outside the range of normal Simpson holddowns. I would try to get the max uplift down to 13 to 15 K if possible to keep it simple.

Regarding the methodology - yes the holddown forces are cumulative, as are the OTM resisting dead load forces. However, remember that for the lower story holddowns, the OTM forces from the shearwall system in the floor above must be added to the overturning force generated by the second story shear forces to obtain the proper holddown force. Otherwise, the holddown will to be too small. This is not good...

Mike McCann
MMC Engineering

 

[boo1]

consider looking at this web site:

http://www.mcvicker.com/vwall/page000.htm

 

[mudflaps]

Don't forget about the foundations. For a 30 kip net uplift you'll need 45 kips of dead load to keep OTM =< 2/3 RM. That's lots of concrete!

old CA SE

 

[NKT]

To get 30k uplift with two stories, you end up with an average of 1500plf in your shearwalls (assuming 10' walls). This is a lot for wood framing; I would try to get more shearwall length or consider another lateral-force-resisting-system.

You can get some of the holdown forces to cancel out if the upper and lower walls are offset with just one end overlapping. In the simple case of one shearwall each level, you get at the foundation: one upper shearwall holdown force, one holdown force equal to the difference between the upper and lower wall holdown forces, and one lower shearwall holdown force.

Instead of using an enormous chunk of concrete to resist the holdown forces, I've used two approaches:
1. Recognize that holdown forces come in opposing pairs, determine the moment, design the footing as a grade beam to resist the moment, and check the entire line of footing for overturning. Reinforcing will be substantial.
2. Install helical piers at the holdown points.