Shearwalls - add in roof uplift?

[CEmonkee]

Hi - quick question for all of you shearwall experts out there....

Do you consider roof uplift when calculating the loads for your shearwall holdowns?

What I usually do when calculating my shearwall holdowns is take the unit shear times the wall height, then subtract out 0.6 times the wall dead load that is tributory to the side of the shearwall I'm calculating the holdown load for. (I usually neglect the roof dead load.)

However, another engineer I recently spoke with said that for shearwalls parallel to the ridge line he always adds in the roof uplift as well.

Any comments?

 

[msquared48]

I don't as the individual studs should be able to take that through hurricane ties to the foudation.

Mike McCann
MMC Engineering

 

[msquared48]

It also seems to me that the uplift you are describing is due to wind normal tu the shearwall. In that the shearwall holddowns are primarily resisting uplift forces due to the wind forces parallel to the wall line, it does not seem logical or necessary to me to add them.

Mike McCann
MMC Engineering

 

[CEmonkee]

Hi Mike - thanks for your replies. I've never included the roof uplift, so I'm glad to hear that you don't either

Re the direction of the wind... parallel to the ridge (the longitudinal direction in Figure 6-2), the wind pressures shown for the 0° roof angle are negative, so I was assuming that an uplift would occur on the roof for wind loading in that direction as well.

 

[JAE]

You absolutely should include wind uplift in your shearwall calculations. A few things to remember though:

1. The wind should not be C&C wind but MWFRS wind as the design you are performing is for an overall main wind design.
2. The uplift is usually taken by the individual studs and the shearwall anchor bolts, not the holddowns. The holddowns are typically there for the uplift due to the overturning of the lateral force.
3. The shearwall anchor bolts between the holddowns would then take lateral shear and uplift combined.
4. The shearwall anchor bolts between the holddowns might then have to be a little more densely spaced than in non-shearwall walls.
5. The wind uplift is based on the tributary area of the exposed structure adjacent to the shearwall. Where roof members are parallel to the shearwall, there may not be much uplift to consider. Where roof members are perpendicular to the shearwall there would be higher wind uplift countered by the roof dead load (i.e. use net wind uplift based on the appropriate load combinations)

 

[CEmonkee]

Hi JAE - thanks for your response. I was specifically asking about the holdown calcs... the other engineer I was talking to thought that the calcs for those should include roof uplifft. Item #2 of your response confirms what I originally thought (the roof uplift is not considered in the holdown calcs).

 

[ishvaaag]

Here (Spain, current code CTE) anything having earthquake as a load wouldn't have concurrent wind action for the strength check. But as always complete load paths to foundation (anchor or counterweight in this case) are required top to bottom for everything figuring in a required hypothesis to check.

 

[msquared48]

I can think of one common scenario where the uplifts would be additive. If you have a shearwall at a building corner, say of a residence, that has a wrap-around roof over a open deck, then the uplift on the roof from the wind parallel to the shearwall would be additive to the corner holddown.

However, I rarely take any contributory dead load into consideration at corner shearwalls either...although there is some, just rarely enough to make a substantial ifference.

Mike McCann
MMC Engineering

 

[Ron]

Mike...I was thinking of similar, except wind at oblique angle to corner on hipped roof.

JAE is correct as usual! The individual load for the anchors is subjected to tributary shear and tension, thus capacity is reduced by interaction.

Keep in mind that, in theory, if holddowns have significantly higher capacity than the anchors, they will accept a portion of the uplift load, as they are mobilized first under certain wind directions and roof configurations.

 

[woodman88]

CE, If you designed your connections for the roof uplift to the foundation (or to where .6 the dead load removes any uplift) than adding the this uplift to the shearwall holdowns would be conservative, in my opinion.

Garth Dreger PE
AZ Phoenix area