Calculating Chord Tension/Compression at Portal Frames with Holdowns (PFH)

[medeek]

Assuming a portal frames header and pony wall act in a rigid manner can someone tell me if my free body diagram for the uplift at the holdowns makes sense. Compare with the uplift calculated by Shearwalls assuming segmented shearwalls at the two locations. The thing worth noting is the significant decrease in the uplift/overturning forces with the use of a portal frame. Mostly I'm just looking for a sanity check here in case I'm missing something.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[manstrom]

I love this stuff, I just never have the time to research it enough.

I follow you. It makes sense.

I would also add in dead load to reduce the hold down force.

Another possible approach would be to consider the frame as FTAO and fix the connection between the wall piers and the pony wall above. It is basically a pin as it is now. Add vertical straps between the piers and the wall above or lap the header into the wall piers a la PFH. It should reduce your hold down force, or at least provide a more redundant connection.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[medeek]

This is without considering the tension straps between the header and the piers, those will further reduce the holdown forces, see my previous post on PFH analysis.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

Actually the shear panel widths are a little less than shown above but the idea is the same.

Here is the final drawing for this particular portal frame garage wall:

Interestingly the contractor is very preferential on how they like to frame things up. For instance they will probably want to raise the garage door header to the double top plate and then frame down the opening to get the correct garage door rough opening height. As far as perpendicular wind loads this is probably a little better but it does change up the holdown forces some.

Technically the new shear panel height for a raised header would be the distance from the top of the stemwall to the bottom of the header however the framed in opening with cont. sheathing applied will tend to stiffened up this portion of the wall and decrease the holdown forces somewhat similar to what I've shown in the free body diagram above. Not sure how to best quantify that yet.

Portal frames for residential construction are a very interesting animal in my opinion and based on case studies like this one and others I have encountered I'm beginning to get a feel for some of the issues and variables that drive these designs, however I feel that I still have a way to go before I can create the next portal frame calculator.

Another issue I've recently encountered is also shown in the drawing above. The contractors in our area like to block down to the footing for their doorways instead of utilizing a deeper stemwall foundation which has more bending moment and shear capacity. Per Malone's book this sort of foundation arrangement at Portal Frames is basically a no go. I'm curious as to what others have experienced with similar situations. On this particular job, after running a few more checks, I felt that I could let this configuration pass given the relatively light shear load to this shear wall line, but typically at a portal frame I like to see a full stemwall underneath the opening with at least one horiz bar at the top and one at the bottom, like the detail below:

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[XR250]

Seems like a statically indeterminate problem as the hold down forces will vary with stiffness of your header. I am pretty sure code portal frames are only intended to take the moment out at the header, not at the base.