Wood column to resist lateral loads. 4

Hello group,
In Florida I notice many situations where porches open on 3 sides have 4x4 post with 2- 2x10 in post caps on the open long side parallel to the front wall of the house.. On the beams is a porch roof diaphragm. The bottom of the post usually has a simple PB 44 base plate which appears to be pinned.
How can these post take the lateral shear from the edge of the diaphragm and also provide any support to the diaphragm?
It seems to me the top of the post is free to rotate since there are no shear walls in plane with this edge of the diaphragm.
Also if the post was embedded in concrete or in a moment resisting base is there an internal moment ( maximum ) in the post at the base AND a resisting moment (thrust) equal to the post moment ( M=PL) at the interface of the post and resisting material being either the concrete or the base plate if one is used?
And last, where does the the P=V shear occur and is it in addition to the thrust force?
Thanks,
Jeff

 

[JAE]

Doesn't the lateral load get taken out via the roof diaphragm tied into the main structural roof diaphragm (i.e. cantilevered diaphragm) as opposed to pinned columns taking lateral shear?


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[XR250]

They don't. The porch roof diaphragm laterally cantilevers off the main roof diaphragm to provide the stability.

 

[ChiefInspectorJeff]

I didn't think you could cantilever a wood diaphragm 10 feet span. These roofs have no blocking in then or provisions at the main house shear walls for rotation transfer.

 

[dhengr]

ChiefInspectorJeff:
The idea is that the porch roof diaphragm can’t parallelogram much in the 10' length, as it acts as a shear transfer diaphragm. It is much stiffer than the columns, so it takes the lateral load back into the main structure. Of course, blocking and connections and the like have to be properly designed to tolerate these loads back in the main bldg.

 

[ChiefInspectorJeff]

Thanks for the response. I had a few other questions about the base moment at the post base:
Hello group,
In Florida I notice many situations where porches open on 3 sides have 4x4 post with 2- 2x10 in post caps on the open long side parallel to the front wall of the house.. On the beams is a porch roof diaphragm. The bottom of the post usually has a simple PB 44 base plate which appears to be pinned.
How can these post take the lateral shear from the edge of the diaphragm and also provide any support to the diaphragm?
It seems to me the top of the post is free to rotate since there are no shear walls in plane with this edge of the diaphragm.
Also if the post was embedded in concrete or in a moment resisting base is there an internal moment ( maximum ) in the post at the base AND a resisting moment (thrust) equal to the post moment ( M=PL) at the interface of the post and resisting material being either the concrete or the base plate if one is used?
And last, where does the the P=V shear occur and is it in addition to the thrust force?

 

[jayrod12]

If you provided a fixed base for the post then yes there would be a moment. M=PL would be extreme conservative as the diaphragm wouldn't allow that much moment into the column before it transferred it back to the house. It would take a lot of lateral deflection at the top of the post to engage a fixed base fully.

I'm not sure I quite understand your "AND" statement. without the resisting moment at the base there would be no internal moment so they must be present together, you can't have one without the other.

P=V shear would in theory happen at the two connections (the roof collector beam to post and the post to base). The shear at the top causes the moment and is resisted at the base. The column (assuming it could see max moment and shear as if the diaphragm wasn't there) would need to be designed for M and V of PL and P respectively.

Perhaps I'm way off base but that's how I see it.

 

[ChiefInspectorJeff]

It seems the shear at the base is a sliding force and the resisting moment (thrust) at the base of the wood connector is similar in direction In other words if a connection is fixed there is always a resisting moment at the interface. But if it is a pinned connection the moment is resolved in rotation at the pin? what is the force at the pin and how is it calculated?

 

[KootK]

In this situation, the force at the pin will be equal to the compression -- or tension -- in the column.

I believe that the porch condition that you're concerned with is that of a primary gable roof running out beyond the exterior wall in a direction parallel with the ridge line. No problems there, as others have discussed.

What is often an issue is canopies where a smaller canopy roof ties into the sloping side of a larger roof. Often, there is no appropriate home for the chord forces generated in the cantilevered diaphragm. It can be done properly but usually isn't as the detailing can be unconventional. It's usually better to make moment frames of the canopy columns in these situations, as you have suggested.

One of my first projects was a two story bank with a drive through canopy open to the elements on three sides. I cantilevered the diaphragm as has been discussed here. Unfortunately, the chords of the canopy diaphragm tied into the walls of the second floor around mid-height. By the time that I caught my mistake, it was too late to spare myself a little embarrassment and some serious self loathing.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[ChiefInspectorJeff]

Interesting. Could you anchor the end of the chord to blocking which would then transfer into the shear wall below?
I was under the impression that a normal wood flexible diaphragm needed support on all sides. If the diaphragm is designed in rotation wouldn't the chord moment would be high at the intersection with the wall?
And I thought there was a limit on the l/d ratio of a diaphragm in rotation?
But all the situations I have seen in the field have no provisions for extra anchorage. So it looks like the pinned post are receiving a lateral load ( either drag strut or chord force) at the beam to post connection and at the base of the post.

 

[woodman88]

If the cover is open on three side than the lateral force at the exterior posts will be minimum. There are many ways to account for this force. Uplift is more of a concern.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[ChiefInspectorJeff]

True, since uplift is on the lower and upper areas of the roof. But lets say the porch is 40 feet wide by 20 feet deep. With a wind load of 32 psf blowing parallel to the post line and with 4 post you could have a point drag strut load of 2.2K? This would induce a moment at the base of the post which requires fixity?
Am I off base?
I have never seen a wood diaphragm designed and built for rotation to avoid the post lateral loads.
Thanks,
Jeff

 

[KootK]

Yeah, there are diaphragm aspect ratio limits. 4 rings a bell.

Given the questions the you've been asking, you need this: Link

Many of theses issues are easier to discuss meaningfully if you post a sketch illustrating your perspective/concerns.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.