Wood column to resist lateral loads.
Wood column to resist lateral loads.
(OP)
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
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






RE: Wood column to resist lateral loads.
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RE: Wood column to resist lateral loads.
RE: Wood column to resist lateral loads.
RE: Wood column to resist lateral loads.
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.
RE: Wood column to resist lateral loads.
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?
RE: Wood column to resist lateral loads.
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.
RE: Wood column to resist lateral loads.
RE: Wood column to resist lateral loads.
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.
RE: Wood column to resist lateral loads.
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.
RE: Wood column to resist lateral loads.
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.
RE: Wood column to resist lateral loads.
Am I off base?
I have never seen a wood diaphragm designed and built for rotation to avoid the post lateral loads.
Thanks,
Jeff
RE: Wood column to resist lateral loads.
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.