Three questions about wood detailing/design 1

[Buleeek]

Hello everyone,

I have three questions regarding wood detailing/design:

1. (See pic. "1") In the situation presented in picture "1", is it okay to assume that both beams (separated by a pile) carry the same amount of load from the joists ? To me it seems like the interior part/ply of the beam takes majority of the load, and the exterior one is being lifted (lever). See picture "1".

2. (See pic. "2") When joists rest on, let's say, 3-ply wood beam, is it okay to assume full joist bearing area across all three plies? To me it is not quite right, the first ply takes the most of the load. See picture "2"

3. Have you ever notched a 6x6 column to a 4x4 post, so it can serve as a railing post at the top deck? I'm worried about concentration of stresses in the notch. If not, what would you recommend to connect a 6x6 column to a 4x4 railing post (railing post on top of 6x6 column).

https://files.engineering.com/getfile.aspx?folder=e891da4d-72e3-44d4-90c3-a7b6883909aa&file=2.JPG

https://files.engineering.com/getfile.aspx?folder=e0f05330-ed4a-4338-98a5-7849c2268464&file=1.JPG

Please advise.

 

[JoshPlumSE]

1) The first one is going to take more of the load. But, let's say the joist spans 10 feet and the beams span 15 ft. The joists that hit the center of the beam span is going to be more evenly spread because the inside beam will deflect more than outside one. My patio cover has lots of connections that look like this. Ultimately, none of it really matters because the beams are sized more to "look right" than they are to resist the load. Meaning they are way over designed.

2) For bearing, I agree with you. Design the outside beam to take the full bearing.

3) I've seen large posts like that notched before to make a better connection. Personally, I think it's more architectural than anything. It's looks sturdy, but I think the bolt is designed to take all the load, not the "corbel" formed by the notch. Granted, I have never done that type of design. And, I would not feel comfortable relying on that notch to resist the shear reaction unless I did some more research on the subject.

That being said, I think there are ways to make the connection LOOK like the shear is being resisted solely through bearing even though there is a hidden connector somewhere taking all the load.

 

[BAretired]

1. (See pic. "1") In the situation presented in picture "1", is it okay to assume that both beams (separated by a pile) carry the same amount of load from the joists ? To me it seems like the interior part/ply of the beam takes majority of the load, and the exterior one is being lifted (lever). See picture "1".

You are correct. The interior beam carries more load than the exterior. Some allowance should theoretically be made for that, unless blocking is installed which make the two beams act as one.

2. (See pic. "2") When joists rest on, let's say, 3-ply wood beam, is it okay to assume full joist bearing area across all three plies? To me it is not quite right, the first ply takes the most of the load. See picture "2"

Under load, the joists slope at the support. If the beam plies are not tied together by fasteners, the inner plies tend to carry more load than the exterior. The code specifies minimal fastening for beam plies, which tend to make plies rotate as one. Normal engineering practice assumes the load is shared equally by each ply.

3. Have you ever notched a 6x6 column to a 4x4 post, so it can serve as a railing post at the top deck? I'm worried about concentration of stresses in the notch. If not, what would you recommend to connect a 6x6 column to a 4x4 railing post (railing post on top of 6x6 column).

Nope, never have. I don't see how it can be done, other than using some sort of hardware.


BA

 

[phamENG]

1) Yes, but I try to avoid such connections, especially when outside the building envelope. When designing with wood, bearing is your friend!
2) Multi-ply wood beams should be fastened together to act as one. The residential codes have prescriptive guidance on this, or you can design it based on the eccentric reaction you're talking about. Size and number of fasteners should get the load on the outermost ply back through the rest and resist twisting.
3) No. Don't do it unless you're going to regrade - and potentially reject - each piece of lumber. The stress values we use are intended to capture the worst case possible scenario. So a Number 2 SP 6x6 has a certain allowable knot size an location. When you cut it down to 3.5"x3.5", that 'acceptable' knot may now be in an unacceptable location and the actual allowable bending stress is something much lower than the published value.

Your guard posts should be fastened into the floor framing and tied into the diaphragm. DCA 6 from AWC has details for this with tension ties, and Simpson has tested at least one arrangement with some of their screws - Timber screws, I think - that meet code requirements for the connection.

 

[dauwerda]

1. I agree with JoshPlumSE, for the sake of analysis and design you can assume that the two beams will take equal amount of load as the interior beam will attract more load, but as it does it will deflect more and the load will redistribute to the outer beam. In reality the interior beam will be resisting slightly more load than the outer beam but it's not enough that you should adjust your analysis/design to account for it.

2. Chances are that when your 3-ply beam is built the tops of the plies won't all be perfectly aligned anyway. The ply that is sticking up the most will initially take the load, but either through fasteners to the other plies or due to deflection, this load will eventually equalize into all 3 plies. On the same note, the bearing may initially be only on one ply, but through deflection and/or local crushing the bearing will eventually utilize all 3 plies (if it needs to). Localized deformations that help redistribute load are perfectly acceptable. It would be very over conservative to design the joist for bearing utilizing only one ply of the beam as a bearing surface (assuming one ply shows an inadequacy).

3. phamENG nailed it.

 

[Buleeek]

Thank you for all responses, it was very helpful.