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I'm working on a lofted bedframe design.

Currently, I'm planning on creating the main frame using some wide boards (10"-12") joined with cross laps, leaving some overhang (from which I can later hang a facade)

Wide boards with cross lap

I'm planning on affixing posts (4x4s or the like) to the corners of the resulting rectangular frame either on the outside corner of the joined boards:

exterior corner post

or on the inside:

interior corner post

Is there any functional difference between these two options?

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    IANAE (I am not an engineer), but it seems to me that the "inside" (second image) joint would be somewhat stronger, although I don't know by how much. This is because the full width of the half-lapped board would be carrying the load, as opposed to only half its width if the connection was on the "outside" of the joint. However, this may also vary depending on fastener position. Maybe?
    – MattDMo
    Commented Jun 17, 2022 at 17:13
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    Hi, welcome to Woodworking. You ask if there's any functional difference between the two options and that's really THE question. How much combined weight does this need to carry? I don't want to make any assumptions about static load given we don't know the size or thickness of the mattress, adult/child occupant etc. But that aside for the moment, and looking at the basic problem superficially, inside mounting is stronger (for at least a couple of reasons) but given you're planning on using nuts and bolts to attach them I'm not sure there's any difference worth noting, expect in looks.
    – Graphus
    Commented Jun 17, 2022 at 17:25

1 Answer 1

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For a 4x4 post, at moderate lengths, you will not even come close to the structural failure points for this sort of project. As a test, put a 4x4 of the length you are using horizontally supported only at the ends. Stand in the middle and bounce. This is the worst case scenario. The more vertical you get, the harder it will be to cause failure.

At the top end, a short 4x4 post, fixed at both ends with load applied to the center can support over 9 tons. As the post gets longer and the load less centered, the capacity goes down. In either of your choices, the through bolts will distribute the load across the width of the post, and the side boards will decrease the unsupported length, both of which will increase the strength.

The important part either way is to make sure the connections are tight to essentially make the entire frame a single structure that so that adjoining sides can help reduce wracking. Beds have to withstand dynamic as well as static loads. The post is more than strong enough to handle the weight as long as the rest of the bed is well enough constructed to handle movement in other directions.

There is a good video on post strength recently posted on youtube by The Engineering Hub called How much load can a timber post actually carry

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    ??? strength question is not about strength of posts I think.
    – Volfram K
    Commented Jun 18, 2022 at 5:15
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    OP is asking about the strength of the entire joint, not the 4x4.
    – MattDMo
    Commented Jun 18, 2022 at 15:42

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