Can someone please explain why the top left image is correct and the bottom left image is not? Both of these images have the bolts on the lower half of the wood beam (tension) side.

enter image description here

  • $\begingroup$ Top right hand or top left hand? $\endgroup$
    – Solar Mike
    Commented Oct 28, 2020 at 19:55
  • $\begingroup$ Sorry. Top left vs bottom left (with big X) - edited. $\endgroup$
    – SSan
    Commented Oct 28, 2020 at 21:05
  • $\begingroup$ Your error is in assuming the bottom half is always in tension. The point is that over a support, if the beam is made continuous (by interference fit and/or by adding rigidly connected plates) then it will be hogging, and then the bottom half of the beam will be in compression. That is, the top left is correct precisely because the bolts are not in the tension half - and the bolts that are in the tension half (which is here the top half) are slotted so they don't split the beam (and also to discourage them making the beam continuous). $\endgroup$
    – achrn
    Commented Oct 29, 2020 at 12:25
  • $\begingroup$ So, is the bottom half of beam in tension for the lower left image? $\endgroup$
    – SSan
    Commented Oct 30, 2020 at 2:28
  • $\begingroup$ The bottom half of the beam is in tension for all of a simply supported span, and most of a continuous span (for most loading cases). So whether the lower left detail is in tension or not depends upon the position within the span, the detail of the beam over supports, and the loading (potentially in both this and adjacent spans). $\endgroup$
    – achrn
    Commented Nov 1, 2020 at 9:00

1 Answer 1


From what I see in the picture I am getting the impression, that these two are for completely different uses.

The one above seems to refer to connection of two wooden beams on top of a column, such as the following


In that case the entire side for each beam will be moving away from the the center line, however the points near the bottom will be moving the least, therefore less tensile stresses will appear.

The second case (lower left) seems to be a bracket which is used to support a hanging weight, somewhere along the length of the beam. If that is indeed the case, I am guessing the reason why they have that as an incorrect, is that the bottom part of the beam will most likely be in tension, and as such positioning a weight there is like putting a wedge. In that case the top half of the beam will be a better position to put the weight is at the bottom of the beam (see below).

enter image description here


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