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I'm working on a project where I need to hang some 6x8 joists off of other ones currently in place, using steel hangers pictured below. These are all 5 1/4" w x 7" d and the first row of 3/8" lag screws would sit at 1 3/4" from the top of the beam while the second row would sit at 5 1/4" from the top, which is below the neutral axis of 3 1/2" (I assume).

Previously, I didn't think much of this until I read the following in the NDS manual Table 12.5.1C.2 (numbered page 88):

Heavy or medium concentrated loads shall not be suspended below the neutral axis of a single sawn lumber or structural glued laminated timber beam except where mechanical or equivalent reinforcement is provided to resist tension stresses perpendicular to grain....

My question is about there being two rows of bolts, one above the axis and one below: because of the upper row is the lower row not a problem? Or must I find a hanger with a top flange, or worse put a larger beam in place?

In the diagram the first dark colored beam is the existing joist that the new, light-colored joists will hang off of. The two light-colored joists to the right will also hang off of their respectively loaded joists.

Steel Hangers Diagram

Jesse Yishaeyn
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Having done some research and having spoken to an engineer it appears it is too risky to make this appraisal on one's own.

The neutral axis is not always the mid-plane of the beam but varies based on the density and structure of the wood. Therefore picking the mid-plane as a reference plane is not useful. This research paper explains this principle.

Jesse Yishaeyn
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the rule is to prevent you from attaching large loads that would tend to cause the beam to split

Looking at the photo this may be too late already. but then again this joint has a header below it so probably not an issue, but shimming under the ends would not hurt.

These hangers have larger fastenings and fewer than I am used to, but (assuming those are through-bolts, and not lag screws,) if you want to ensure that the load is on the top drill the bottom hole oversize.

regular nail on hangers have the bulk of the nails go in the top half for a reason.

Use taller hangers so that the load is higher on the joist.

Jasen
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