360-16 'I'-section with bolt holes in one flange - should it then be considered singly symmetric?

[12345abc6ttyui67]

Hi Folks,

I have an existing structure which we are strengthening. The strengthening is to I shaped beams and columns. We intend to drill the existing sections and install haunch brackets and/or cover plates to the section flanges. These would be connected via appropriately sized HSFG (slip critical) bolts. Welding is not permitted on this site. In some cases the strengthening will be doubly symmetric (cover plates on both flanges) and in some cases it will be singly symmetric (either a cover plate on one flange only, or a haunch bracket on one flange and a cover plate on the other).

I will be sizing the strengthening such that utilisations for (Original Loads / Original Properties) + (Future Loads / Strengthened Properties) < 1.00. This is to account for the strengthening being "ineffective" for the loads which are already supported by the structure, and only being "active" for any new loads added after the strengthening is installed.

Where I am struggling to interpret the code is how to consider the section properties for the "holed" or "drilled" sections. We will be removing material from the flanges and obviously there is a temporary condition where the original loads are applied to the drilled, but not yet strengthened, structure.

I have seen the provisions for built up / cover plate impacts on tension capacity in 360-16 Chapter D3, compression in E6, and flexure in F13.

What I am not clear about is:
1) For cases where we are only drilling one flange, hence making the section singly symmetric, should I be considering it as singly symmetric and using the corresponding checks/limits from the code for singly symmetric members? Or is this still considered as a doubly symmetric member so long as I also follow the various checks for built up members / cover plates / effects of bolts holes where specifically mentioned etc?
2) I am aware of using the net area for the tension checks, but should I also be calculating reduced properties for all the other parameters (elastic/plastic modulus, 2MOA, radius of gyration) etc, calculated with the material at the holes removed? Or does the code assume a 'whole' or 'complete' section and then make whatever reductions are necessary for the bolt holes within the limits for built up members / cover plates?

I am comfortable that if the post-strengthened structure is singly symmetric (i.e. cover plate on one flange only etc.) then the singly symmetric checks will apply. It's specifically the pre-strengthening I beam/column with bolt holes in one flange I am unsure which way to classify.

Thanks in advance for any comments!

 

[Newmark918]

1) Likely I would analyze as singly symmetric for modifications to only 1 flange. Not sure what structure you're modifying, but maybe that could change the approach. You could analyze both ways for a fun comparison. In general, I can't imagine you're drilling enough under your temporary loads in an area of max internal loads such that F13 will control the design. E.g., max design live, snow, seismic, etc., can be treated with reduced load factors for a quick upgrade.
2) F13-1 limits Mn with a ratio of A.net/A.gross, so I read that as use the net flange area for that check, but use the gross and built-up properties elsewhere. This is assuming you're using typical holes and typical gages. If you have something atypical like slots longer than listed in AISC or squeeze 4 bolts into a gage instead of 2, I would reconsider.
3) Maybe I misunderstood your usage ratio up above, but I would take a close look at the strain profile before and after. I've only worked on a couple modifications like this, but we didn't use that form of an interaction equation. Adding on "future load / future properties" to something that was already near 100% DCR doesn't feel right since the future properties may already have a tension flange near yield. So the original flange could yield quicker than your equation might imply, and eat into the additional moment capacity you thought you had from the assumed "unyielded" composite section.

Have fun, neat problem. Let me know if I'm off base or missing anything.

 

[12345abc6ttyui67]

Thanks Newmark, and apologies, I don't know how to quote people on here...

1) I think we agree on that. The loads are actually supplied by another contractor, so for simplicity (and conservatism) I am aiming to make things work using the maximum loads, but if I need to fall back on reduced load factors in recognition that it's a temporary case then agreed it's something to look into. From what they've said I think it's predominantly driven by pure dead load though, so there's maybe only a bit of wiggle room there.
2) This sounds like what I am thinking as well, gross properties unless specifically stated to use something else. I've just skim read over the relevant sections and haven't gone through and applied the numbers in earnest yet but will see how I get on. Bolts and gauges are all standard for the section size, nothing too funky going on.
3) I think we are saying the same thing but I worded it poorly. For tension for example I am going to check (original tension force / original tension capacity) + (the additional 'future' tension force / strengthened tension capacity). My intent is this covers the fact the tension flange already has load in it (the first half of the check) and the fact there will additional tension applied later (the second half of the check). If I limit this to less than 1.00 I believe I have covered the full tension envelope for current and future cases. I'd do the same for compression, flexure, etc. These would then get rolled up into the combined utilisation checks (per Chapter H) in the same manner, where tension, compression, bending utilisations would be combined for both current and future. Short-handedly I wrote this per my OP, but I appreciate it might not have been the easiest to follow.