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I have an unfinished two-story detached garage and (if possible) I would like to make the structure more resilient by going a step beyond the basic/minimum building code safety standards for my area (presuming it makes good sense to do so). Is it possible to make structural improvements to the I-Joist being used between the first floor garage area and the second floor studio apartment area above?

Current installation is 14-inch TJI 210 16-inches apart (on center) with about a 20 foot span. I need to run 4-inch sewer line through the I-Joists. The PDF document I reference below shows where those 4-5 inch holes can be cut into the I-Joist. Will this weaken the I-Joists far into the future (20-30 years)? Can I (or should I) do something to strengthen the I-Joist in locations where the 4-inch sewer line will be passing through the I-Joists?

Patching the I-Joist appears to restore the integrity of the I-Joists, but can the same principles be used to strengthen the I-Joists as well?

TJI® JOIST SPECIFIER’S GUIDE Featuring Trus Joist® TJI® Joists for Floor and Roof Applications http://www.weyerhaeuser.com/woodproducts/document-library/tj-4000

BTW, I am not trying to make the building resistant to a meteor crash with withstand a MOAB (Mother-Of-All-Bombs), but I do want to make smart improvements and retrofits beyond the basic building code safety requirements.

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    Were the joists installed knowing that there's an apartment area above, or was that added afterwards? Reinforcing the joists just for reinforcements sake seems like a waste of time and money, but if there's a structural reason for the reinforcement, that's different
    – mmathis
    Commented May 15, 2017 at 17:01
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    Are you asking about strength in terms of collapse resistance? "Minimum code standards" almost never allow for collapse of a floor. Unless you plan on keeping a fleet of large aquariums up there, it's not a concern. Or are you trying to reduce flex/bounce? Please edit your post to explain your mission a bit better.
    – isherwood
    Commented May 15, 2017 at 17:26
  • 14" on 16 centers I believe is well above the minimum. When I built my shop I thought I could have used 12" on 24" centers but that was almost 20 years ago my memory may be foggy because I did go with 14" on 16 centers, the unsupported span was only 16'.
    – Ed Beal
    Commented May 15, 2017 at 19:04
  • Just put the holes in the right place per specs and it does no harm to the joists at all.
    – Ecnerwal
    Commented Jun 17, 2017 at 23:32

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Page 9 of the specification guide you linked describes Allowable Holes. If you comply with that specification then you are not considered to have weakened the joist. If you violate that specification then you would need new sealed engineering prints to stay compliant.

If you just want to increase rigidity or load-bearing capacity there are a number of common practices:

  1. "Sistering" joists: I.e., running an identical joist alongside an existing joist.
  2. Reducing the unsupported span of joists. E.g., most simply, by adding a load-bearing post under a joist span.
  3. Upgrading joists. E.g., replacing existing members with joists having higher specified capacities. Or adding supported joists under the existing ones.

You would need an engineer to quantify the effects of any of these reinforcements. Some of these are very implementation-specific. For example, depending on the construction, sistering might provide much more support when the sistered joists are cross-bolted in a particular way.

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You're thinking vertical loads. A much bigger problem is lateral loading (loading that moves the structure side to side), especially because your space is over a detached garage.

A garage (and especially a "detached" garage), by its very nature, has few (and often skinny) "lateral resisting walls". If you want to worry about something, worry about a force (wind, earthquake, meteors) hitting your building from the side. The weak link to your building is the holddowns from the lateral resisting walls by the garage door openings to the footing and the "tie" from the top of the wall to the beams and floor above.

If the garage was built more than 3 years ago, (most Building Codes are revised and updated every 3 years,) then the lateral resistance does not meet current code. (This is the one area that they revise almost every 3 years. To test my theory, check to see if the walls by the garage doors are 8" thick concrete with steel anchors and plates from the wall to the foundation. If it's 6" or concrete block, or no steel connectors, then it's not to code. )

Btw, the size and location of holes in engineered joists are determined by each manufacturer...and no, they don't "go bad" after 30-40 years.

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