Help Me Ask My Engineer If This Is Built Right
February 22, 2012 4:20 PM   Subscribe

Can you keep this old house from falling down -- or help me stop pointlessly worrying about nothing -- by helping me ask my licensed structural engineer this question the right way? (Long.)

History, and why I am asking Metafilter instead of just the engineer
I'm rehabbing an old house. One to two rooms in particular seem to have questionable framing. We invited the structural engineer out to give us advice about how to fix it. We've been working with him since we did the foundation. He gave us advice, which we passed along to the builder. In my understanding of things, it seems like the builder misinterpreted the advice. A builder friend came over. On the way in, he gave a speech about how homeowners often get hung up on irrelevant details. Then he saw what we were asking about and agreed it looked questionable.

The engineer came back out, after multiple requests, and pretty much immediately said "yes, it's fine." He didn't seem to look very closely. My eyes hadn't even adjusted to the dim light yet, so I'm not sure how he even could see. That's part of why I'm still thinking about this. ("Did he even look at it?") But he's a smart guy, and I'm no engineer. Maybe it was just obviously fine, and he could easily see what he needed to see.

I asked about what I see as the potential fatal flaw, and he said, "oh, hmm, yeah, maybe put in a clip there." We asked him about putting a clawfoot tub and tile upstairs, and he'd said it was fine. He supplied us an official, stamped letter saying that the framing improvements were accepted.

I later emailed him to say "just to confirm, this will support an additional 1000 lb clawfoot tub full of water, and deflection will be less than the L/360 maximum for tile?" (An online deflection calculator had given me an idea that our deflection would be too high.) He wrote back, "yes," and then wrote again a few minutes later to say "actually, you probably don't meet L/360, so if you want that, add an extra 2x8 in between each of the floor joists." (This is pretty difficult given all the piping, so we didn't immediately just do that.)

So, on the one hand, it makes me relieved to know that at a certain point his "it's fine" will change to "it's not fine." On the other hand, it makes me nervous to hear "sure, you're fine for tile" followed later by "oh, you want L/360? No, you don't have that," when L/360 is the standard requirement for tile. That's according to the tile association, and it's all over the internet. It's not that I care so much about my precious tile cracking. It's that I wonder, what standard is he using to say that we're "fine" on other fronts?

And if he's quickly saying "you're fine," then revising that when thinking about the details a bit more specifically, then could that also apply to the construction details? Like if my follow-up email had said "just to clarify, the 2x6s are fine?" might he have said "oh, 2x6s? No, you need 2x8s." But again, maybe I'm worrying about nothing.

What the room in question actually looks like
The floor joists are 24" on center running from the east end of the house to a partition wall to the west. They're 2 x 5.5" in actual dimensions. The span is 9'6".

These are interrupted by a beam running perpendicular, N-S, which I'll call "the header." It's maybe 3' from the west wall. It's 2"x7.5" in its actual size, but notched up to 4" in places.

So, we have tail joists running from a partition wall on the west, 3' east to the header, where they attach with clips. Then on the other side of the header, joists run another 6.5' to the east wall. There they rest on the top plate and are cut diagonally to accommodate the roof slope. These joists are offset by about 12" at the header (it's not straight east-west lines intersected by a header).

My concern: the header isn't supported vertically by anything. At best it's transferring the load. It T's into a floor joist in a room to the north, attached by 2 nails on its end. On the south, it just sort of ends about 8" past a tail joist about 2/3 of the way across the room.

The other big question is that we don't know what's going on with this partition wall, since it's covered with drywall. Some of the tail joists rest on what seem to be posts, while some seem supported more from being sistered to the joists spanning the adjacent room -- making the effective span of some joists more like 22 feet.

My understanding was that the engineer's advice was "get this header out of here and run some normal floor joists across the entire east west span affixed well to a ledger nailed to the posts there at the partition." What actually happened was that they replaced the ceiling joists (a second layer of joists), creating continuous east-west connections with them. They used nominal 2x6"s and attached them to a nice ledger on the west wall with hangers. It surely helps stiffen the room against twisting forces. But it added additional notching to this header. I had thought the goal was to add some vertical support, but there is a 1" gap between the header's notching and these ceiling joists that would keep it from transferring load vertically. (This is where the engineer said "oh yeah, add a clip there.")

My questions to you
Am I worrying about nothing? The engineer's behavior would make sense if I'm worrying about something that's obviously fine.

Above this floor, there's one 450 sf. 1 BR apartment, then the roof. Maybe I'm naive in thinking that the big engineering challenge is to keep things from "falling through the floor." It may be that what's there is messy but will completely work. (There is quite a bit of wood.) And it may be that a 1000 lb. tub, etc., are nothing for some old 2x5's at 24" centers intersected by a 2x8 notched into a 2x4. The header probably doesn't need to provide vertical support, though I do see online white papers recommend doubling-up headers in this situation. The room above doesn't even cover the entire room here; maybe 15% is not livable floor space due to the sloped roof above.

If you think this is something worth asking about further, how do I ask the question with adequate specificity? Since he gave us a better answer when I switched from "we want this to work for tile" to "we need deflection to be less than L/360 so we can use tile" (notwithstanding that the tile association officially recommends L/360), maybe I just need to ask better questions.

Thanks in advance for reading this long question and any suggestions you have.
posted by slidell to Science & Nature (11 answers total) 3 users marked this as a favorite
So is the partition wall a bearing wall? What's underneath that? I think you need to figure out what's going on in there. I'm also having trouble visualizing how "the header" is actually doing anything.

The AWC Maximum Span Calculator says DF-South 2x6 "select structural" is good for 9'4" at 24" spacing, L/360 40lbs/sq.ft live load and 10 lbs/sq.ft dead load, which is the usual for a second floor.

That is, of course, assuming "select structural" for those joists...

The 1000lb tub occupies at least 18 square feet, or 55 lbs/sq.ft. so you're over a bit but still in the ballpark for that region, as long as you beef up the floor sheathing to handle the point loads from the feet.

You could get more specific, figuring out where the tub is in the room, but as long as that partition wall is a supporting wall I think you're not going to have the tub crashing through the floor.
posted by straw at 4:40 PM on February 22, 2012

Response by poster: Thanks. I don't know if the partition wall is a bearing wall, though it probably needs to be one. There is a wall below it, uh, at least along most of its length, and that wall below is bolted to a deeper footer below the new slab. I'm not sure how/if the load is transferred from the 2nd floor down to the first, nor how well that 2nd floor partition is built.

I'm also not sure what the joists are made of. They're old and seem to have tight grain and be in good shape. The house was built in 1890, though I don't know if it was rebuilt at any point. The span might actually be more like 9'4" than the 9'6" I stated above, so maybe we're not too far off.

Is there any chance that the second layer of framing (the ceiling joists) could help bear the load? Right now, they don't touch the floor joists, but what if some sort of clip, tie, or blocking(?) connected them?

Thanks for the reminder to take a look at the floor sheathing and point loads at the feet.

This might be too much information, but I threw the photos I have onto a slideshow here. It shows the wood and might answer other questions that could come up: Engineering AskMe Slideshow.
posted by slidell at 6:38 PM on February 22, 2012

Just to be clear, I'm not a structural engineer, just a guy who's paid a little attention to such things because I just built (actually am in the process of completing) a permitted living roof structure that's got a design load of 120 lbs/sq.ft. on the roof.

In looking at those pictures, I'm now super confused: Let's ignore the new wood that's the ceiling: That's not tied to anything, right?

From your description above, I thought those 2x6 (nominal) floor joists were continuous across the 9'6" span, but it looks like they're terminating (in clips, have you gotten out the Simpson catalog and looked at the numbers they quote for those yet?) in that "header" you show on image 6 that looks like it's now severely compromised because of the notches for the ceiling "beams".

I'd say the partition wall definitely needs to be bearing and supported. I think you can figure end loads for your floor joists based on 50 lbs/sq.ft, each joist end is bearing about a thousand lbs (50 lbs/sq.ft * 2'*9'6"/2). If those joists are anywhere near studs and your studs are attached to drywall (to provide some buckling resistance), there's no way you're going to overload the studs (I believe an 8' 2x4 stud with any sort of shear attachment is good to 3000 lbs prescriptive).

If those 1x5 floor planks are T&G, I'm less worried about the claw-foot point loading.

So, yeah: What's the actual span for those floor joists, and what's holding them up at the ends? If it's 9'6" and a wall that's supported underneath, you're golden, if they terminate at that compromised "header", without cantilever, I'd be concerned.
posted by straw at 8:20 AM on February 23, 2012

Response by poster: Let's ignore the new wood that's the ceiling: That's not tied to anything, right?


it looks like they're terminating (in clips, have you gotten out the Simpson catalog and looked at the numbers they quote for those yet?) in that "header" you show on image 6 that looks like it's now severely compromised because of the notches for the ceiling "beams".

You're right. And thanks for the catalog tip.

What's the actual span for those floor joists, and what's holding them up at the ends? If it's 9'6" and a wall that's supported underneath, you're golden, if they terminate at that compromised "header", without cantilever, I'd be concerned.

Let's say the span is 3' to the header, then another 6'6" from the header to the opposite wall. The header itself is not supported other than by the joists themselves, so the header is transferring the load (one hopes) but not supporting it. On the 3' section, some of the floor joists don't attach to anything. Also, I don't know what you mean about "without cantilever."

Thanks for the thinking, straw -- I appreciate it.
posted by slidell at 10:53 PM on February 23, 2012

So do the joists terminate at the header, or are they continuous through it? If they terminate at the header, you can calculate the tensile load on those hangers, and it's gotta be huge and scary and I'll bet more than they're spec'd for.
posted by straw at 8:44 AM on February 24, 2012

Response by poster: They terminate there. How do I calculate the tensile load?
posted by slidell at 12:43 AM on February 25, 2012

There are a couple of ways, again, I am not a structural engineer. Further, for rigid bodies in complex structures better calculation is still an area of research.

If you think about dangling an object from the center of a doubled-over rope, if both ends of that rope are directly over the object, then each end is supporting half the weight of the object. If you move those ends away so that the ropes meet in a "V" of 90°, 45° from vertical on each side, each end is now supporting sqrt(2) times half the weight of the object, because you've now added enough force to pull those ropes into an isosceles right triangle. Pythagorus must be paid.

You can also think about the geometry of simple structures in terms of triangles: If you have a right triangle where the 90° side is the bottom against the support, side A is against the support, side B is down, side C is the hypotenuse going out to the tip where your point weight is hanging. You know the ratio of A to C is (from measurement), so you know what the pull on that top support is.

And the shorter A is relative to C, the more leverage the structure has and the more tensile strength along C (and compressive strength along B) you need.

If you go search on beam calculations you can find examples of this, except that beams are volumes, so there's a lot of calculus involved. You, however, are in luck, because your hangers are screwed on, so you have point attachments. Which reduces this back down to geometry. This is also why it matters whether your beams are laid across and free to pivot (in which case the load is internal to the beam), or whether they're fixed from pivoting at the ends.

Needless to say, you're going to be in 10-20x multipliers real fast.
posted by straw at 7:34 AM on February 26, 2012 [1 favorite]

Response by poster: straw, you are the best. Thanks. I'll try to think this all through. I'll be talking to the engineer later this week, so this gives me good leads for asking my questions more clearly.
posted by slidell at 9:01 PM on February 26, 2012

Response by poster: Alright y'all. Not to over-update, but I found another engineer to come out and give a second opinion. The site visit fee is so worth the piece of mind. I'll let you know.
posted by slidell at 12:00 AM on March 3, 2012 [1 favorite]

Response by poster: The second engineer said it should be replaced and also found a number of other things to think about upgrading. Good news and bad news: we were right to be concerned. Thanks again for all your help on it, straw!
posted by slidell at 9:43 PM on March 6, 2012

Glad to have helped!

Yeah, that notched header and the terminating joists bothered me too...
posted by straw at 9:36 AM on March 8, 2012

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