Floor Joist Notched for Ledger 3

[SteelPE]

I have a project where I have to calculate the capacity of an existing wood framed floor (should be simple enough). I made a visit to the site yesterday and took a few photos and took some measurements. I noticed that all of the floor joists were notched in order to rest on a flush framed wood beam (see photo).

The notches on these joists is over the allowable notch located in the building code (which I believe to be 10% of the overall depth of the wood member). NDS does give an allowable shear equation for notches of fv = [3V/2bdn][d/dn]. Using this equation I am calculating an allowable end reaction of 154# which gives me next to nothing in capacity for the floor (17.5 psf).

I know this method of construction was used in the 1970's (my parents house has this exact detail). Question is, do I ignore the detail based upon it's the historical use of the detail or do I report the capacity of the floor as 17.5 psf? I am leaning towards the 17.5 psf as it's really not my problem to deal with at this time.

 

[IceNine]

You could recommend they add some 2x4 joist hangers and more screws in the ledger to increase the capacity.

 

[phamENG]

Depends on what they're doing with it.

It appears they're spiked into the ledger, so the 154lbs is probably a very conservative estimate, but a reasonable one. If the use isn't changing, then I would warn the owner that it can technically continue in service under the existing building code, but it is not a reliable condition and these are prone to fracture (assuming, of course, that your state has similar EBC provisions to mine). Look for any that appear to be splitting (even just a little) and have them repair those, at least.

If they are changing the use, then 17.5psf is your new capacity and you need to upgrade all of the connections if it needs to increase.

 

[bones206]

and a ledger connection fastened below the beam neutral axis, is that reliable given the tension induced perpendicular to grain? I see this type of joist ledger used all the time around here to save a few bucks on hangers, and always think it's a bad detail.

 

[SteelPE]

So currently we are just being asked to calculate the capacity of the floor for a proposed new "workshop" that will be on the elevated floor. I verbally told them that the building overall was not in good condition and putting any money into the project was going to be a waste.

At this point, I will just give them reduced capacity (I don't even think this is the lowest capacity member in the floor).

For a repair I was thinking of adding some solid blocking in-between the floor joists and then adding proper joist hangers to the ends of the joists..... but again, I was just contracted to calculate the capacity of the mezzanine.

 

[phamENG]

Sounds like 17.5psf is a good number, then.

 

[Eng16080]

Based on the current NDS code, I'd say that the capacity is zero. The notch exceeds 1/4 the depth, which is not allowed, per Section 4.4.3. And "heavy or medium concentrated loads shall not be suspended below the neutral axis..." per Table 12.5.1C, Footnote 2. I would just leave it at that, because if you tell them it's good to go with 17.5 psf and then there's an issue, how will you justify it?

A repair detail something like what you described above sounds reasonable.

 

[gte447f]

SteelPE, I would do as you have done in calculating the shear capacity of the joist per the notch equation from NDS, and I would use this as the basis for the allowable capacity of the floor, assuming there isn't an even more limiting condition.

If you want to reinforce this condition, I have seen calculations performed for a screw driven into the bottom of the joist. The screw must be long enough to cross the crack or potential crack and have adequate penetration on the top side of the crack plane. I don't have a copy of the calculation though.

 

[bones206]

Similarly, you could also reinforce the beam with screws going in from the bottom for the aforementioned tension perp to grain.

 

[XR250]

Had a floor collapse in a Sorority house once. Most of the joists failed at a similar 2x4 ledger notch. Alcohol and dancing had nothing to do with it!

 

[KootK]

Piggybacking on bones206's concern for fastening below the neutral axis, the worst version of that here may actually be in the ledger block rather than the beam per the sketch below. Kinda depends on where the joists land relative to the ledger fasteners.

These are always uncomfortable situations, having to explain that something that's survived a long time may in fact be quite fragile. I find it useful to tell clients that it's pretty common for actual service loads to be much less than code design loads. I think that, statistically, most 40 psf residentially loaded floor systems will see about 6 psf load in service. As such, the situation can change markedly if the occupancy changes to produce a load condition that will push the service load much closer to the originally intended design load. That's when weaknesses in the load path start to show their true colors.

 

[lexpatrie]

This was done back in the day, it's still in the code (see 2021 IRC Table 602.3(1) I know, Wall Construction, bear with me). It's item 29 in that code. If you prefer a more graphical depiction, look in the ISANTA ESR-1539, Figure 12E.Link Thing is, at least in the ISANTA graphic the joist is supposed to bear on the ledger strip and the flush header (girder?) is supposed to be deeper. Sure it goes into shear that tends to try to split the wood apart, but it's permitted.

In an engineering sense, this is difficult to justify (as has been stated above), and what is on the site doesn't actually meet the code (nailing is missing/not spaced the way the code currently wants it, nails appear to be maybe even cut nails (bolts?), and there's that deep notch in the joists. The construction technique looks like 1940s yet the wood isn't rough sawn. I guess maybe it's "nice" rough sawn? You could maybe palm nailer and get some A34 clips each side of the joists? The two joists you show don't look like they have already split, and if the load isn't transferred by bearing on the ledger strip, the split shouldn't develop in the future?

I thought that ledger looked "too wide" but I guess they want 2x2 minimum (2021 IRC R502.6.2)

Regards,
Brian

 

[wrantler]

A straight 90 degree notch will have stress concentrations at the notch. It is not a good detail and usually cracks appear here and can slowly creep out into the span. Failure is usually sudden as XR250 said.

If it was me I would state the facts and give the most reasonable repair option.

 

[LittleInch]

What is the actual depth of the joist and the notch? Picture kind of implies the joist is bigger/ taller than the cross beam?

Is that a wedge or shim on the right hand joist under the notch?

The condition looks remarkably good - no splits, dry rot or water?

I do like KootK's comment though about what loads the floor has actually seen compared to "design" and now you might be a lot closer to allowable than it has seen before so might just suddenly fail.

Those connectors look like they were screw bolts driven in and then someone has cut off the heads to allow the joists to fit?

What's on the other side of the supporting beam?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bones206]

lexpatrie - that’s interesting that the ledger strip is permitted in IBC/IRC. Seems to conflict with the NDS prohibition on medium/heavy loads acting below the N.A. Maybe this is one legacy detail that should be removed from the prescriptive provisions...

This recent thread had some good discussion on the topic thread507-507651.

 

[Eng16080]

that’s interesting that the ledger strip is permitted in IBC/IRC. Seems to conflict with the NDS prohibition on medium/heavy loads acting below the N.A. Maybe this is one legacy detail that should be removed from the prescriptive provisions...

I suppose the code never shows a detail with the ledger strip below the neutral axis, or with the joist notched, but it's definitely implied. I agree this should be removed from the code. I'd much rather have the detail here than a 2x2 ledger strip nailed to the bottom of a beam which the code implies is ok.

 

[SteelPE]

So in the end, I ended up going back and giving them the allowable loads for the floor based upon the different failure modes. This detail actually did not create the lowest failure mode. That belonged to a post that was bearing on a beam that was supporting 1/4 of the attic and generated an allowable floor load of 0psf. Definitely some issues on this project and I wasn't happy with the architect for bringing me out here to waste my time. It was a new client (warning sign #1) who does a lot of work with a competitor (warning sign #2) and another engineer was in here a few months ago doing some shoring work on the floor..... but they couldn't help the architect with the loading question he was asking (warning sign #3).

In regards to some questions above.

Yes, the joists are higher than the main support beams
Yes, it looks like they shimmed that floor joist.

There were some brand new Timberlok lag screws installed on some of the ledgers (see warning sign #3 above).

Hopefully they pay my invoice for the field visit and the report I prepared. I have my doubts.

 

[phamENG]

Hey, it's a new client, right? Then no harm filing at small claims court!

 

[bones206]

Found an article where they actually tested this type of ledger strip connection for PT deck configurations. According to the article, apparently PT deck ledger strips were removed from the code starting with IBC 2018. Interesting to see that the failure mode was a roll-over/fastener withdrawal mode - although it was only one test.

https://www.jlconline.com/deck-builder/ledger-strips-and-the-code_o

 

[lexpatrie]

Egn16080 - That idea that the code "implies" a notched joist isn't founded in the code itself, I don't have the IRC commentary, and there's no figure or language in the code itself permitting it. Isanta doesn't show it, either. The "typical" construction is supposed to have a girder that's deeper than the joist by 2", nominal. If they wanted to approve a notch, there would be a figure or language in the text of the code.

Excellent article Bones206. It also has great language about how you aren't supposed to "rip" dimensional lumber and presume it retains the same grade. Plus, you need to treat the cut face. But they don't report a load?

Incised "wet" Doug-Fir wood would perform differently, if you are out West it's a whole other thing. This testing likely would be not applicable. Not that you should really see incised wood inside a building....

Regards,
Brian.

(I'll check out that other thread for the prohibition, revise if needed, but there are a lot of things in the IRC that don't "meet the NDS", while these are prohibited for "us" (professional engineers) they aren't exactly defects. The code supersedes the NDS, not the other way around, if I recall correctly.)

Can I just moan a moment that people constantly introduce an acronym without defining it? PT - post-tensioned, pre-tensioned and, based on context here, pressure preservative treated (PPT). It also looks like they used treated southern pine (the photos don't look incised and Frank and Friends are out in Virginia where that's the typical species as far as I know). Technically the Southern Pine would potentially be stronger than, say, Hem-Fir or Spruce-Pine-Fir that are more commonly used for interior framing.