Houses Are so Resilient 3

[XR250]

Looked at a good one the other day. A customer wanted to take out a wall in the kitchen. It appeared to be supporting a 25 foot long beam supporting 20 ft. trib of roof, a second story floor system and finished attic floor system. I was able to identify the beam as a W8 x 15 by looking through a plumbing access. For the life of me I could not figure out how a beam of this size could be supporting that kind of load over 25 foot span. The homeowner finally fessed up that there used to be a mid-span support wall, but it was removed last year. I went in the crawlspace and found a pier directly under where that beam would’ve been supported. Construction photos show (5) studs used to support the beam. Apparently that was not a red flag to the handyman. Amazingly, there was no noticeable sag in the floor system. I guess the second story exterior wall was acting as a deep beam.

 

[Once20036]

I have a shed behind my house with a lean-to attached to the side. One side of the lean to frames into the shed roof, the exterior edge is supported on two posts.
One of those posts is completed loose/carrying no load. The roof must be acting as some sort of torsional member, with load on one post, the opposite corner, and uplift at the third corner.
Its been on my list of things to do something about for years, but I haven't seen any signs of movement or distress, so I haven't acted.

 

[GeorgeTheCivilEngineer]

When extending and renovating my house a few years back it took me hours to demolish just a few stud walls. I was trying to limit damage to surrounding areas, but still everything was just connected to everything else! Structural wall finishes...

Mind you, the wind and bricks are stronger and heavier than I am.

 

[IRstuff]

We replaced a load-bearing wall in a remodel with a W8 x 12 beam that supported a 2nd story and attic; added columns and footers on the ends to support everything

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[DoubleStud]

Definitely. Also I feel like 40 psf of live load that we typically design for a residential structure is a bit conservative. You have to have a big party where everyone stands really close together the whole room. Almost never happens especially in bedrooms lol.

 

[phamENG]

They are. I do plenty of historical stuff, and have seen plenty of things that "shouldn't" work. Of course they do and, if I felt like taking the time, I could figure out at least a plausible explanation. Usually, it can be chalked up to not having seen a full design load coupled with inherent redundancy.

Put in a beam that's at the limit for camber in A6 (or beyond it - most residential contractors probably don't know what ASTM A6 is or that limits exist), and there's a real chance that intermediate wall/post wasn't doing anything anyway.

DoubleStud- have you ever noticed Table C4-2 Typical Live Load Statistics in ASCE 7? It lays out the source of the 40psf and shows why it seems 'conservative'

There are those cases, though, where somebody puts in a beam with posts in lieu of a bearing wall and half the second floor cracks apart. So I tend to think of the redundant load paths as a source of ductility - the house is more likely to warn us that there's a problem than collapse outright. At least, classical designs and methods do. A lot of modern architectural practices are driving us away from those redundant systems and into more discreetly engineered buildings...all the while the same contractors are building them and assume there's no real difference.

 

[NorthCivil]

A real common post war method to build around here is:

24 foot wide house, bearing walls down the exterior, with one line of beams/posts up the centre.

usually the beams are 6x6 dfir spanning 12' between 6x6 dfir posts. 8" joists forming the floors span perpendicular everywhere.

I was called out to a 3 level house once built this way. The owners wanted to open-concept the basement, so they just removed all the posts. 2 full occupied floors above the basement, bearing down on nothing. the floor was deflecting obviously, about a foot from flush.

the whole house, just being held up by the broken floor plate, acting like a kind of tent/shell. the plywood over the joists creating a "net" so to speak.

of course i went into panic mode, evacuated everyone, called a carpenter friend to come in and shore the place as an emergency.

but it had been like that for a couple weeks before they called me, with people living upstairs.

having seen that, i have no time for arguments like we had here the other day, about how long we should calculate header spans over windows, giving or taking 3".

 

[DoubleStud]

NorthCivil, I had a similar story. The basement had an interior concrete wall. The contractor decided to remove it because above the wall, it is not really supporting anything because the joists run parallel to it. He just decided that's not a load bearing wall. Well, he was correct that it was not a load bearing wall. It is actually a buttress to support the foundation laterally. The basement wall started bowing in and you can start seeing cracks on the finishes. Fun.

 

[Settingsun]

My parents' house had a 20' timber beam with midspan steel splice, originally two 10' spans with a post in the middle. After doing some timber design, the splice appeared so inadequate I would've evacuated immediately if not for 20 years of personal knowledge of its soundness.

 

[JStephen]

My father-in-law was a jack-of-all-trades. Some 25 or 30 years ago, they moved halfway across the county to some property owned by their son. Their method of building a house there was to bring in an old single-wide mobile home, and then they basically expanded that into the equivalent of a modular home. So lots of work involved, but they got it done. Then a few weeks later, the roof started sagging. I don't know the details (and didn't want to!) of what they did or didn't do, but they got busy adding some exposed beams on the inside to remedy the situation. Net result looked okay and lasted 20 years or so until my father-in-law passed away. Fortunately, that sagging was a somewhat gradual process that gave them time to remedy it instead of just a "whoomp".

One other drawback to that house was it was built on cement blocks on whatever clay soil happened to be there, and so things would move around as the soil dried or got wetter. When my father-in-law was more active, he'd periodically crawl under there and relevel things. At some point, that didn't get done anymore, so the front door was permanently stuck shut. Meanwhile, they were both having mobility issues. So they would go out the back door to the carport, get in the car, pull out on their side street and then across the road to get the mail- and that mailbox was maybe 100' from their front porch.

 

[kissymoose]

Not to mention the 100+ year old roofs that use 2x4 hips, valleys, ridges, rafters without any rafter-tie action from the ceiling joists or bracing anywhere. Those can make you look real bad if you end up having to qualify the roof and start describing what needs to be done (like tracking new bracing loads down through the house) and they look at you like you're out of touch, pointing out how the thing has stood for a hundred years.

 

[phamENG]

Those can make you look real bad

I used to feel this way. So I spent some time figuring out why the codes are what they are. Biggest thing: reliability. I've found, recently, that if I describe what reliability is, what the loads are that we use and why, and how they translate to the real world, people are no less pleased with spending the money but most (if they have a thread of rational thought in their brain) can at least appreciate what I'm doing an why.

Once I point out that the only reason that their house has stood so long is due to diligent maintenance by past owners (especially easy when I can show that the neighboring houses built around the same time are long gone) and they're potentially one roof leak away from a failed connection in some critical area, they're a little more willing to listen to why adding some code mandated details are a good idea.

 

[phamENG]

100+ year old roofs

It's also important to be able and willing to look at these beyond the narrow scope of new construction. Our simplifications err on the conservative side for a reason. We have to be willing to do the hard work of figuring these things out. An understanding of true strength and variability of wood is also critical. I've been in old churches where large, heavy furniture and frequent assembly loads were being carried by 3"x6" joists spanning mind bogglingly long distances, while the same joists in low traffic areas with nothing on them had long since failed. The reason? The low strength area had a few joists with big knots. Local failure lead to overloading of those that were in good shape and the floor sagged significantly as a result.

Old lumber doesn't always mean stronger lumber, but a clear specimen of old growth timber will often be a bit stronger than new growth lumber.

 

[XR250]

but a clear specimen of old growth timber will often be a bit stronger than new growth lumber.

A "bit" seems like an understatement. I would guess 100% stronger. I agree on the knots though. Def. have seen that numerous times.

 

[phamENG]

A "bit" seems like an understatement. I would guess 100% stronger.

Disagree there. The biology and physical "construction" of wood hasn't changed in the last 2 or 300 years...perhaps in the last 2 millennia? The fundamental drivers of strength - grain direction/angle, grain spacing, and water content - are going to be the same in a clear specimen of loblolly pine harvested in 1720 as one harvested in 2020. So if you cut a piece now with growth rings comparable to a piece of 300 year old lumber and dry it to the same equilibrium moisture content, the strength will likely be comparable. The behavior may not be - the older stuff will be significantly less elastic and more prone to splitting, and may even have had its strength degrade due to moisture or thermal cycling.

The issue is that finding those comparable growth rings would be quite difficult. But that can be resolved with a standard visual grading process to give usable numbers for design. If not critical, I just assume the old lumber (if the knot locations are acceptable) is No.1 based on current design values and move on.

 

[XR250]

The issue is that finding those comparable growth rings would be quite difficult.

I agree, but yeah, finding anything but widely spaced, fast grown rings nowadays would be a challenge and uncommon to find on any job-site.

 

[phamENG]

There's one contractor around here I've worked with, doing exclusively high end custom homes in the 6,000sf+ range that only lets their framers by machine graded lumber. That stuff is pretty nice - looks good, too. Growth rings are very tight compared to typical SP No.2, and are much nicer than some of the historic wood I've worked with.

Another important factor to consider - our design values are 5th percentile of all tested specimens involved in the SPIB inspection program. So what if the stand of trees that building came from is superb and actually lands in the 95th percentile (maybe 2-3 times stronger than the 5th percentile)? And we are naturally cherry picking here...the only building left standing from 200 years ago are the ones that were either built with better materials/craftsmanship or better maintained AND weren't in the way of a newer, more valuable building.

So a lot of what's left is likely much stronger - and could be 100% stronger than our current design values - but that's because the buildings made with the lower end materials collapsed or were torn down decades ago.

Sorry - I've given a lot of thought to this. Working with historic structures has given me an awesome view into what can work, but I'm also a believer in improving building reliability. Combine that with dealing with people who only think of the immediate consequences and fail to consider second or third order consequences or reasoning shortens my patience for those who flippantly dismiss modern code requirements with the 'it's worked for all these years' arguments. (XR, I am NOT saying you're doing that.) There is value in the historic performance of a structure, but it's folly to use that exclusively to write off the need for repairs or improvements. And again, as design trends move away from those inherently redundant framing schemes, I'm concerned about how these discussions will go for the next couple generations of structural engineers.

 

[SWComposites]

We used to own this house in Seattle:

House was built in 1914. It needed a new roof, and old roofing needed be removed; well, when the roofing contractor tore off the roof they found two! layers of asphalt shingles on top of the original cedar shake roofing. There was an unused chimney in the center of the house for the original coal furnace in the basement and wood stove in the kitchen. We had the roofers remove the chimney below the roof line. When re-roofing was all done I go up into the attic to start removing the rest of the chimney. Looking up at the rafters and framing, I have no idea how that roof stayed up (particularly when a couple of years before we had a big snow storm followed by rain which put a huge layer of wet snow/ice on the roof) as the rafters are 2x4's, many of which did not connect to the ridge board, and many of which had large splits sometimes completely severing the rafter; there wasn't much in the way of cross ties or bracing. The flat-ish dormer areas on the sides were particularly concerning. But I figured, well, the roofing weight is now a lot less than previously, and it held up somehow with a foot of ice on the old roof, so why worry. Probably the fact that the framing was all old growth Douglas fir helped a lot.

 

[CrabbyT]

Contractor cut out a tension rod that supported a 30' long 6-ply 2x10 beam. That beam supported 2 additional stories and a roof.

He said the floor deflected like crazy when they cut the rods. I'm amazed the whole building didn't collapse.

 

[NorthCivil]

I've been up in the guts of hundreds, of hundred year old roofs, framed from 2x4's.

I've seen 2x4's @ 1200cc spanning ~16 feet, from ridge to outer wall(yes, a floating 2x4 ridge supported by nothing) . We're talking the good old Dfir 2x4s, not 1.5x3.5s, with the roof sloped at about 35 degrees. It had a big 1 foot permanent deflection, giving it that nice cambered pitch on the roof, that looks so good on historic houses. (when the roof looks like a skateboard ramp)

Still standing after 100+ years, all kinds of snow and windstorms.

definitely gave me the heebie jeebies a bit, but also gave me a lot of perspective when doing alterations to 2x8 framed roofs built in the 70's. Everything you touch doesnt always need to be reinforced to the modern code.