Structural Analysis of 100 Year Old Structure 10

[pittguy12]

I'm running into an issue on an analysis that I am doing for a 100 year old 4 story building....and I'm wondering what I could be missing that's causing it to blow up on me.

General description of the building:
- 24' x 90' rough dimensions for first two levels. Then upper levels step in on one side and building is 20' x 90'
- Steel Lumber Floor system (basically cold-formed steel back to back channels) with 6" concrete slabs on each floor
- Upper floors span the narrow direction across the building and bear on exterior masonry walls
- Exterior walls are 12" - 16" brick
- Where the building steps in, the exterior wall (and thus half of the floor load from upper floors) is supported by a pair of steel I-Beams which, in turn, frame into 4 18" deep floor girders which also span across the narrow width of the building.
- No obvious signs of structural problems...bowed floors, cracked plaster, etc.

Issue:
I've taken into account the soft steel of the 1920's. I've found good design data on the steel lumber product and deduced it was designed for the typical residential 40 psf live loads plus the weight of the concrete floor. And I've done select demolition in different parts of the building enough that I have a pretty reasonable feel for how the building was framed and how it all works.

The problem is that when I run the analysis of the steel beams which are supporting the stepped in wall, it is blowing up on me. Stress ratios 2-3 times what the allowable was for 1920 steel (16,000 psi according to old literature). So much so, that even just running a dead load case it is still 1-2 times allowable.

I refuse to believe that the building has been standing 100 years and could be this far off. Even if you include the possibility that they ignored live load, it still seems unreasonable to assume they didn't account for the building materials they used! So I believe the issue is on my end.

I also am not eager to be the engineer who cries wolf that the building is unstable when it has stood for 100 years without any visible signs of structural distress.

Anyone with experience in similarly aged structures able to lend some expertise?

 

[r13]

pittguy,

Please have a quick look on the 2nd link I provided. Interestingly, composite beam has been used in that time period successfully, and no stud, which came to the picture in a latter date.

Also, does the height of the brick wall above exceed one half of your beam span? If so, you may consider to take advantage of arch action - assume the wall has cracked diagonally, thus the load above is passed to the more rigid end supports (a heavy concentrate load on the 18" beam). I learnt this from an old colleague, not through practices. For your case, rather than call it magic beam right way, the use of some bold methods approaches should be acceptable/justified.

 

[bridgebuster]

Allow me to muddy the waters...

The attached is an excerpt from the first AISC Manual, published in 1923. On the second page under "5 ALLOWABLE STRESSES" it says maximum static stresses. In school I was taught static = dead. On the first page of the attachment "4 LOADING" contradicts Section 5. Perhaps I'm just having a bad day but this is puzzling. I've been going through the manual but I can't find anything.

Also, in the Manual, it does permit a 30% live load reduction for a 4 story building.

 

[r13]

Where the building steps in, the exterior wall (and thus half of the floor load from upper floors) is supported by a pair of steel I-Beams

Where is the step-in occurred, and how that exterior wall is supported laterally - no intermediate supports along entire 90' length?

 

[JAE]

Are you sure there aren’t steel beams and columns framing up above your transfer floor and buried in the masonry out of site?

 

[HouseBoy]

OP
I'm not clear if the failure is under a full design load or just the dead load and I wonder if the actual design load is something that is not ever realized. Not saying that should not be a concern, just trying to understand the load case.

I think I understand that the "failure" is in the double beam that is spanning 16'-6" located about 4 ft in from one of the side walls and that is supporting brick walls and floor (etc.) above.
The double beam is referred to as a Steel Timber" beam and is a cold formed steel product.

I would go along with the suggestion that some form of composite behavior and arching action is happening. Curious that they are 6" apart and I wonder if there is something located in there that is contributing to the needed shear transfer.

Very interesting condition. A couple of photos might be helpful.

 

[JAE]

Even if arching action was happening the end connections of the beams would still take the majority of the load. Arching tends to only help flexure, not shear if the end of the arch is also the end of the beam.

And arching wouldn’t matter a whit to the supporting perpendicular beams.

 

[SlideRuleEra]

It is highly unlikely that a meaningful steel-concrete composite floor was intentionally used in a 1920's house in WV (I read the attached paper). However, I won't dispute that composite action could (unintentionally) be taking place. With that said, I would not rely on composite action several reasons, here is one:

It's the 1920's. Concrete is proportioned by volume, say 1:2:4 (cement:fine aggregate:course aggregate), and it mixed on the job site. Some of this concrete can be surprisingly good... say on bridge projects with skilled workers. For residential type work... I would not count on it. Has the OP tested the concrete? What were the result?

http://www.SlideRuleEra.net

 

[HouseBoy]

JAE and SRE
I'm not disagreeing with you but I AM wondering how the structure is behaving "so well".
Not saying it is a reliable system either (although we all know how difficult it can be to defend unintentional structural systems that appear to work fine but that we don't want to rely on).

Sounds like the system is not very redundant so... I'm just wondering about it.

I have not considered what stresses ought to be but I assumed OP was referring to bending stress on the double 10" tall beams that ar 6" apart.

 

[r13]

If I remember correctly, 1:2:4 mix can produces a concrete strength of approximately 2500 psi. 1:3:6 mix can produces 2000 psi concrete. The actual strength depends largely on quality control then.

 

[SlideRuleEra]

HouseBoy - Sorry for any confusion I caused. I was addressing the OP's question, but failed to say so:

Does anyone know if composite beams were used? If so, did they utilize something other than studs or coils to transfer the load?

From the attached paper, in the USA, here is the one and only worthwhile step towards development of satisfactory composite construction during the 1920s:

Well, here is Mr. Kahn's patent.

If the OP wants to go on a wild goose chase... do some exploration in the slab looking for the "bent-up flange cut-outs" shown in the patent.
If the detail does exist in this house... be sure to submit this house for inclusion as an ASCE Historic Landmark.

http://www.SlideRuleEra.net

 

[rowingengineer]

I would look more into the arching action JAE is suggesting.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[human909]

I would look more into the arching action JAE is suggesting.

Absolutely. How could it not be behaving like this?

If I understand it correctly, you have an extremely stiff wall sitting on some very flexible beams across their full length. The assumption of uniformly distributed load goes out the window the second the beam starts to deflects slightly and the stiff wall doesn't.

 

[JAE]

One other aspect of this to consider, when talking about a supposed arching action - is that with 100 year old brick, you've got masonry with much softer units that what is used today along with a very much softer mortar.

So instead of a highly rigid wall you actually have masonry that tends to slowly deform when its support moves, rather than crack.
Look at old masonry and sight down the mortar joint lines and you'll see this kind of effect. No control joints needed in these old masonry walls because they aren't rigid/brittle like today's hard units with high strength mortars.

This would slowly negate some of the arching action as the wall deformed downward onto the supporting beams.

Also - just to clarify - I wasn't suggesting that arching action was occurring but I was saying that if it does arch, then the full loading (dead and live loads from the walls and floors/roof above) will still come down on the ends of the steel support beams and are probably then taken by the beam end connections to the columns.

I still wonder if there isn't hidden steel columns and beams in the stories above and the beams that are reportedly failing here on the "transfer" level are only taking the weight of the single story wall directly above it.

 

[r13]

In the early development days of composite beam (a clip from paper linked above). Shear connector was invented/used in the later date as pointed out by SRE.

Another example is Mathias Koenen’s (1849–1924) flat-soffit, later (from about 1892) ribbed, floor in which the underlying steel sections carry the tension and the arching concrete infill sections are solely responsible for carrying the compression. Despite the lack of shear connectors, tests confirmed Koenen’s assumptions regarding the structural behaviour (Christophe 1902).

 

[pittguy12]

I'm following the arching action concept and believe it is probably happening...though I don't think that could be something intentionally built into the design of the building. What makes it even more difficult to assess...I do know that there is at least a course of terra cotta clay block used in that wall that I've observed from some of my test holes. But to what extent that is used is hidden behind plaster walls. Unfortunately, one major limitation of this whole effort is that I cannot rip down all the plaster to expose everything. The labor cost involved in removing plaster, which is all tied to metal lathe interestingly enough, would quickly exceed the total value of the building.

Similarly with the composite beam idea. I think there is probably some composite action happening...but do I make the leap to assume that is how it was designed?

Based on the exploration I have done, I'm reluctant to assume there are hidden beams and columns at work that I can't see. I've exposed enough of the wall and the beam pockets where the steel bears on the wall to know that they aren't there...or, if they are, are not located in the areas that would make the most sense for them to be!

Some progress sketches attached.

 

[pittguy12]

Last link was a picture of the Steel Lumber.

Here are the drawings...

 

[phamENG]

During your field investigations, did you measure the deflected shape of the framing? If the building is currently empty and is essentially nothing but dead load right now, you can fine tune your analysis until the calculated deflected shape closely approximates the measured deflected shape. That will tell you that your current loading assumptions are at least tolerably accurate.

From there, you can apply your new loading and see what the building will do. If you can still get it to pass serviceability requirements but stresses are too high, then try some material sampling and testing.

 

[kipfoot]

I think it's time to tell the owner about the columns they need to add. It's as if they built to the second floor and said, "you know what?, let's add a light well and some windows on this side" and added the 10" beams to support the upper floors and wall.

The 18" beam looks about right at the ground floor, right? Then the same shape was used at the second floor with a significant additional point load! You don't know the original camber of the beam, but if those second floor 18" beams are at what you think they should be for dead load, then I think that confirms your assessment.

I'll add that I don't usually buy the argument that "it's been fine for 100 years" because you don't know the load it's seen. In this case, live load just pushes the stresses closer to yield. If you're thinking about testing the steel, take your coupons from the ground floor level.

 

[JAE]

phamENG - good point but you should keep in mind there could have been specified initial camber in the beams, and or natural camber, so your fine tuning calcs will have to be in a rough range anyway to acknowledge that the initial beam, before any loading, might not have been perfectly straight.

 

[pittguy12]

Thank you everyone. I actually sent a summary to the owner this morning explaining where I was with everything and what would be needed to take this further.

I've done a lot of history renovations and assessments in my career...this was the most interesting.