Working Stress Analysis Procedure for Fully-Encased Concrete WF Beams

This might be more conservative than necessary, but one might assume that the concrete adds some fire and impact resistance, as well as cosmetics (no dust-collecting lower flange); but no appreciable strength. The lower chord of the concrete is in tension, and unless it is reinforced there.... Also some doubts about slippage (shear) along the lamination axis.

I couldn't find a B16 x 35 shape in my old AISC book; the closest (?) was W16 x 36 (and next lighter W16 x 31).

This makes me wonder if the floor was grossly overloaded at some time in the past.

Or is this whole thing now a problem in plastic design?

Good Morning Tornado!

Wowsers, I can always count on you being in the thick of the fray, eh? *LOL*

Looks like we're thinking along the same lines regarding this floor girder problem that was dumped into my lap by a good client of mine.

You'll have to use the AISC Design Guide 15 (Copyright 2003), Table 2.3.1 "Dimensions and Primary Properties - Steel Sections 1887 - 1952" to find the B16 that I referenced in my OP. Even the oldest AISC Steel Manuals may not include it.

One of my first thoughts upon observing the existing construction was that the concrete encasement around the steel girder may only have served as a fire inhibitor and nothing more, at an uniform 1 3/4-inch thickness all around, except that the upper surface of the top flange is at the same elevation as the underside of the concrete floor slab. But who knows for sure what the original designer was thinking as he designed the building framing. Possibility does exist that the floor framing may have been designed as full composite, partial composite or as no composite section. I feel that I have to look into all scenarios just to play things safely...a bit conservative, but safe none the less.

Currently, just the sides of the girder are covered with concrete. The concrete that had covered the underside of the bottom flange had fallen down some time ago onto the plaster and metal lathe suspended ceiling. This was discovered a few weeks ago y the client's Contractor when the old ceiling was removed. The entire exposed underside of the bottom flange has heavy, though not deep, surface corrosion. We've have drilled the existing concrete in selected low stress areas along the span to ascertain girder dimensions and insitu metal/corrosion states. Also, there are regions along the span at high bending moment locations (where Junior Beams frame into the girder at 1/3 span points) that are exhibiting a large amount of concrete cracking, especially as one gets further away from the PNA and closer to the flanges. I believe that at least in these high bending stress areas the concrete encasement has de-laminated from the steel girder web surface and has slipped or creeped somewhat. The high shear regions (at supports each end of the span) do not exhibit any sort of concrete cracking. The General Contractor has measure the vertical deflection of the steel girder along its span utilizing a laser level. Span is 25'-0" (+/-). Maximum deflection occurs at midspan and was found to be (-)7/8 inches. Deflections at the third span locations (8'-4" inward each supporting girder CL) was found be be (-) 3/4" each. Ends of the girder are dead level and even, hence there was no slope in girder originally. These measured deflections are only due to supported Dead Load. No Live Load was applied to the existing floor system. In fact, prior to my arrival to the school, I asked the Chief Financial Officer of the facility to have the GC remove all stockpiled floor tiles, bags of mortar and all other construction material and tools from the existing Shower Room slab system. This was done immediately following our telephone conversation.

IMHO, the girder and slab have not acting as a composite section, but the possibility does exist that during construction the steel girder may have become overstressed if: A). temporary shoring had been removed before the concrete slab compressive strength was adequate enough to support itself or the girder to support the contributing floor slab area, or B). Inadequate temporary shoring.

BUT, the General Contractor had reported that when they were rehabilitating the existing Shower Room they discovered numerous shower pan leaks, cracks in the existing cast iron drain pipes, open/failed drain pipe joints. Also, several of the existing cast iron steam pipes had split open together with several open and failed joints. All of that moisture had to go somewhere (see my hypothesis below). I learned that this leakage problem had occurred for about 4 or 5 decades and was never fully addressed by the former owner of the Building.

The existing building was built in 1927 by the Christian Brothers and had served as a school classroom building until the early 1980's. It sat vacant and unused for a half decade before the current owner acquired it. Only a few of the original Architectural blue prints of the building are in the possession of the current owner: a few electrical and plumbing drawings and they have been found to be fairly inaccurate by the owner's Facilities Supervisor and the GC. I am currently in contact with both of the Archdioceses of NYC and of Albany NY trying to locate a set of the original construction drawings (if they exist). The Architect of Record has gone out of business long ago in the early 1950's and it is not known where that firm's records are keep or were even kept.

The existing floor is constructed as follows, top to bottom: glazed clay tile in a mortar bed; approximately 2-inches of gypsum leveling grout; approximate 10-inch thickness of moderately coarse floor ciders (this acts as a plumbing and electrical case), all supported by a concrete slab approximately 4 1/2-inches thick. Known slab reinforcement is 10-gauge welded wire mesh. There appears to be no other steel reinforcement in the slab or in the concrete encasement around the existing steel girder, which to me somewhat bolsters my hypothesis that the girder and slab do not act as composite section. Also, there is no evidence that shear tabs, pins, studs or reinforcement exists at the top flange of the steel girder.

I believe the existing steel girder had become overloaded. I further believe that the water leakage from the shower drains and the steam pipe found its way into the layer of floor cinders over a substantial period of time, thus overloading the existing steel girder substantially. Using the known girder deflections and framing layout, I was able to ascertain the sum total Dead Load of the floor system in order to produce those deflections. Again, no Live Load was involved in this analysis. Also a key factor in determining the total DL was the fact that much of the existing cider layer is still very damp and moldy as evidenced by visual observations made at the dozen or so locations in the floor slab were the exist floor drains and piping have been removed and small holes exist.

My calculations show that the Superimposed DL on the contributing floor slab area to produce the deflections as measured along the existing steel girder (no composite action taken) resulted in a calculated bending stress of (fb) 21.02 Ksi. AISC Allowable Bending Stress (Fb) for ASTM A-9 steel circa 1927 is 18.0 Ksi, with a corresponding Fy=30 Ksi (> 1/2 * Tensile Stress which is tabulated at 55/65 Ksi). Bending Stress represents a 16.8% Overstress in extreme fiber bending in the existing steel girder due to applied Dead Load alone. It gets much much worse if Superimposed DESIGN Dead Loads plus Live Loads are utilized in the analysis.

At the present time, pursuant to my suggestion, the existing floor girder has been temporarily shored with timber its entire span, with shoring placed from floor to floor all the way down to an existing concrete foundation walls in the basement tunnel under a central corridor. I have preliminarily recommended that permanent structural steel shoring columns be installed at the underside of the existing girder at it's 1/3 span locations in order to pick-up the reactions from the floor Junior Beams. The new columns will be installed all the way down to the existing foundation walls. The existing girder will be striped of it's concrete encasement and abandoned in place. I am recommending this action rather than removing it because 1). Logistics of cutting up a 25 foot long girder and removing it from the building, and 2). leaving it abandoned in-place will not disrupt the existing concrete floor slab and new Shower Room constructs, including new finish glazed tile work on the floor and the dozen or so shower stalls.

That's it for now....

Good morning CaptMoosie,

Since the 10" of floor ci(n?)ders was holding water for maybe 82 years, and is the space where the plumbing and electrical circuits are located, I would imagine the pipes carrying water and or gas are about to start leaking big time. I would also guess the logical location for replacement lines would be to attach them to the bottom of the subject sagging I-beams. Therefore I would add these future weights to any solution.

For calculation purposes of your new solution, I would imagine an assumption that the weight of 10" of cinders full of water, would not be excessive.

Additionally, I would recommend totalling all the weights of all the floors and design an appropriate footing for each column. IOW, saw-cutting the basement concrete floor to accomodate a new strong steel-reinforced square footing for each column.

Hello FlyingHigh and others,

Thanks for the comments. I do have to address some of them for clarity sake, okay?

The cast iron drain pipes have already leaked of a number of decades. Ditto with the broken steam pipe, which passes vertically through the floor system. That's the most reasonable source for finding wet and damp floor cinders in the existing floor system. All piping in the area has been replaced in accordance with the NYS Building Code.

I have already taken into account the weight of saturated cinders in my calculations, including any subsequent calculations.

The future column will only support the existing 3rd Floor Shower Room slab and Junior Beams + partial load attributed from the to-be-abandoned girder + column self-weight. I do not want to support other lower floor. The new columns will pass down through the lower floors via cut-in holes.....lateral restraints, each axis, will be provided at the top of each column segment (at underside of floor slabs) to prevent translation and rotation. Provisions for vertical and horizontal field adjustments will be provided throughout the height of each new column.

New reinforced concrete column piers will be constructed in the existing foundation wall to transfer the loads from the new columns to new individual reinforced concrete column footings. The new foundations must be in-line with the existing (non-bearing) foundation walls because they cannot be constructed within the basement tunnels. Reason for that provision is that a myriad of existing utilities are hung from the First Floor concrete slab and would interfere with any installation of the new steel columns.....cannot re-route any of these pursuant to client's request........stream pipes, domestic cold and hot water piping, electrical conduits of all types, sanitary/waste piping, gray water waste lines, and roof drain leaders.

The building was dsigned in 1926 and built in 1927. All structural steel connections are hot rivited. So far, there has been no evidence available of any welds being present in the building.

Hope that explains things better, and shows that I have already though out the structural solutions weeks ago.

Have a great sunny day!!!

Hi CapMoosie,

Dare I ask.....................

....."fully-concrete encased steel wife flange beams" "wife flange"? Is that a technical term? My wife had that problem, and she also thought she was a Wolf, but she is alright nowaaoooooohhhhh!

Take care.

I am not a civil or structural but a lowly chemical and metallurgical guy, but it sounds to me that you are experiencing shear collapse at the weld joint girder to column and it could be on all the floor connections if this is stress corrosion from your description. In all that time it could be a combination of a lot of things, building wind deflection, heavy movement on the floors, blah, blah.

After the 911 collapse of the twin towers, Homeland Security came out with a modification strongly recommending support gussets welded under all floor girder attachments to columns. (to us is sounded like they were trying to tell us something.) If I can find it, I'll send it along. Send me your email addresses to send it directly as it is quite lengthy.

There is one old friend of mine, one of the best civil guys and teacher of the arts that might have some input. You might send him a note with all this stuff and ask him to comment or consult. His name is Bill White, email: w3white@carolina.rr.com
. You can mention my name but he may not do it then, just say Hi from Me. I believe he's retired by now.

Tom Coyne

Email: tcinc002@aol.com , Phone: 503-630-6759. Oregon time.

Hey CaptMoosie,

I found the following in my e-library that may at least be helpful in summarizing the general building design philosphy at the time of construction: "Structural Engineer's Handbook" Second Edition, 1918, by Milo Ketchum, which you can download in PDF from googlebooks: http://books.google.com/books?id=64yVDHoW_tcC

On a quick scan of the book, I didn't find any info specific to WSD of fully encased WF beams requested in your OP, but it may be in there somewhere...

Good Luck,

spak

The problems you describe indicate that the structure in question is suffering from iron corrosion fracture fatigue. The oxidation of the iron in the steel causes swelling which fractures the concrete from the inside substantially reducing the integrity of the structure without rusting through the beam, or anything close.

Visual inspection is where you start with this. Which seems to be underway. There are ultra sound based scaning machines available but they have limited provability in terms of the definiteness of the output.

X ray inspection is used more reliably when both sides of the structure are accessable and the cross section is not too thick. Check with the vendor on this, I don't have the specifics.

Back scatter X ray analysis scanning units are under development, but I have not heard of a production ready system.

This problems is common in the analysis of the structural integrity of bridges but it is more art than science. The I-36 bridge in Minnesota was extensively analyzed and deemed sound but collapsed anyway recently!

Do a thorough job of documenting the problem to CYA visa visual and structural loading and then focus the customers attention on fixing the problem instead of analyzing it would be my heartfelt advice.

Best wishes for the season and coming year,

Mr. Gee

Thank you Spak & Mr. Gee,

The referenced book I'm sure will come in handy next to a circa 1922 Structural Engineering textbook (has approx. 1700 pages!!!!) that I have that's worth it's weight in gold, and then some!!!! Found that text quite by accident in a Estate sale for $0.75. I had it appraised by several highly respected book dealers in NYC last time I was there and found out it's worth at least $10k (USD) in Auction!!! Ohhhh mmmmmmyyyyy my jaw nearly dropped down to my toes when I heard the value the first time!! Maybe I should visit estate sales more often, eh????

Mr. Gee, many tanks for the advice regarding non-destructive testing. I have previously discussed this with the client and steered them away from any further testing of the girder itself due to the cost, and IMHO, the existing girder is a "Dead Duck" (ie, not viable) and not worth saving whatsoever as I feel that it has experienced severe overloading for an extended period of time, that this overloading has pushed the encountered stress/strain into the plastic zone and that the girder no longer ductile, nor capable of carrying the Design Superimposed Live Load and additional Design DL, so here's what will most likely happen next month after the client's Board of Directors meet and make a decision following my presentation to them:

(1). The existing girder will either be removed after proposed structural mods are completed, OR,

(2). The existing girder will abandoned-in-place after floor loads are removed and little if any disturbance to the existing remaining structural framing and floor slab is held to a minimum.

I will not be investigating the possibilities of a composite section any further for the sake of preserving the client's budget, although it would be nice to have the Working Stress analysis procedure in-hand for a "next time"!

Have a Merry Christmas everyone, and many thanks for the help and advice offered here!

==signed, CaptMoosie

Good Day All, I have been asked by a professor at school to perform a literary review on experimental data of fully encased composite beams utilizing high strength concrete (great than 60Mpa). If anyone can point me in the direction of an article, or report that would be very helpful. Thanks!