Composite action of wood floor systems

[connect2]

coming from another post about floor vibrations in wood floor systems this;

So there are system factors that vary from a simple joist with a nailed plywood sub-floor to a field glued and nailed sub-floor with an attached drywall ceiling all of course sufficiently nailed. As we progress through the floor types the I of the system increases for a number of reasons not the least of which is the 'Stressed skinned Panel' effect of the sub-floor 'becoming one' with it's support, the joist or the floor truss...composite action, a stiffened orthotropic plate

To the best of my knowledge the various floor systems and the degree of composite action assigned to each was based on testing and field applied glueing. Comments.

To have a conforming stressed skin panel only phenol-rescorcinol glues are permitted and they must be used under specific conditions one of which the glue line pressures required are huge and not accomplished with a nail or a screw.

To move forward. In the calculation of a floor system how does one calculate the I of a field glued nailed sub-floor? How does one know what the contribution of the sub-floor is to composite action. To calculate the fundamental frequency of the floor system we need to know I of the system. Do we ignore the possible contribution of the floor sheathing? Seems like everybody feels a nailed glued sub-floor will increase the stiffness and decrease the vibration. But can it be quantified?

 

[XR250]

I just consider it a bonus and never try to calculate it.

 

[CELinOttawa]

Okay, well I think you're confusing things... If you are composite, you have access to the increased I value. Non-composite is just the joist's value.

For vibration, I will consider an in-between value. It is not something I would do for ULS.

It is not accurate, and most engineers do as Excel does - Don't include the added effect of glue, but be glad it is there.

Two separate things are at play. Gluing or nailing gives you access to composite action. Screws alone can do a good job of giving composite, but will almost never yield fully composite behaviour without reflection. That defection is not an issue for strength (ULS), but can certainly be an issue for vibration. As such adding the glue gives you more reliable/less flex required in order to achieve composite behaviour.

Properly you are composite, or non composite. For ULS I go 100% or 20% if considering composite action at all. For SLS I go from 50% to 90% to 100% depending on build quality, detailing decisions, etc. Yes, these relate to the same floor since, as previously laid out, the speed and relative movements at which each is effective is very important and not at all the same for ULS and SLS even given the exact same floor.

 

[connect2]

CEL and anybody else for that matter,

Not confusing things. This is exactly what I am saying. If you have full composite action then you have an increased EI_eff, re b_eff, and if you don't then your perhaps somewhere between that, full composite action, or the joist/truss acting itself, alone.

In order to have full composite action between the joist and the floor sheathing they must be glued together. The only permitted glue is a glue conforming to CSA 0112.7 which is a rescorcinol or a phenol rescorcinal glue. The use of this glue requires a press in order to achieve the high glue line contact pressures and set times required by the standard. To the best of my knowledge it is still the only permitted glue for use in the manufacturing of plywood, OSB, gluelam, LVL's, and all other composite structural building components etc., etc.

Codes do recognize partial composite action with their System factors K_H, but the use of a system factor can not include considerations of composite action between the floor sheathing and the joist and is restricted to dimensional lumber and common nails or their equivalent for the system Case 2. So quite restrictive and likely based on testing. Which was one of my questions, how were the system factors developed?

Everybody agrees that a field glued and properly fastened sub-floor will improve the performance of the floor system be it ULS or SLS. Question is; By how much? and How do we, if we can at all, quantify it?

Long span wood floor systems are typically SLS problems with vibration a key concern. CEL you say that for SLS considerations depending on the build quality detailing descisions, etc.., you will assume anywhere from 50% to 90% to 100% composite action. Do I understand that correctly? What do you base that on?

So here's the scenario. Your standing on an installed floor that obviously has vibration problems. You turn to the Contractor and say 'The subfloor should have been glued and properly nailed, the strong-backs are not correctly installed and you need to have the drywall ceiling installed. If you do all those things Mister Contractor you won't have a problem with vibration.' Meanwhile the Joist/truss Manufacturer is standing there yawning and not saying much. The Contractor does all those things and 2 weeks later you're all standing on the floor and vibration is acceptable, that's good, OR the vibration is still unacceptable and of course the Owner by now has developed inner ear problems and Vertigo and having trouble standing at all and informs you his partner has a glass and rare antique Chinese tea cup collection that they had planned to put on display in the center of the room on glass shelving. That's bad.

Guess I'm trying to get some comments from others on their possible experiences and knowledge of similar situations and as well be able to crunch some numbers and examine their various effects on the possible solutions BEFORE I start proposing them. We are not the 'designers' of the floor joists so are somewhat at the mercy of their comments and directions. We are however the EOR.

Excel your direction is likely the safest, easiest and quickest at the design stage.

Enough said. Thanks for any comments that follow.

 

[CELinOttawa]

The general numbers quoted are based on nothing but field experience and the advice of a grizzled old, fantastic and lovely, senior engineer in my first year of internship.

The code requirements for super high reliability glue is in order to consider composite action at ULS, and should not be thought to apply to requirements to achieve composite for SLS.

These are based on my professional judgement, and I am very clear with any Owner about the limitations to any attempted repairs. It is not something I am able to back up with research reports, etc, and may very well get me into a dispute at some point. So far, however, so good and I have been able to successfully repair a number of floors.

 

[manstrom]

At best, the floor is partially composite. The nails will have a little slip and they do not have the shear capacity to fully develop. Also, consider that there are gaps in the sheathing that prohibit moment development at those locations.

Glue is tricky. Yes, it helps, but by code, you can't consider it becuase it's application varies from installer to installer. I always neglect it, but require it for floors (reduces squeaks)

I have done some calcs for wood sheathing on a stud as a T section. I used an equation that considers the partial contribution of the two members as a partially composite section. It doubles or triples the I of the wall.

Regardless, I consider the composite action a "bonus" but I would never rely on it. Too many variables.

 

[EngineeringEric]

I suppose you could explore the option of increasing the stiffness of the floor system by utilizing a composite action. to get complete composite action you will need adequate shear flow (VQ/I....). Once you increase your stiffness (note that E_ply =/= E_joist) then i suppose you will have an change in floor system fundamental frequency, f=sqrt(M/K). All of that with a reduced stiffness capacity in your back pocket seems like a bad approach, I would say that the joist/trusses/other alone need to be good for the vibration/load.

As for how much, see what amount of load transfer you can have with your field glue and nails, use that modified semi-composite section and do the same thing. but the I_semi-composite is the variable you are solving for. load transfer = VQ/I_semi

note that 'liquid nail' has some >450psi shear transfer, so it is pretty good actually. Do not use low VOC.

 

[connect2]

Ok CEL, thanks for the comments.

Couple of comments back first the adhesives/glue. These adhesives, resorcinol/phenol resorcinal, are the only permitted adhesives for structural wood connection and have been the only adhesives used for a long, long time in the gluelam industry, in the plywood/OSB panel industry and now in the world of composite wood members be they I-joists, floor trusses, LVL's, PSL's, stressed skin panels, SIPS, etc etc. To these industries they are not super high reliability glues, they are just the only adhesive that are permitted to be used. They are the only adhesives considered to be 'waterproof'. If you examine 086 closely no other adhesives are considered. There are and have been attempts to get a standard going on 'Adhesive for Structural Wood Products (Limited Moisture Exposure)', but there is still much disagreement on long term durability and capacity when exposed to moisture. All structural adhesives are considered to be potentially exposed to water/moisture during the construction process, re couple of weeks of rain/snow, before the buildings is closed in or sitting outdoors somewhere for several months, and therefore their adhesion can be considered compromised.
Secondly, in the case where these adhesives are used in the fabrication of structural members, composite action can and does apply to SLS criteria. As an example check out 8.3.6.3.8 of CSA 086 where the standard tells you that you shall use the calculated (EI)_e. Be that as it may.
Finally, yes engineering is part codes/standards/regulations, part intuition, part just plain experience, part art....combined with statistical probability based loads ... to come out with a solution that in ones estimation, after doing a risk assessment, will make/keep your insurer happy.
At this time it is my conclusion that there is absolutely no way to consider composite action in these floor systems with ExcelEngineering's one liner being to the point.

Again thanks for the input.

 

[CELinOttawa]

In your opinion. Having seen floors calmed down by simply increasing the number of screws / decreasing fastener pitch, I have to disagree strongly.

As alluded to adobe re Tau discussion by EngineeringEric, glue is not the only way to provide composite action.

Words like absolutely do not apply to debatable parts of our practice.

 

[KootK]

About eighteen month's ago, a close colleague of mine designed a single stringer cantilevered stair for a well to do home owner. It's nothing short of thrilling visually. When one steps onto the cantilevered landing, however, the landing swings abruptly to the side by about three inches and then swings around for about thee seconds afterwards. It's brutal. Over the last year and a half, my colleague has tried:

1) All manner of FEM modelling, verified by me.
2) Filling the HSS stringers with concrete.
3) Turning pinned connections into moment connections.
4) Installing horizontal bracing in the landing.
5) Adding mass to the landing.
6) Torsionally stiffening the supporting floor structure.

Fixes one through five produced no effect at all. Fix number six reduced the motion noticeably but didn't eliminate it altogether by any stretch. I'm sharing this story because there a couple of lessons to be learned from it:

1) Solving vibration problems post hoc tends to be mostly about intelligent experimentation. I wouldn't knock yourself out trying to get too fancy with the numbers.

2) It sucks, but I don't ever tell an owner "I've run the numbers and I know how to fix this". Rather, I tell the owner what I know to be the truth: that I have some thoughtful ideas for a solution but I'm not able to guarantee the results.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[CELinOttawa]

Yup... I have designed five such stair cases, and each one was more stiff than the last. Now I have a nifty spreadsheet and refined details.

Getting it right, or at least right enough, the first time was the subject of a great number of consultations, field reviews, discussions with the client, and a goodly number of sleepless nights. I can say with a great deal of pride that I know how to do one of these, and that each has led to other business. They are not trivial.

If your colleague wants a second (make that third) opinion I am not hard to get a hold of.

 

[KootK]

Offer of help extended...

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[connect2]

CEL, I don't disagree at all about what you're saying. Should have said there is no way to actually 'calculate' the composite action effect. That it can, and does, contribute to the overall performance of the the floor system is agreed by all, including myself. I am absolutely not a part of your practice, so you can relax. I have my own, thanks.

KootK, No not going to knock myself out with numbers, just was hoping someone might have had some 'black box equation' out there that would have shed some light.

 

[CELinOttawa]

Lol... The practice of professional engineering, not your individual shop. I find we don't identify readily as members of a community of colleagues... Yet another advantage the medical folks have over us.

 

[connect2]

CEL, Sorry I call my 'shop' my practice and what I do my profession.

 

[CELinOttawa]

You're quite right Connect... Likely a language thing. My apologies; it isn't always easy keeping the languages, not to mention dialects, straight.

 

[XR250]

@CEL;

Here is one I recently did.
Stringer is 5/8x11 steel plate welded to 5' long vertical angle irons @ 16" o.c. bolted to 2x6 walls studs.
The studs were notched 5/8" for the plate so it could flush out and be covered with the sheetrock.
The treads are C7x9.8 with wood on top.
Slight bounce, but very acceptable to the homeowner.
Very difficult to model, but took a leap of faith, and it seems to work.

 

[CELinOttawa]

I've never done a full cantilever like that Excel... My single stringers have all been a central stringer with the treads cantilevering off to the two sides.

That does look fantastic, however, and I would think the fastening to the wall (not to mention the wall detailing) could be tough.

With a central support you face two other issues: step resonance, and the sheer point that you're twisting in opposite directions time after time. Add another person or two and the least bit of softness really gets things going...

 

[XR250]

I understand what you are saying about the central spine setup.

I attached this to the wall by bolting 5' long L3x3x1/4 vertically to each wall stud and then welding the plate to it. This kept the connection loads low and stiffened the studs at the same time. Added a Simpson L50 top and bottom of the studs to address the couple which was not that significant anyway.

 

[KootK]

That is a bold stair and some clever detailing. I'm going to file that away for future off-ripping. The stair eventually got a handrail, right?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.