Code Deflection Issues. Local + Global

[EngStuff]

This is more of a discussion coming out of frustration and also trying to get a clear understanding. Have our company standards been conservative all this time?

I think the new Code books that roll out should specify Tile deflection. In a majority of firms that i worked at, we use L/480 for tile loading all the way up to L/600(ceramic). Now we are Runing into a situation because of one structural firm using L/360.

A little background and information. The situation is one Architectual firm we work with are our bread and butter, so we cannot just tell them to kick rocks. We have always worked on their office buildings which a lot included tile loadings(ceramic). No issues, However, recently another engineering firm was hired to do something small scale in UTAH(we did not have time to take the project), the situation now is that they are questioning if we are conservative because for office buildings, we have a standard for composite deck on steel beams. Our tile deflection is L/600 to .3~.5 to 1" (depending on span and situation) we can have beams spanning anywhere from 10 to 20 feet, and girders spanning 20 to 60 feet.

so, the came to us questioning our designs based on the structural firm that was hired to do the small-scale project. it is a composite deck with w8x15 beams spanning about 10 to 12'-0" with a live load of 50psf. The deck spans 8'-0" with 2-span to 3-span conditions.

So at the end of the day, if we are going by code, It works. It meets the Live Load of L/360 and TL/240. They want us to design the same way. We tried pushing against it. We explained and justified the situation, but it came down to "does it meet code, yes then design it that way." So now there are local deflections of about .8" and global deflections of over 1", but Per code It WORKS!!!! Code only specifies Local of L/360 not Global.

So at the end of the day, we did design our office building Per IBC Code Minimum. Which yes, they saved TONS due to reduced steel weight. They increased the clear distance of their floors which is also great, and we understand we won't be responsible for any damages or cracks or other issues that might occur due to the low deflection limit. But we still don't like the idea of designing it per that low of a limit.

The lead engineers in the office are basically thinking about it and are deciding to just start designing per code minimum. Because if our standard doesn't change, the company loses over millions of dollars a year of revenue from the client.

Unless we have been conservative all this time? We can only justify it as a rule of thumb and experience. Essentially, it's not enough, and per code it's L/360. Also, the L/600 comes from masonry design, so it's something that we cannot really justify. It's based on engineering judgment, but It still came back to code minimum is L/360.

Btw, i don't mean to make it sound like the architect firm are A*Holes. We had a civil and good discussion and they have listened fully because it's an important issue for us and them. They understand the code very well, but it's coming down to the client being saved tons of money and it still meets code minimum.

We have come so far with code books, yet code still does not give us a definite answer and are leaving it up to us. That's great and all, but now another firm might be added to a list of firms who design tile loading deflection of L/360 and a not-so-great global deflection. FYI, we still make sure vibration is okay!

Unless this is truly okay? Maybe our firm has designed everything conservatively?

 

[human909]

I haven't dealt much with tiles but that sounds pretty unconservative when it comes to deflection requirements. "It meets the Live Load of L/360 and TL/240" Those sort of values are generally my upper limits with steel floors! That said tiles are by definition segmented so are likely alot more tolerable of movement than a stiff masonry wall.

Deflection serviceability criteria is something that engineers/builders can push and push to save money until clients get unhappy and start the law suits or enforce rectification.

On one job for me last minute changes by the guys on site ended up neglecting a minor support on a walkway and thus ending up with a longer span. I reluctantly approved after checking it wasn't a strength concern. Months later during commissioning of the plant I ended up having to get some guys in to put some steel for that 'minor' support, fortunately it was a simple rectification exercise. The walkway was deflecting too much and felt like you were walking on a rope bridge. The vibrating conveyor didn't help!


The point is that it is often hard to tell what is unconservative until cracks start appearing or the floor starts bouncing! And once that starts happen rectification can be expensive!

 

[rowingengineer]

Ceramic is normally better tile at handling deflection compared to stone or similar in our area of the world (Aust). We have incremental and total deflections, however l/360 would be considered satisfactory for total local span with a check on vibration.

Don't know if our tiles have a different bedding system that makes them more flexible however I am yet to see an issue in 20+ years from this sizing method.

The only time I had issues with deflection is on steel joist (yes different situation) when you can see a noticeable deflection change when you walk across the room in each joist.

 

[TheDW]

I'm with you on keeping tighter deflection limits for brittle tile. Granted, I'm mostly working on SFH, so the scale of $$$ that conservatism costs my clients isn't as significant. I have successfully held my ground on this issue, but again...smaller scale.

Sounds similar to this discussion, in case you missed it:

https://www.eng-tips.com/viewthread.cfm?qid=489853

I think the comment got deleted, but there was a reference to another forum with a discussion about a recent lawsuit. The issue was out of plane wind deflection of an exterior metal stud wall with brittle stucco finish. At a window, 3 king studs (which met code) next to a single stud (which met code). The different stiffness's of the triple next to the single supposedly allowed cracking to propagate in the finish.

My point is that the engineer lost the lawsuit even though his design met code. All it would take in court is the plaintiff to get another engineer to say 'yeah, we usually design for tighter deflections for tile. They should've done the same'

 

[phamENG]

Deflections should generally be tighter, but it depends on the tile.

Boise Cascade put together a technical note based on information from the Tile Council of America and the Marble Institute of America. Ceramic tile in thin-set mortar can handle regular deflections pretty well - L/360 is fine. Stone tile, on the other hand, cannot. They recommend L/720 or 7/32", whichever is less. Here's the PDF BC put out. EDIT: These are TOTAL load values.

As for global vs. local, I'd only be concerned about local for the tile issue. You're concerned about cracking being caused by a curvature with a too small radius. Now there could be some cases where you want to look at a sort of 'composite' global - if you have a bay supported on each corner with a column and beams between them, joists running between two of the beams, look at the global deflection in the middle of the bay based on a 'span' from corner column to corner column. That will capture the curvature at 45degrees from the joist span direction.

And the you guys not being responsible for cracking...pretty sure that wouldn't hold up. If you're not responsible for the performance of the structure, who is? Now you can say 'we designed it code minimum' and maybe get off, or maybe they'll get another engineer to say no, you should have considered these other items. Like in 1604.3.6 Limits: "The deflection limits of Section 1604.3.1 shall be used unless more restrictive deflection limits are required by a referenced standard for the element or finish material." So it's important to look at the floor material they are specifying and read what the manufacturer requires.

Edit: If you want a referenced standard calling for total load of L/360 for ceramic, here you go: ANSI A108A-99 (AN-2.3)

 

[JoshPlumSE]

As for global vs. local, I'd only be concerned about local for the tile issue. You're concerned about cracking being caused by a curvature with a too small radius. Now there could be some cases where you want to look at a sort of 'composite' global - if you have a bay supported on each corner with a column and beams between them, joists running between two of the beams, look at the global deflection in the middle of the bay based on a 'span' from corner column to corner column. That will capture the curvature at 45degrees from the joist span direction.

That was pretty much my thought as well. Apart from that, can't you have your standard contract with this firm include a disclaimer that this client "has requested that our firm use the least stringent tile deflection criteria allowed by code"? Maybe not so harsh. But, just some words in the contract template for this particular client so that you don't have to wrangle over it every time and don't have to change your company standards.

 

[bones206]

I'd also pay attention to negative moment regions in the deck. That negative moment cracking that propagates along the beam centerlines could be mirrored in the tile and be a huge problem. Supplemental negative moment bars would help that issue. If you design these types of slab systems a lot you are probably already aware of that, but I think it's a problem that could be exacerbated by a reduction in overall framing stiffness. May not be as much of a risk if you have a bond breaker under the thinset, like that orange Kerdi Schluter waffle membrane stuff.

I don't think there is anything inherently wrong with the tug of war between conservative design and client's need for economy. You need that tension in the marketplace to keep things pushing forward and promoting innovation. I think the trick is staying within the "kern" of the standard of care, not just meeting minimum code requirements.

 

[jayrod12]

Regarding the potential for cracking in the negative moment regions, which is highly likely when you're playing at code minimum deflections, see if you can talk the arch into a control joint in the tile over the beams. I've had some success with that, albeit it was with an Architect that had previously dealt with cracking tiles on other projects. So they weren't too keen on it happening again.

I can't imagine you're having deflection issues with the deck spanning between the joists, but more likely joist deflection. Please correct me if I'm wrong.

 

[EngStuff]

Thanks for the response you guys,

Looking at what you all said, L/600 is way too conservative, based on the codes, we will start using L/360. That can be more justified, besides code, it seems like a lot of you guys have done it without issues.

 

[phamENG]

Make sure that L/360 is TOTAL load, though. That can make a difference. And I'd some sort of disclaimer saying that the design assumes a tile assembly of X, Y, and Z. If anything else is used, they need to contact you. Last thing you want is for a tenant to convince the owner to upgrade the floor to marble tile in their office after the frame has already topped out and nobody asks you "because there was already tile there."

 

[JoshPlumSE]

Make sure that L/360 is TOTAL load, though.

I have to disagree with that as written.... I believe we said this was a composite beam floor system. If so, then then the only deflection that could lead to cracking of the tile is due to load applied AFTER the tile is placed, right?

That's not entirely correct, because there is probably some "creep" in the composite beam. But, for the most part, I don't see a good reason to include the entirety of the dead load deflection that occurred during construction. Certainly you include the full live load, and any POST-Composite dead loads (including the weight of the tile and an allowance for some additional superimposed dead load).

 

[phamENG]

Josh - while I agree in principle, the warning is coming from the building code via referenced standard. See my post above (18 Jan 22) for a link to ANSI A108A-99, which is referenced b the 2018 IBC.

Floor areas over which tile is directly bonded to subfloor shall not have a deflection greater than 1/360 of the span when tested per ASTM C627. Make allowance for live laod and impact as well as all dead load, including weight of tile and setting bed. (Emphasis mine.)

Now, there may be something buried in C627 that would break it out some, but as I don't often design ceramic floor systems or have a need to test them, I don't have a copy.

 

[bones206]

It sounds like that particular requirement is for superimposed, post-composite dead load like Josh was describing. Maybe just bad/imprecise wording choice saying "all dead load", but reading between the lines I think they meant all post-composite dead load.

 

[EngStuff]

Make sure that L/360 is TOTAL load

We are using it for the total load. I did read the standard which i really appreciate that! It seems like we will be updating our standard to that deflection limit.

I think they meant all post-composite dead load

We think it's better to play it safe with tiles. After reading it multiple times and thinking about it. We are going to design it with all dead loads. We came to that conclusion because we feel like that as a standard, they would have stated to include post composite dead load only. Also, when they stated "Including weight of tile and setting bed" one can only assume the "all dead load" they are referring to is everything.

However, it does make sense to check the deflection for post composite deadload.

 

[bones206]

I'll never fault anyone taking a conservative interpretation, but I don't see how pre-composite dead load could affect performance of a floor finish in this scenario. Presumably the slab will be finished level and flat, so any deflection that could affect the tile would happen after the slab has set.

Maybe if the slab was to be placed with a constant thickness and not required to be level and flat, that would be a reason to include pre-composite dead load to limit the extra material (thinset or self leveling compound) needed to level the floor for the tile. I would think that would be an undesirable scenario though because you would be causing more deflection as you add more material to try to level the floor.

 

[XR250]

With concrete and steel I would be less worried about it than wood due to creep and the larger dead load.
What are the chances that the space will even see 25% of the design live load?

 

[8thStreet]

Great Discussion and I always think of this quote when people bring up code minimums: "A Code Minimum Building is the lowest quality building that you are allowed to build."