Building Code deflection limits perpendicular to joist/truss spans 5

[ArchGuy]

The IBC Floor and Roof deflection limits are frequently applied in the following manner for wood framing. The deflection of each floor joist or roof truss is checked to meet L/360 for Live Load, and L/240 for Total Load (I know the roof limits can be different, but for simplicity and for purposes of this discussion, lets assume the roof supports a plaster or stucco ceiling). If each joist or truss meets the deflection limits under the applied loads, then the IBC deflection criteria are considered to be satisfied.

There is a serious flaw with this approach, and creates a scenario where I believe the IBC code is not met. To illustrate, consider the simple framing plan below (which represents common scenarios).

Assume that the IBC deflection limits along the span of each two-point bearing joist or truss are just barely met. This satisfied deflection in one direction. However, the floor or ceiling that these joists or trusses support is, for service, meant to function as a two-way system (people using the floor above or the ceiling below expect the floor/ceiling to function the same regardless of which way the members frame). So, consider the floor span between bearing walls in the section below perpendicular to the truss/joist direction:

IBC Section 1604.3.1 states:

"The deflections of structural members shall not exceed the more restrictive of the limitations of Sections 1604.3.32 through 1604.3.5 or that permitted by Table 1604.3"

The floor itself is a structural member. Therefore, the Code deflection limits for the floor in the direction perpendicular to the joist/trusses are not met in the above scenario, because the floor span perpendicular to the joist/truss span is half that of floor span parallel to the joist/truss span for the same deflection. In addition, this creates a sudden drop in elevation from the 3-point joist or truss to the 2-point joist or truss, which violates the curvature limits on which the code is based. It creates a noticeable floor slope issue for people using the floor in the case of joists, and it often results in ceiling cracks in the case of trusses. Therefore, it doesn't meet the intent of the code either.

Do you agree that the IBC deflection limits language and intent are not met by scenarios such as the one outlined above?

 

[Tomfh]

You are supposed to satisfy deflection limits in all directions, and yes, if you have a hard joist (Eg supported on a wall) adjacent to a soft joist (Eg a long single span) then you can easily get unacceptable deflection perpendicular to the joists.

 

[MIStructE_IRE]

Rightly or wrongly this issue is ignored in most domestic cases.

 

[phamENG]

You're spot on. This issue is ignored by most "designers" (the bulk of the unlicensed home designers, quite a few licensed architects, ALL of the EWP supplier techs driving their sizing software, etc.). Quite a few structural engineers don't pay it enough attention either.

When faced with the code provision, most don't consider the floor sheathing a structural member. If you do believe that it is, you can look at internal deflections (deflections between two supports) which are typically assumed to be rigid vertical supports for the purpose of designing the sheathing. It's only after you accept the fact that the important thing is global deflection and, still more important, the shape of the floor for the sake of comfort and preventing damage to finishes that you have to be honest and consider the joists to be spring supports that you understand the true shape.

In your case, it may be hard to satisfy. You'd have to a) do special detailing to remove the walls below from the load path (out of plane bracing, truss clips, etc.), run a beam from the middle bearing wall perpendicular to the joists to smooth out your curvature, or use stiffer/deeper joists to limit mid-span deflection in the longer span joists. All of these have potential constructability/architectural implications that may make them a tough sell - especially for a problem that most people don't understand (until their floor separates and their drywall cracks and they look for somebody to sue...)

 

[Klitor]

Is it really such a 'tough sell' in usa-residential to run a simple perpendicular beam in a case like this?
Seems like a non-issue thread.

 

[phamENG]

Depends on the span. I've had cases where that beam needed to be deeper than the floor joists. Deeper floor joists to hide it mean more money. If you're building a $6M custom mansion, no problem. Mid-level tract homes for a developer that pinches every penny? Tough sell.

 

[XR250]

I typically ignore this but do see an issue with it occasionally in older homes once creep deflection has occurred. It mainly shows up as a sheetrock crack in the ceiling or noticeable differential floor deflection.
I try to never design to code minimum deflection - which can help prevent this issue.

 

[TheDW]

Seems like a non-issue thread.

The 'tough sell' comes in when you have to tell a homeowner that the visible hump in their floor meets code, so they have no legal leg to stand on to make the builder fix it.

We will usually put a double joist right next to the rigid wall to hopefully minimize this effect. Seems to work okay. Same thing can happen in a crawlspace with interior piers.

I've seen it in new houses, the most recent had the floor trusses at 24" spacing and a kitchen island on the long joists. That doesn't help.

 

[ChorasDen]

I see similar issues with long span floor joists framed near a a 90 degree corner stem wall at the foundation level. Regardless, code deflection is really intended for one thing: preventing damage to finish materials. I do not consider deflection to be truly a structural concern (it can cause other issues, however, such as moisture intrusion). With that said, differential deflection as you have detailed in the OP does cause perception concerns for the building owner (rigid support below one joist, with a long span below the next, often noticeable by those that walk across the floor). However, I rarely see this causing issues on finish materials. Gypsum rarely cracks under the situations I see, but more often it negatively affects how the floor feels.

 

[dhengr]

ArchGuy:
With the last two ‘3-point support’ trusses or joists, at the end of the wall, why not notch the top plate on the wall 1/3rd and 2/3rds of the deflection of the first full span truss, so they are set lower? Then that deflection change will take place over there truss spaces, instead of just one space. That wall should probably have a double top pl. There are also some good framing, sht. rkg. and truss deflection details out there which allow this relative movement to happen with minimal damage to int. finishes, etc. This is a fairly common problem in many U.S. houses with a central hallway which ends at a larger open room (open living & dinning rms. for example). Intended or not the clear span roof trusses bear on one or both of the hallway walls, and don’t take their full deflection until they span the full length in the open room space. And, there is invariably some ceiling cracking at the end of that wall.

There is a similar, but opposite, roof truss movement problem, in the same room/ceiling plan location. This one is due to roof truss uplift (some may call this truss jacking, truss humping, I think). It is caused by length changes primarily in the top and bot. chords of roof trusses during the colder seasons in the north. The top chord lengthens due to increased moisture content up near the roof sheathing, while the bot. chord doesn’t change much in length, since it is in the insulation and near the warm ceiling. The outcome of this top chord lengthening is that it moves up, and through the web diags., it also pulls the bot. chord up with it, into an arched shape. A good vapor barrier at the ceiling, good insul. and venting the attic space can usually resolve this issue, for the most part.

 

[BAretired]

In the situation shown in the OP, it might be better to leave a gap between joist and wall so that all joists can deflect equally. However, there is still a problem with the joists adjacent to the exterior bearing wall.

BA

 

[ArchGuy]

Thank you everyone for posting responses. Regarding this comment from Daywalker,

"The 'tough sell' comes in when you have to tell a homeowner that the visible hump in their floor meets code, so they have no legal leg to stand on to make the builder fix it."

This is the case I'm making - that the homeowner does have a legal leg to stand on, because his floor deflection actually does not meet code. It meets neither the intent nor the language of the code.

The intent of the code is to limit the curvature of members supporting components sensitive to such curvature (hence, we have tighter limits for members supporting stucco or drywall, for example). Excessive curvature causes damage to finishes and other components. This violates the serviceability section of the code: Section 1604.3 - "Structural systems and members thereof shall be designed to have adequate stiffness to limit deflections and lateral drift".

The Section which follows is to quantify what this limit should be "The deflections of structural members shall not exceed the more restrictive of the limitations of Sections 1604.3.32 through 1604.3.5 or that permitted by Table 1604." Again, examining the table and footnotes, these limits are intended to limit curvature. Limiting curvature in one direction but not the other direction does not meet the intent of the code.

My other argument is that the span ratios are not strictly met because the floor itself is considered to be a structural member, even though the components making up that member are discrete. We consider a shear wall to be one structural unit, even though it is make up of discrete panels, and we treat it as a unit for in-plane deflection. We do the same thing for the floor sheathing when considering diaphragm deflection - we treat it as a unit. Why would we not do the same for out-of-plane deflection of the floor system? Furthermore, take an individual 4x8 plywood sheet spanning over four trusses spaced at 2' on center. That member itself does not meet the span ratio in the scenario I cited above.

By the way, this question came out of a forensic investigation of a new house with this issue, where walking the floor perpendicular to the joists makes one feel dizzy and disoriented.

Thanks again for your input everyone. I would appreciate any additional answers to this question - do you agree the IBC code serviceability limits are not met in the scenario I cited above?

 

[XR250]

This is the case I'm making - that the homeowner does have a legal leg to stand on, because his floor deflection actually does not meet code. It meets neither the intent nor the language of the code.

Well I certainly hope y'all lose your case because you can find this issue in most houses if you look hard enough. All they have to prove is that it is standard practice NOT to check this - which is my experience.

 

[jayrod12]

I agree it is standard practice not to check this. At least in my experience.

Personally I detail around it. In your case, I likely wouldn't of used that wall as a bearing wall unless absolutely necessary, and then if I did, I would've done something like some others have recommended above. But what I do often gets called overkill by other engineers/contractors.

 

[TheDW]

I don't think it is reasonable to argue that having a floor joist parallel to an exterior wall doesn't meet code.

 

[XR250]

But what I do often gets called overkill by other engineers/contractors.

Yea, if I showed that detail on my plans, it would be the last time I worked for that contractor.
I am pretty mindful about deflections however.

 

[phamENG]

I think you're right on the intent of the code, and AISC/SJI has included this issue in a couple of webinars I've seen. XR, jayrod, and Daywalker have all brought up great points, though. The code isn't explicit on this, and it would be VERY easy to find enough expert witnesses to say no, they don't check that either, to establish that doing so is above and beyond the standard of care.

I try to make it part of my standard practice, but that's only because I've seen the issue enough times to understand that it's real.

Quite often, though, I'll check dead load with creep only. That's because the live load component is typically so low that it gets lost in a rounding error. They may not always be the case (bookshelves along a wall, etc.), so you have to be careful and consider the intended use. But trying to hold D+L to L/360 perpendicular to the joists when a long span joist is 16" from a wall is usually not worth the expense. You can help to control those situations with strong backs, bridging, etc.

 

[Brad805]

"where walking the floor perpendicular to the joists makes one feel dizzy and disoriented." One or two human's walking around a floor << design load. This suggests either an overreach on the description or a deeper underlying problem. If long span and the floor coverings are very light it could be vibration, but most pay attention to the floor rating you calculate.

 

[TheDW]

Thanks dauwerda.

Standard of care is what a 'reasonably careful and competent engineer would do in similar circumstances'. Judging by the responses in this thread and Ron's thread, most people don't pay it much if any attention, so I'm pretty surprised at Ron's conclusion, and understand why the engineer would file a complaint.

I'll be very interested to see what the code writers come up with, because I absolutely agree that its a problem that we should pay attention to in a perfect world. But like even Ron said, its not codified. So, it is very tough to win the argument with the contractors. I run into this on a pretty regular basis and would love some clear guidance on it to come to the table with.

Maybe some general notes about the stucco guys paying attention to crack sensitive locations would've saved that engineers butt.

 

[phamENG]

TheDaywalker: I wouldn't say Ron hung the guy out to dry. He was asked if he would check it, and he said yes. The standard of care is really a pretty nebulous thing, and as I understand it the definition is typically determined by the court after hearing from a number of engineers. Unless I'm the head of the only engineering firm in the region, or every engineer practicing in the region has worked for me at some point in the last 10 years, I really can't define the standard of care on my own. I can only say what I do and those of whom I have a close working knowledge would likely do. From there the court has to decide if I'm the model of that 'reasonably careful and competent engineer', if I'm myself subpar, or if I'm an overachiever.

I haven't been directly involved in a case (yet), but I did do support work for a previous employer for a number of cases he testified for. It always seemed that the plaintiff sought out the best, most expensive engineers for their side to prove that it should have been done (forgetting, of course, that they shopped the original fee and beat the sucker who took the job down until he barely had two pennies to rub together when the design was complete) while the defense scraped the bottom of the respectable barrel of engineers to get somebody to justify why it was okay they didn't do it.