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?

 

[TheDW]

I wouldn't say Ron hung the guy out to dry.

Yeah that was pretty strong phrasing so I deleted it. Not quick enough though!

From Ron's thread:

I stated in deposition that they did not follow an appropriate standard of care in evaluating deflection in two directions, not just one, and that even though the building code allows a load reduction for serviceability, that they had an obligation to evaluate the building, not just meet the minimum mandate of the code.

He said more than he would check it. Maybe he's paraphrasing and there is more to it. But that statement in a vacuum I disagree with.

 

[phamENG]

I can see your point. According to what I've been taught, meeting the minimum mandate of the code is what we're legally required to do. It's good practice to do more, but only required if mandated by the owner/client.

I think better phrasing would be "in failing to evaluate the total deflection of the wall system, they failed to meet the minimum mandate of the code."

In saying they didn't follow an appropriate standard care, I interpret that as "I consider myself to be a reasonable, careful, and competent engineer and therefore believe that the actions I would take are a reasonable standard of care, and they didn't meet my standard."

 

[Celt83]

this would be an instance where I would say the IBC minimums are in fact met, within the current language of the span ratio and deflection limitations. Meeting the serviceability requirements for the space is a longer discussion but if the arch/owner did not set serviceability criteria beyond the code minimum then any suit will hinge on the standard of care.

side discussion:
lets assume all those joists are the same size, and the joists can span and meet the two-way deflection checks Ron notes in that other thread across the full width. The interior walls are now non-load bearing walls but still need to be there for the architectural layout. Those walls are going to try to support the joist regardless, and with this discussion we are almost trying to say reducing the joist span makes for an inherently worse structure, that's a bit of a tough sell.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[jayrod12]

Yeah, that's where the detailing should require a slip joint between that wall and the underside of the floor framing to prevent exactly this issue. Assuming of course that they are all the same size and strength of joist.

 

[TheDW]

Is anyone out there calling for the first wall stud past every wall corner, or past a perpendicular interior shear wall, to be reinforced?

 

[Celt83]

Slip joint in wood construction, best your going get is the framing sub toenailing the plates to the joist and sheathing and relying on flexibility of the single plates to allow some movement.

Unless we've all switched to spec'ing these things, which I tried once and Owner/GC VE'd it off the job real quick: [URL unfurl="true"]https://www.strongtie.com/trussclips_platedtrussconnectors/tc_roofclips/p/stc.stct.dtc[/url]

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[phamENG]

Celt - I don't think we're saying it's an inherently worse structure. My point is that serviceability as it pertains to in-use structural performance is a structural issue whether we want it to be or not. If it were intended to be dealt with by somebody else, it wouldn't be in the structural section of the code. So we need to consider how our structure will deflect in use and how it will impact the non structural parts of the building and the comfort of the occupants.

I've spec'd the truss clips, but they also have screws available now that are a bit less labor intensive.

 

[Celt83]

Oh I'm in agreement, serviceability performance absolutely needs to be considered. The criteria defining that performance is just way to loose and open to interpretation, I mean shoot for years the IBC table for D+L in plain text said that dead load deflections of steel are 0 in the deflection calculations, footnote g "For steel structural members, the dead load shall be taken as zero".



My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[ArchGuy]

Thank you Dauwerda for citing this discussion:

"Ron posted a very similar discussion a few years back, here."

Do you (or anyone else on this post) have the contact information for Ron (or even just a last name)? I'd like to discuss my scenario with him.

Great discussion - I'm hoping for more comments on my arguments regarding whether this scenario meets building code. I'm not making a case regarding "standard of care" or "what most engineers do". Regarding these points from my previous post:

"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."

Are there rational arguments against my assertion that this scenario meets neither the code intent nor the code limits?

 

[phamENG]

I agree with your premise, but I'll play devil's advocate. Let me put on my best sea lawyer hat...

I disagree with your use of the term "member" to describe the floor. "Member" is not specifically defined anywhere in the code that I'm aware of, but it is used extensively. It is also used extensively in other definitions. Therefore, I think we can determine an intended definition as the word is used in the code. That definition is: a discreet component of a building structure such as a beam or column. What you are describing is a structural system. A system is comprised of several component members. I would also contend that member applies to those building materials that are marketed and manufactured as a 'closed system' - like prefab roof trusses. Those are self contained, whereas a custom timber truss for a church or a steel transfer truss might be comprised of members as it is custom designed and integrated with the structure in a more interdependent manner.

So checking the members - commonly taken as the joists in the floor system - can be easily satisfied. The question then becomes whether or not the floor sheathing is a member as it pertains to 1604.3.1. If we go back to our cobbled together definition of a member and the sources used to produce it, I don't believe anything like sheathing is ever referred to as a "member". That absence of usage could be used to justify saying it is not a member in the 'eyes' of the code and therefore isn't subject to the deflection rules.

There...I feel dirty typing that. It's all hooey - but the fact of the matter is that it happens all the time, most houses are built without engineering input at all and this happens. It will result in cracks eventually. It may result in sloping floors eventually. But it's done and done frequently. What I wrote above shouldn't be used to justify that it's okay - it should be taken as an indictment of the wording in the code that allows it. I don't think a code revision should be necessary, but such is the world in which we operate - we need a rule to force people to pay attention.

 

[phamENG]

And by the way - Ron is a regular contributor here. Perhaps he'll pop in eventually.

 

[dauwerda]

ArchGuy, I see my previous post was removed, I'm sure because of the reference to the other site. But I know for sure you can DM him on that site if you join it.

 

[Celt83]

IBC defines L as the length of the member between supports and sets deflection criterion as a ratio of L/some number.

In the case of the floor sheathing the joists are the supports, so with a literal interpretation of IBC the floor sheathing deflection ratio would be checked based on the local deflection of the sheathing between each joist, the fact that the supports are at different elevations does not apply otherwise no stair stringer in existence would pass deflection checks. The floor is not a member, your examples of shear walls and diaphragms is apples to oranges. Shear walls/diaphragms are tested composite assemblies that take advantage of the connections between all of the elements. The engineer designed the floor on a component bases if you want to say the floor as a whole is a member then you need to do a much more advanced analysis taking into account the composite effects which probably yields a better calculated deflection than what the engineer got with the individual members, more detailed analysis takes more time which requires longer schedule and more fee.

It is also important to note that the deflection limits which defined based on some statistical analysis of comfort and/or finish damage may or may not consider various aspects of the loading. A tile floor won't crack because of the deflection caused by the self weight of the structure that existed before it was placed only the loads causing additional curvature after placement would need to be considered for the serviceability of the tile.

Without knowing any specifics of this situation what is built may very well satisfy the code limits and intent, however those code minimums may not be serviceable to you which in that case alternate criteria should have been conveyed to the engineer in design.

...forensic investigation of a new house with this issue,...

This all assumes we are talking IBC and not IRC also, IRC has all kinds of prescriptive items that would fail IBC checks but the serviceability requirements of a single family home have historically been deemed to be much lower to allow for those prescriptive designs.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[TheDW]

There is no reasonable way to take the current code serviceability provisions and strictly apply them to this scenario.

If the floor perpendicular to the joists has to follow the IBC serviceability criteria, then two adjacent joists have to have deflections to within L/240 of 16", or .066".

If one joist is 10' long and one joist 12', one is allowed to deflect .5", and one .6". And the points of max deflection are different so you're even more screwed.

The 'intent'? Sure.

How its written? Absolutely not.

Edit: This would make unbalanced live loads much more complicated as well.

 

[XR250]

.... Or as you previously pointed out, a joist that is parallel to an exterior wall could only deflect .066" – which is impossible.