Responding to Plan Checker's Comment (the polite method) 33

[SE2607]

I have a plan check comment that one of my beams is 2% overstressed. While I always try not to exceed a CSR ratio of 1.0, it sometimes happens (putting a bigger beam is expensive, not available, etc.). This is a wood-framed residential remodel, which should be as safe as any other structure, I find this ridiculous. I could reduce the assumed dead loads enough to make this work, but I'm wondering if there are any provisions in the code which allow me to do this.

TIA

Sorry for the double post.

 

[RipPECity]

Sometimes I find it better to just do what they ask rather than argue and potentially get more comments back. I'd just sharpen my pencil a little on the loading/tributary area/length and show it works...

 

[jayrod12]

I agree just sharpening your pencil on your calcs and then sending them in should do it. 2% isn't enough to be worried about that it doesn't work if you trim sig figs more accurately this time rather than the first time.

 

[Eng16080]

Others will likely disagree with me, but I agree with the plan checker here. Technically, the beam fails based on the analysis that you provided. End of story. The code draws a hard line and you crossed it. Well, just barely, but you still crossed it.

Certainly the odds of the beam failing are nearly zero. We all know that. But what should the plan checker do, let a beam at 2% over pass, how about 5% or even 10%? Will a beam overstressed by 50% even fail? Probably not.

So, if you're like me, you probably made some conservative assumptions concerning the loads getting to the beam and perhaps a few other things. If you refine your numbers, I'm guessing it will work just fine. And if it doesn't, then I'd just upsize the beam, reduce the span, or something so that you don't cross the line.

 

[milkshakelake]

Some of us overdesign by a few percent once in a while. Like those cases where you're on the precipice of specifying a 10" LVL that's a few percent over, versus a 12" beam that works nicely but would piss off the architect/owner. I know one engineer who never does that, no matter what, so it varies person to person.

When it comes to finding that extra 2% to pass a code review, it's a matter of refining all the little things, like:
-The NDS factors (size factor, temperature factor, repetitive member factor, etc)
-Playing with the support conditions (clear span vs "regular span" vs full span); 2" refinement might make it pass
-Honing in the loading
-Reducing the superimposed dead load by a few psf with a more exact calculation
-Live load reduction
-Slap on a 2x8 sister joist in the midspan (kind of a complicated solution)
-An engineer I know considered the plywood to compositely act like a T-beam in a tight situation like yours, but I don't recommend this (hard to justify)
-Any moment continuity you can use? Like over a 3 span condition.
-I've never heard of anyone doing this, but it's a crazy idea I had. Let's say it's bearing on a 5.5" post. If you nail it nicely, can you consider it partially fixed, like 5%? This is probably more trouble than it's worth.

 

[JoshPlumSE]

I remember putting together a calculation where I came out slightly overstressed. I didn't try to sneak it past the checker or anything. I highlighted it and then added an explanation about where I felt I'd been overly conservative with my applied loads, unbraced lengths or such.

 

[dik]

Can you re-evaluate your design dead load to pare off a few pounds? I generally design things quite close, except for columns and cantilevers. If there's any chance for redistribution... I'm not so fussy.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[phamENG]

I'm with Eng16080 on this. The code isn't about 'what can I put up that won't fall down', it's about 'what can I put up that will reliably stay up'. Sounds similar and a bit pedantic, but they are subtly different. The codes are based on a target reliability - designing for a DCR greater than 1.0 reduces that reliability. You may feel that the line is arbitrary, and perhaps it is, but I'd prefer we all use the same arbitrary line than everyone getting to arbitrarily adjust the arbitrary line to their own liking.

If you've sharped your pencil down to a nub and you're still 2% over, then use a bigger beam. Otherwise, sharpen away.

 

[Tomfh]

If by your own admission you have crossed the line I don’t really see an issue with a reviewer raising questions at this stage of the design process.

Exceeding the code limits by 2% is a matter of engineering judgment. While some engineers are very flexible, many will only consider it under certain conditions, and others might not allow it at all.

Is it truly impossible to reduce the loads by 2%? Or alternatively show that the beam has more capacity than assumed? Is there really no margin for adjustment?

 

[ChorasDen]

This is a wood-framed residential remodel, which should be as safe as any other structure, I find this ridiculous

This statement is a bit of a reach in my opinion. As PhamENG eloquently put it earlier, the intent of code is to hit a target reliability. Complicating that, you are working on a remodel, which presents additional issues with trying to deal with historic design reliability targets. Additionally, you may not be aware of changes that previous owner/occupants have made to the structure that also hamper hitting the reliability targets.

I sit on the side of the fence that 2% over needs to have more than just a simple hand-wave to justify.

 

[human909]

I think others have hammered home the point so I won't labour it. Personally I agree with your handwave, but I wouldn't ever submit a design for review that clearly shows an underdesigned member. You are putting the checker in awkward position.

Regarding margins what margins do most people operate on? (I like to call it my sleep comfortably at night margin.)

I prefer 65-80% range for most of the steel industrial structures I design. I don't see the cost savings in pushing things to 95%. Also the steel structures that I design often are redesigned and reconfigured with additional loads.

As far as residential timber goes I'm more happy with 90%, this is driven by following the approaches of a mentor rather than highly experienced design in this are.

 

[phamENG]

In wood I design for serviceability, and then verify it meets strength afterward. It's very rare that I need to up-size for strength.

 

[XR250]

In wood I design for serviceability, and then verify it meets strength afterward. It's very rare that I need to up-size for strength

Same here unless the span is short and it is dimensional lumber.

 

[canwesteng]

2% would round down to 1.0, I'm not sure in any application we have the accuracy to determine the difference between 1.02 and 0.99... that being said, I'm not sure what is expected of the reviewer since he probably can't ignore it, and I don't know that this is justified for new construction anyway, when you could just bump up the size.

 

[Aesur]

I agree with the majority of everyone else here, sharpen your pencil or try to explain to them as JoshPlumSE mentioned why you are accepting this. I have on occasion done the same, just put a note in the calcs, structurally acceptable as I used conservative loading, etc..

There is a PEMB document about significant figures out there that is used by the PEMB industry for 103% utilization rounding down to 100%. While they are providing designs with almost no extra loads, etc., I can't necessarily argue with the position paper, although it did make me add to my GSN that we will not accept any member over 100% as a few other engineers have done. I have only had this be an issue once, and they were actually able to adjust one little thing and all of a sudden everything dropped a few %.

 

[Greenalleycat]

I'm on board with everyone else here - I'd sharpen the pencil and just include calcs that work if possible

However, I know that sometimes your spreadsheet will just spit a fail
A classic example for me is that I like to design all my beams assuming the same dead load (floor or roof say)
I usually assume values that I know are higher than reality to give options in the future/sleep at night factor/flexibility on site
This sometimes causes a problem where one beam fails narrowly and you're stuck either including a 'failed' calc or changing the assumptions that you have used across the board
My preferred approach here is to include the failed calc then write/annotate in PDF an explanation as to why this is actually OK
That way the design assumptions are consistent but the one outlier is appropriately explained

 

[hokie66]

I'd just add a note to the calculations, "Within tolerance, accepted".

 

[Euler07]

1.02 is 3 significant figures. Is every single value you're using in your calculation 3 significant figures? If only one value is 2 significant figures then your answer is rounded to 1.0.

This is one of my pet peeves when it comes to engineering calculations. I see calcs where, for example, the input is 0.5 kPa and then the answer is 3.52465kNm.

 

[Tomfh]

Strictly adhering to significant figures rules can overlook precision nuances. E.g. 0.99 and 1.0 have two significant figures, yet 0.99 has about 1% precision, while 1.0 only offers 10%—a major difference. So I suspect they’ll need a better argument than rounding down to 1.0 on the basis of significant figures.

 

[jerseyshore]

Who submits calc's that say failed on them? You gotta know better than that. What's the reviewer going to say other than not-approved?