IBC Existing Structures - load increase 5

[amendale]

I am adding a new platform to an existing steel structure. The IBC specifies a 5% and a 10% allowable increase in load/capacity ratio for gravity and lateral loads, respectively. Does this mean that I can add 5% or 10% to the capacity under new loading conditions, or a proportion of 5% or 10% of the old loading conditions? For example if the original structure was designed for an interaction ratio of 0.7, am I able to increase the ratio to 0.7 + 0.1 = 0.8 OR 0.7*1.1 = 0.77? These two calcuation would yield very different results under low design ratios, in which case the latter calculation would be very strict.

 

[SteelPE]

I'm not an expert in AISC 341 as I usually use R=3.... I'm sure nutte and others know a ton more than I do. But OCBF refers you to section 8.2 which specifically mentions compression elements and not tension elements. Table I-8-1 references limiting width-thickness ratios for compression elements. If you element is in tension I would think you would not need to comply with these two sections.

You have to remember that these limits are in place to insure that the members do not buckle locally prior to the brace buckling globally. If that is the case, and you don't any braces in compression there is no fear in having the brace buckle locally.

At least that is how I interpret this section of the code.... but again, I may be wrong.

 

[amendale]

In 2005 AISC 341 yes, in 2010 they got rid of the compression specification and assigned to all elements. I think the logic behind this change was that when the load reverses, the brace is going to buckle. When buckling occurs, you want to avoid local buckling in order to reduce the extent of plastic deformation. If you have too much plastic deformation, it might reduce the tensile capacity as well.

 

[SteelPE]

aaahhh, I don't have a copy of AISC 341-10. Are you sure they removed the word compression? A quick search online yields the seismic provisions and table D1.1 gives the limiting width thickness ratio for compression members. If you are sure they include tension members then you will need to make them comply with the provisions by reinforcement or remove and replace.

 

[amendale]

Actually yes you are right, I double checked and they did change the title but the word compression is still in there. I got I was overanalyzing the problem. Thanks.

 

[dhengr]

I think Nutte’s 17:27 post gives a darn good explanation of the code situation and SteelPE and JAE also give good added explanations of the problem. It seems to me that we have become such slaves to these overly complex codes, which should (which we want to) spell out every nut and bolt or detail, that we’ve lost all engineering perspective and the ability to apply any engineering judgement.

My understanding of the original intent of these types of allowable overloads, over stresses, or allowances for minor changes in loads, or structural configuration, etc. was the following: we are usually conservative in our code required design loads, we design to a min. Fy (whatever other criteria) which is almost always exceeded in the actual materials supplied, our design approach and formulas are generally conservative to account for typical expected variations, etc. etc.; So we’ll give you 5 or 10% without forcing you to redesign and reinforce the entire structure, that’s a small percentage of the safety factors we still have in our back pocket. Obviously, you can’t take 5% of the total uniform load on a beam, then apply it as a concentrated load right near one of the end connections, and say that’s o.k. That certainly violates the spirit or intent of the allowance, as well as any common sense or engineering judgement. You can take some new concentrated load, plus the current uniform loads on a beam, and be o.k. if that doesn’t change the stresses by more than 5%, or I dare say, overstress the beam by a few percent. All it says or does is... we’ll allow you to use a few percent of the safety factor for this change without a lot of extra rework effort; and sets some limits on these so people don’t go crazy. But, it is still your responsibility as the engineer to be sure that this change isn’t a killer someplace else, down the load path. If everything else checked out and was conservative, I might exceed those limits a little to save a bunch of rework for my client, but that is becoming more dangerous in our litigious society. I’m actually getting afraid to design anything to code any more, because I may have missed a new word or phrase on page 4278, in an appendix which was referred to in a new tabulation footnote associated with a sub-section which was referred to by that other chapter. I don’t have enough fingers to hold the page locations for the various sub-sections which a code paragraph refers to these days. And, while holding those page locations, I also need to reach for three different standards, and the AISC and ACI codes too.

One solution to the b/t problem might be to stitch weld a ½" sq. bar to the tips of the angles as a stiffener. You might also look at how you structure or support your new platform to lessen its impact on any one member.

 

[SAIL3]

dhengr just described exactly where I also find myself in regards to the recent code complexity and trivialization....with over 35 yrs of engineering experience, I have witnessed the codes becoming less useful and just about imposssible to understand the intent/engineering logic behind the numerous trivial details...what I try to do, currently, is try and parse any new code for any meaningful change and incorporate that into my design going forward and ignore the rest...so I basically design based on sound engineering theory and use my own engineering judgement....I have tried to use the new codes and have found myself becoming paralized when trying to interpret or use my own engineering judgement and am left with the option of becoming a "cookbook" engineer attempting to follow every letter of the code without a clue where or how many of these endless poorly written requiements originated from....for example, I tried to use the ASCE7-10 this week for what should be a simple wind/seismic calculation and just gave up...chucked it and used the ASCE7-05...in case one has not noticed, the schedules on projects are really becoming brutal...as a result, my designs are probably 10 to 15% overdesigned and I am ok with that...what is driving projects today are the financial costs of the upfront funding of projects, so that cost mounts up for everyday the project is being constructed...this cost blows away any dickering around with any minor weight savings..steel/concrete is relatively cheap when compared to all the other costs involved...I would love to see all the codes revert back to a 10 page document..this would force everyone to seperate what is really critical/important and get rid of all the other dross....

 

[SteelPE]

Sail3

This is getting off topic but I somewhat agree.

Before being laid off in April I did a bunch of work for a design-build company. The schedules were so compressed that it was often useless to try and refine a design. I had one engineer who reviewed my work make fun of me because I typically designed with an interaction ratio of 0.7. Well, when I have 2 days to get out a preliminary design for a 3 story building you can bet that I am not going to design to an interaction ratio of 1.0. It's not even worth the time as we typically had a 10%-20% chance of even landing a project. When the project comes through on the other end I don't really have the time to refine the design to save some $.

I have only briefly looked at ASCE 7-10. I never understood why the needed so many more chapters for wind load.... and why they need to change wind loads to ultimate loads. Seems like someone wanted to justify a paycheck..... or maybe force me to buy a new code.