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IBC Existing Structures - load increase
3

IBC Existing Structures - load increase

IBC Existing Structures - load increase

(OP)
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.

RE: IBC Existing Structures - load increase

That's not the way I have interpreted this. If I know the member sizes, I hold to current code requirements. If I don't know enough specifics about existing structural members (i.e. exact joist designation), then I use the 5%/10% allowances.

RE: IBC Existing Structures - load increase

I have always take the 5% and 10% as the lazy way out (which I use all the time). Basically you take the loads you have on your member using the building code today then add your new loads. If the ratio of new loads to existing loads > the 5%-10% then you need to take a closer look at the member and possibly add reinforcing. Upon closer look you may find that the column was over designed an you don't need to do anything to the column to accommodate the new loading criteria.

RE: IBC Existing Structures - load increase

I am not familiar with the document to which you refer, but based on what you quote from that document, I cannot see how it could mean 0.7 + 0.1; the only meaning that I would take is that it is 0.7 x 1.1.

RE: IBC Existing Structures - load increase

(OP)
But this does not make sense. If a member is originally over-designed, say to an interaction ratio of 0.2, using a 10% allowance in increased load I can only at 2% to the capacity of the member. However if the original design was more critical, say to an interaction ration of 0.9, I would be able to add 9% to the capacity of the member. This results in a reasoning that says I can add more load to a member the more critical it is. I dont see the logic benind this. Am I wrong?

RE: IBC Existing Structures - load increase

Yes, you are wrong. I am going to try and use an example off the cuff here so hopefully it isn't wrong.

Say I have a beam in a roof that is 25 feet long in an existing building that the client would like to hang a 1k load on the underside. I go through the code I would use today (using today's snow loads and live loads) and figure out that I have 2k/foot load applied to the beam. This means I have a 156.25 foot-kip moment in the beam. The new load applies a 6.25 foot-kip additional load on the beam bringing up the moment to 162.5 foot-kips. 162.5/156.25 = 1.04 < 1.05 therefore the beam is acceptable.

Now lets say that you have the same beam but you figure it has 1k/ft load on the beam. This means the existing beam is supporting a moment of 78.125 foot-kips. Adding in the 1k point load brings the moment to 84.375 foot kips. 84.375/78.125 = 1.08 > 1.05 NG. Something needs to be done. You go out onsite an find that the existing beam is a W18x35 50ksi member with a moment capacity in accordance with the AISC of 101 foot-kips. 101 foot-kips > 84.375 foot-kips therefore the beam is OK.

Now if you go out onsite an find a W12x35 50kis which has a capacity of 79.6 foot-kips then you will need to reinforce the beam to support the new load.

I have always though of the 5% limit as trying to help out with a difficult situation. In my area they are constantly messing around with snow loads. If a client wants to hang a 1psf ceiling from the underside of the roof only to find out he can't because the code people decided to increase the snow load by 5psf and his roof now doesn't work. What does he do????? reinforce the whole roof because someone decided to increase the snow load..... but the building has been standing for 50 year w/o incident..... Very difficult to explain this one.

This is how I interpret this section of the code. If someone doesn't agree please let me know.

RE: IBC Existing Structures - load increase

Concerning the 5% increase for gravity load, the IBC sections in question are 3403.3 and 3404.3. They state that if the addition or alteration causes an increase in design gravity load of more then 5%, the member shall be strengthened, supplemented, replaced, or otherwise altered as needed. Note that it refers to load, not stress, or moment, or interaction ratio. So if your beam has 1 k/ft originally, you can go up to 1.05 k/ft without having to modify the beam. If your beam is supporting a concentrated load, I would apply this 5% increase to the new concentrated load.

For elements resisting lateral load, the code refers to the demand-capacity ratio, and says that this ratio can be increased by 10%. Here, you would multiply your demand-capacity ratio of the existing member by 1.1 to find the new limiting demand-capacity ratio. If it's at 10%, you can up it to 11%. If it's at 90%, you can up it to 99%.

What this tells me is that for gravity members, you don't even need to worry yourself with the interaction ratio. I guess it's assumed to be OK by virtue of being "existing." As long as you increase the applied load by no more than 5%, you're OK.

For lateral members, though, you do have to check the interaction ratio. It's not enough to simply limit the applied load in a member to 110% of the original.

RE: IBC Existing Structures - load increase

(OP)
Nutte - increasing the load is the same as increasing the interaction rate, as capacity doesn't change.
SteelPe - I understand your example, however you didn't address my question. Say I have a drawing of the existing building and I know all the member sizes. When I look at the existing braces in the structure, under existing loads, I find that some braces have 10kips in them and some have 2kips, all the same size with the capacity of 30kips. Now I want to apply 1kip additional load. According to your interpretation of the code I am allowed to apply this load to the braces that have the higher load in them, but not the others ones, even though they are both under capacity.

RE: IBC Existing Structures - load increase

nutte,

It's interesting to see that the IBC chapter 34 is in reference to load while the IEBC 707.4 exception #1 specifically references stress. I never noticed that before. The jurisdiction that I practice in replace chapter 34 of the IBC with the IEBC.

amendale,

Does your building fall under IBC chapter 34 or the IEBC. In referencing the IBC, if your demand capacity ratio > 10% then you need to make sure the members comply with the building code today (sections 1609, wind and 1613, seismic). In your example this should not be a problem.

RE: IBC Existing Structures - load increase

Sure, if it's a linear relationship, then you have the same effect. I was just pointing out the code refers to load, so as to simplify the discussion.

And SteelPE did address your question. Unintuitive as it is, you are allowed to add less _load_ to the member at 0.2 than the member at 0.9, without having to possibly reinforce the member. You can still add more than 2% to the 0.2 member; you just have to check it, reinforcing it if necessary. Reinforcement is not required, if the member can be shown to be adequate with this increased load.

RE: IBC Existing Structures - load increase

nutte mentioned the key here in understanding the supposed "un-intuitive" nature.....

the code uses this 5% and 10% as a general check - assuming existing members are designed near the 1.0 unity value.

The code also says that IF you exceed these general exceedance numbers then you simply do the engineering to check them for capacity

- for your 0.2 unity member this would be a no-brainer.

RE: IBC Existing Structures - load increase

(OP)
Do you know if in the case the prescribed limits are exceeded, do the existing members with the addition of new loads have to comply to the code at the time of their design, or to the new current code?

RE: IBC Existing Structures - load increase

(OP)
In my case the existing structure is located within a seismic zone and was not designed in accordance with the current AISC-341 requirements, because it did not exist at the time. If it is checked against curent code requirements, the bracing does not even meet the compactness requirements of AISC 341

RE: IBC Existing Structures - load increase

As far as I know, if the limits are violated the structure needs to comply with the current code.

What are you using for an R factor. Would it be possible to use R=3 and ignore all of the AISC 341 requirements?

RE: IBC Existing Structures - load increase

New, current code.

Section 3403.3: "... as needed to carry the increased load required by this code for new structures."

RE: IBC Existing Structures - load increase

(OP)
We are in seismic zone D. Our R factor is 3.25 for OCF frames. The problem I am facing is that we need to add a platform to support some equipment onto an existing structure. That structure was built in 2001, before AISC 341. The bracing that they use is based on maximum slenderness ratios, and they do not meet the compactness requirements of AISC 341. The load that we are adding is well below the capacity of the braces, but exceeds 10% of the existing load. If we need to comply with AISC 341 then we would need to basically redesign the existing structure.

RE: IBC Existing Structures - load increase

Are we talking compactness or slenderness ratios.... or both? Is there any way to cut down on the length of the brace to get it under the slenderness ratios? What type of bracing members are you dealing with? tubes, angles, WF?

Other than that, I am all out of ideas...... I guess it is up to you to figure out now what you need to do. The code basically says that you need to comply with the code as it is today. If you can't then you need to do what is necessary to get it to work.

RE: IBC Existing Structures - load increase

(OP)
The slenderness is fine, its the compactness ratio of b/t < 0.38sqrt(E/Fy) that needs to be satisfied. The existing X-braces are angles - L3x3x0.25 and L4x4x0.25. They fall outside of the compactness limit. So we definately need to satisfy the NEW code even if we determine that the addition of new loads is well below the capacity of the existing members as per the old code?

RE: IBC Existing Structures - load increase

You will either need to replace the existing bracing members or come up with a way to make the existing members comply with the b/t ratio.

Also, I don't see any requirements on the amount of braces needed in compression for a OCBF system. Would it be possible to use tension only members and opt out of the b/t requirements as the b/t requirements are for compression elements?

RE: IBC Existing Structures - load increase

(OP)
They are already tension only braces. But I think the b/t requirements in the AISC 341 apply to both tension and compression no?

RE: IBC Existing Structures - load increase

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.

RE: IBC Existing Structures - load increase

(OP)
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.

RE: IBC Existing Structures - load increase

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.

RE: IBC Existing Structures - load increase

(OP)
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.

RE: IBC Existing Structures - load increase

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.

RE: IBC Existing Structures - load increase

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....

RE: IBC Existing Structures - load increase

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.

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