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
RE: IBC Existing Structures - load increase
RE: IBC Existing Structures - load increase
RE: IBC Existing Structures - load increase
RE: IBC Existing Structures - load increase
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
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
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
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
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
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
RE: IBC Existing Structures - load increase
RE: IBC Existing Structures - load increase
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
Section 3403.3: "... as needed to carry the increased load required by this code for new structures."
RE: IBC Existing Structures - load increase
RE: IBC Existing Structures - load increase
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
RE: IBC Existing Structures - load increase
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
RE: IBC Existing Structures - load increase
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
RE: IBC Existing Structures - load increase
RE: IBC Existing Structures - load increase
RE: IBC Existing Structures - load increase
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
RE: IBC Existing Structures - load increase
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.