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.

 

[hawkaz]

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.

 

[SteelPE]

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.

 

[ajk1]

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.

 

[amendale]

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?

 

[SteelPE]

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.

 

[nutte]

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.

 

[amendale]

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.

 

[SteelPE]

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.

 

[nutte]

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.

 

[JAE]

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.

 

[amendale]

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?

 

[amendale]

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

 

[SteelPE]

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?

 

[nutte]

New, current code.

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

 

[amendale]

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.

 

[SteelPE]

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.

 

[amendale]

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?

 

[SteelPE]

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?

 

[amendale]

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