2012 IBC Chapter 34 - Existing Structures

[azcats]

I'm working on a project for a sign. Existing sign is ~15 years old and we have the engineering that was used at the time of installation. It was built in compliance with the current code at the time.

The sign owner would like to cut down the top of the sign and install a new LED display of the same size. Finished height will remain the same.

When checking the pipe column against current wind loads It is 15-20% over-stressed. Even though the projected area and height of the sign remain unchanged, the current wind loads are greater than the previous code. Any additional gravity load is negligible as the column is primarily a flexural element.

Is invoking the exception in section 3404.4 acceptable in this situation? The demand-capacity ratio before and after the addition is the same. However, the demand-capacity ratio is 1.15-1.20 under the current code. However, we're not increasing the demand-capacity ratio into the 10% allowable. If the ratios were under 1.0 there wouldn't be a question.

Are situations like this exactly why this section is included in the code?

Thoughts welcome.

 

[straub46]

azcats,

It sounds like the situation you're describing is why the Existing Structures provision is included. If the LED display is the same size (has the same wind area), and doesn't increase the lateral loading from what is currently there, the exception can be applied. I'm assuming the main cause for the wind load increase is either due to:

1) Increase in wind speed
2) Change in the way the demand or capacity is determined

or a combination of both.

I used to do analyses on telecom towers, and my company frequently used the existing structures exceptions to save $$ on upgrading the towers when a code change occurred. Provided the tower was designed appropriately originally, and the area after the new installation didn't cause the loads to increase by more than 10% from the original design (under the current code), we would invoke the exception.

 

[azcats]

Thanks for your thoughts.

The derivation and application of wind loads in the 1997 UBC was quite a bit different than the current building codes. Often quite a bit less back then. And that was also back in the days when there was a 1/3 increase of allowable stresses on load combinations using wind.

It was fun to pull out the old UBC for a peek. Back in the good old days when the wind design section was only two pages. I now sound like the old timers when I started in this business. "Back in my day the wind load was 15 psf. None of this two page nonsense."

 

[BUGGAR]

If you believe that the new code is more accurate than the old code, can you ethically design based on the old code (assuming life safety issues only)?

 

[Ron]

Look also at the administrative portion of the code. There is a provision of "area of replacement" and a provision for "cost of replacement". If the area of replacement exceeds a certain percentage, you must comply with current code. If the cost of replacement is above a certain relative threshold, then you must also comply with current code. Since this is a sign structure, those provision might not be applicable; however, you should check.

 

[azcats]

BUGGAR - you've touched on the conundrum here. It's always easy to take the more conservative route citing ethics and life safety. I guess the counter argument to your question is why would section 3404.4 be included if not to be used in exactly this situation? And technically, I'm not designing based on the old code, simply comparing demand-capacity ratios of an existing structure. Seems like invoking this section would make my design in compliance with the current building code. Maybe not up to current code wind loads, but in compliance with the code. That's really the root of my question. I'm not going to posit on the accuracy or my belief of said accuracy of the old or new codes. Nor do I really care to rely on my belief of the codes accuracy as to whether my design is code compliant.

Ron - I've searched for the provisions you mention as I've heard of them before. However, I can't find them in un-amended versions of the code. And they are not in the local code amendments to the 2012 IBC. Do you know if they exist (and I just can't find them) in the 2012 IBC as published by the ICC?

 

[straub46]

I recommend reading the IEBC (Existing Building Code), published as part of the ICC code family. It goes into further depth about what is and isn't acceptable for alterations, additions, and repairs. Specifically Section 401.3, "Except as provided by Section 401.2 or this section, alterations to any building or structure shall comply with the requirements of the International Building Code for new construction. Alterations shall be such that the existing building or structure is no less conforming to the provisions of the International Building Code than the existing building or structure was prior to the alteration."

I've read that in the past as any alteration (new structural member) needs to be designed to the current code (load determination), but any existing structural member can be left as-is provided it's demand-capacity ratio is no greater than what it was before considering the alteration under the new code. The exception (IBC 3404.4 or IEBC 403.4) allows for small increases on existing members, but still requires the new members to be designed to current code.

Seems to me that if the LED is a like-for-like swap in terms of loading induced on the structure, the alteration is in compliance with the existing building code, because the sign with the new loading is no less compliant than the existing sign. If the new LED wasn't going to be installed, the demand-capacity will still be over 1.0 under the new code, correct? So the LED alteration is "no less compliant" than the current sign.

Use of the existing structures causes of the code are up to the engineers discretion/comfort, but I feel that you are well within the code allowance to invoke the exception.

EDIT: Of course, this is all based on the original design being in accordance with the building code at the time.