2012 IBC Chapter 34 - Existing Structures
2012 IBC Chapter 34 - Existing Structures
(OP)
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.
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.






RE: 2012 IBC Chapter 34 - Existing Structures
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.
RE: 2012 IBC Chapter 34 - Existing Structures
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."
RE: 2012 IBC Chapter 34 - Existing Structures
RE: 2012 IBC Chapter 34 - Existing Structures
RE: 2012 IBC Chapter 34 - Existing Structures
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?
RE: 2012 IBC Chapter 34 - Existing Structures
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.