Addition to Existing Structure

[RFreund]

Regarding the evaluation of an existing building for the force increase in an existing building or element that is part of the existing LFRS:

How do you evaluate this increase:
1. Analyze the existing building with current design loads and determine forces in existing members of the LFRS.
2. Analyze the existing building and addition with current design loads and determine the new forces in the existing members of the LFRS.
3. Compare increase to 10% allowed by IBC.

or

1. Analyze the existing building with original design loads and determine forces in existing members of the LFRS.
2. Analyze the existing building and addition with current design loads and determine the new forces in the existing members of the LFRS.
3. Compare increase to 10% allowed by IBC.

Why?

Thanks!!


EIT

http://www.HowToEngineer.com

 

[JedClampett]

What is the purpose of step one in both methods? The structure plus addition needs to meet current codes. Whether the original structure met past or current codes is not material.

 

[RFreund]

Oh sorry, I forgot to state the original clause.
Essentially The IBC states that if lateral force increase in the existing member is less than 10% than it can be considered OK// as is.

What I'm getting at is a situation where you have an older building not designed for seismic and you want to put a small addition on it. So using the above procedure you may have the following outcome when evaluating an existing masonry shear wall:

1. current code wind 8kip ,current code seismic 10kip
2. current code wind w/ addition 8.2kip, current seismic w/ addition 10.5kip
3. 3.5%, 5% - OK//

1. old code wind 7.8 kip wind, NA (seimic not in old code)
2. current code wind w/ addition 8.2kip, current seismic w/ addition 10.5kip
3. 5.1%, 33% - NG//

EIT

http://www.HowToEngineer.com

 

[woodman88]

Under the IBC Chapter 34 (2009 ed) IMHO you do not need to "Analyze the existing building..." you just look at the existing structural elements supporting the addition.

You check the gravity loads for the existing structural elements that are supporting the addition. If you are increasing the loads (I use the current code loads requirements) to the existing structural elements more than 5%, you need to provide calculations to show the existing structural elements works under the current code or you need to provide a repair for those structural elements to the current code.
You do the same for the lateral loads, but with a 10% increase allowed.

The assumption is that the existing building was designed and built per the original building code requirements. You are only checking the design capability of the existing structural elements that are being effected by the addition. But this means that all the additional loads (from the addition) are going into just the effected structural elements. So, in the case of seismic, you can not take the existing building base shear value and compare it to the existing building plus addition base shear value for the increase.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[JAE]

The way I read IBC Chapter 34, the comparison (for the 10% increase check) is between [red]Original building/current code loads[/red] and [blue]Building with addition/current code loads[/blue].

 

[woodman88]

I agree with JAE. You just check per the current code loads. No need to look up and calculate anything from the original code, for the addition.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[msquared48]

That's the way I've always done it too.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[RFreund]

Ok, I had thought the same. Actually I didn't even think to consider the other scenario until my boss questioned me. I think the verbage is such that I can safely convince myself and him, that the only current loads need to be used.

Thanks guys!

EIT

http://www.HowToEngineer.com

 

[delagina]

I'm adding additional cantilever beams to existing structure/columns. This beam will support loads less than 5% of the original load of the gravity column. Should I need to re-analyze the whole structure/columns

or just locally analyze the new cantilever beam and connection?

 

[RFreund]

My understanding is that if you are not increasing the gravity force on the column than the column is considered OK (unless you are otherwise modifying it in someway to reduce its capacity i.e. removing a lateral support). Then you only need to design the beam and the connection.

EIT

http://www.HowToEngineer.com