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Constructing Additional Level on an Existing Building and IBC Chapter 34's 10% Rule

Constructing Additional Level on an Existing Building and IBC Chapter 34's 10% Rule

Constructing Additional Level on an Existing Building and IBC Chapter 34's 10% Rule

(OP)
I've got an existing 5 story concrete building where the owner would like to construct an additional level at the roof. I do not have any plans yet for the building, but I was told that the original building was designed for an additional level. The building was constructed in the mid-60's under the old Standard Building Code. The building is located in SDC = C.

So let's assume that the original building was designed correctly under it's governing code and the gravity system can handle an additional level. The building was probably designed for 15-20 psf wind. I don't have a copy of mid 60's SBC, but I doubt seismic was even considered in the design, due to the location of the building and local practice by engineers during that era.

Obviously adding an additional level will bump lateral loads on the building's lateral load resisting system by more than 10%. If it's an ordinary reinforced concrete moment frame, it would not be permitted for SDC=C today. If the building is an ordinary reinforced concrete shear wall, it would be permitted.

I know there were some issues with ACI's stirrup spacing requirements back in the 60's and that this building would most likely not meet today's seismic detailing requirements.

IMO, even though the building was designed for an additional level, and if the original plans say what additional loads (or levels) the building was designed for, IBC says I must run a new analysis of the existing building to see if it can handle today's loading. Is this correct? And the original frame or shear wall system must be detailed for todays seismic detailing requirements???? Do I need to get an FRP engineer involved?



Thank you in advance for your suggestions!





RE: Constructing Additional Level on an Existing Building and IBC Chapter 34's 10% Rule

I think you are correct. This is always a tough response to give back to your client - difficult to describe the reasons why a building "designed for an additional floor" can't carry an additional floor.

Meeting current codes with older buildings is very difficult.

RE: Constructing Additional Level on an Existing Building and IBC Chapter 34's 10% Rule

(OP)
JAE,

Thanks for your response. I just really have a problem with how prescriptive our codes have become. I completely understand the logic on the 10% rule, but there should be some discretion left to the PE/SE. For gravity loads, the current code does allow us to use the legacy code loading if there is a occupancy change of use (may have to post loading), so why not lateral in this case.

So a past owner spent the extra money (this is a state building so that means we the people paid for it) on the foundation and framing for future use, the building is in outstanding condition, is in an area that has never had seismic issues in recorded history that IMO would endanger the public.......yet we can't add a level to the building?

So how does this FRP work? You hire some firm that is experienced with it. They wrap the columns/beams/shear walls that they deem critical and then they say it "meets code"?

Thanks!

RE: Constructing Additional Level on an Existing Building and IBC Chapter 34's 10% Rule

I agree with JEAE here, and that is the risk you take by designing for an extra story, and waiting 50 years to do something about it.

Had it been done in only 10 years, the code would have changed, but not to the degree of today.

Mike McCann
MMC Engineering

RE: Constructing Additional Level on an Existing Building and IBC Chapter 34's 10% Rule

I agree with JAE, There is a reason the codes evolve. Majority of experiences I have had with adding new story on, or building over old buildings etc always end up with the same situation. knock it down and start again.

However in saying that I have also designed 10 story building with the capacity of a future 10 story building on top.
When I completed the analysis I ensured there was plenty of factors and to an extent redundancy in the columns/ pads and core wall to avoid this problem.

However it is not always the case!

"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976

RE: Constructing Additional Level on an Existing Building and IBC Chapter 34's 10% Rule

structuralengr89, your point is well taken.

I have a potential project in my own city which is a two story cast concrete building (with basement) where they want to add three floors of wood framing on top (the existing roof would be the new third floor.

The issue is that once I act as EOR on this project the whole baby is mine with respect to structural performance. This is an older building with limited plans with respect to reinforcement layout, cutoffs, etc. Based on its original use as a heavy industrial building and some back-calcs of live load capacity I have a good idea of the live load capacity and how I can share the excess with the new loads above.

But with the lateral, if you now currently have a more significant seismic criteria, I think you are taking on a lot of risk by discounting the possibility of a seismic event in the future. If the building were to have structural damage/collapse in minor event you would have little to defend yourself with.

Despite the codes - seismic events never happen...until they actually happen.

In the US - you would have to defend yourself based on the standard of care - what would a "typical" competent engineer do in a similar situation. My view is that to protect the public an engineer wouldn't discount the required, current seismic criteria simply because it is financially difficult for the owner.

Especially if the extent of work on the structure was as much as you suggest on your own project.

RE: Constructing Additional Level on an Existing Building and IBC Chapter 34's 10% Rule

I was approached with a similar project as JAE about 1.5 years ago. The client owned a 5 story building from the 50's or 60's and he wanted to add an additional 2 stories to it. He had hired an engineer (on the cheap) who decided that attaching to the parapet of the building was a good idea. The additional two stories was going to be framed out of wood and be very light. I was being asked to take over the project because the owner was nervous with the design provided by the original engineer. I was nervous about it as well..... and it seemed like the engineer never addressed the additional lateral loads produced by increasing the height of the structure by 30-40%. From conversations with the owner, I knew he didn't want to spend the necessary money to do a proper analysis, I wasn't comfortable with the existing structure and backed out of the project (after about 4 hours of phone conversations over a week with the owner).

As JAE states, if you add to the structure, you need to be able to by responsibility for the whole thing and if necessary, justify your method if something happens. I believe your interpretation of Chapter 34 is correct. You may want to take a look at the IEBC. This book will have the same 5% and 10% limits as shown in Chapter 34 together with a much more in-depth set of requirements for existing structures.

On a side note: I've seen many different projects done by many other engineers. I've seen project where the engineer just went up with no reinforcing for the existing structure (did they run an analysis... who knows). I've seen projects where the engineer added 252 pieces of diagonal bracing (HSS8x8) to reinforce an existing 3 story building for an additional 3 story addition.

RE: Constructing Additional Level on an Existing Building and IBC Chapter 34's 10% Rule

Refer to ASCE 41 - "Seismic Rehabilitation of Existing Buildings". They have guidelines for how to analyze and retrofit existing buildings that may not be detailed to the current code. You could also add lateral elements to meet the code.

If you want to go the FRP route, there are specialty engineers that can design the FRP systems, which are proprietary. You just need to indicate the strength, ductility, etc. requirements on your drawings.

Keep the new level as light as possible, especially if seismic controls. If you are just adding one level, can it be a bare steel deck supported by joists?

RE: Constructing Additional Level on an Existing Building and IBC Chapter 34's 10% Rule

The RFP is probably going to be your best option. Alternatively, you could drop steel columns down through or on the outside of the building (depending on footprint of structure) and independently support the addition using independent braced frames. However, at five stories the size of these framing members may be problematic with the existing building use.

Considering that the design life for most buildings is 50 years, it seems like a stretch for the owner to believe that he can now take advantage of the addition the building was designed for.

PE, SE
Eastern United States

"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi

RE: Constructing Additional Level on an Existing Building and IBC Chapter 34's 10% Rule

I am working on a project renovating a reinforced concrete flat slab system constructed in 1916. Four stories. The building was constructed and used for Ford Motor Co. automobile manufacturing for several years before switching manufacturing to engines. The current owner wants to convert to a hotel. We checked the gravity loads and lateral loads. For gravity loads, we do not exceed the 5% load increase criteria. For the lateral loads, we are not increasing demand/capacity ratios by more than 10%; however, the original building demand/capacity ratio is about 2.5 for seismic. In my opinion, there is a life safety issue as we are in a moderate seismic zone. We have to do something and there are discussions about what that "something" needs to be. The 2009 International Existing Building Code refers to the possibility of using 75% IBC prescribed forces, but so far I cannot determine what qualifications have to be met to use the reduced IBC forces. We purchased ASCE 31 and ASCE 41 to see how that might help or hurt us. Anyone have any comments regarding the 75% rule?

RE: Constructing Additional Level on an Existing Building and IBC Chapter 34's 10% Rule

We had a similar case recently where the building seemed to have sufficient strength even though there were some detailing issues with the rebar.

We talked it over with the Building Official and he said that we would allow our judgment as the new EOR to prevail.

RE: Constructing Additional Level on an Existing Building and IBC Chapter 34's 10% Rule

Unfortunately, we don't have sufficient strength to start with. I am reading through ASCE 31 for the evaluation part. Whew. Some dry stuff. ASCE 41 next on the reading list for the rehab part.

RE: Constructing Additional Level on an Existing Building and IBC Chapter 34's 10% Rule

(OP)
I found FEMA 547-Seismic Rehabilitation of Existing Buildings a great read (and is free to download). There is a new ASCE 41(13) that should be due out soon...but they keep pushing the date back.

Most of my work is low-moderate seismic level (typ SDC=C), and here I would look at it differently. There are many concrete building that would not meet today's code...flat slabs with no shear walls and questionable moment frames. If you are below the 5/10 percent rule (as I understand it) you don't have to upgrade the building. I would let the owner know, by letter, that the building would not have the safety of a newly constructed building. You can send the info you have and loading required to one of the FRP guys and they will typical evaluate and give you an estimate for free. But at 2.5, I doubt FRP will give you the resistance you need...it will help with confinement of your ties, hoops, stirrups, etc. You might want to look at adding steel x-braced frames at the perimeter of the building...and make sure you can get the loads into the bracing

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