Addressing 6" seismic gap

[KassandraK+]

Hawaii County Structural is asking for a 5.5" gap between existing building and outdoor eating lanai. Engineer is asking how to redesign for add criteria.
Research suggest using double beam which seams costly or a Flexible Joint system. What is the typical solution? There is a 12' doorway at the existing building out onto
the lanai addition with a long step to accommodate height needs for outer lanai roof slop. Photos may allow for better understanding.

 

[redeyedfly]

Hawaii County Structural is asking for a 5.5" gap between existing building and outdoor eating lanai. Engineer is asking how to redesign for add criteria.
Research suggest using double beam which seams costly or a Flexible Joint system. What is the typical solution? There is a 12' doorway at the existing building out onto
the lanai addition with a long step to accommodate height needs for outer lanai roof slop. Photos may allow for better understanding.

That sounds like a weird ask for such a low mass addition. Your engineer should design the lateral system combined with the existing building and an expansion won't be necessary.

 

[Yankee Chronicler]

What is your role on the project?

Why is the structural engineer asking you how to accommodate the gap? Why didn't the structural engineer know that the seismic isolarion would be required, and why didn't he/she design it from the beginning?

 

[KassandraK+]

That sounds like a weird ask for such a low mass addition. Your engineer should design the lateral system combined with the existing building and an expansion won't be necessary.

The old buildings in Hawaii are not very well constructed although they have held up through years of natural wear. The building is on post and peer and single ply framing. The Structural reviewer at the county is a hard ball and is insisting the addition must be free standing with 5.5" gap for seismic pounding.

 

[KassandraK+]

What is your role on the project?

Why is the structural engineer asking you how to accommodate the gap? Why didn't the structural engineer know that the seismic isolarion would be required, and why didn't he/she design it from the beginning?

I'm the designer, working with a green architect. We brought the structural engineer in late in the game. The addition is designed to be free standing of the (E) building however we did not take into account the need for seismic pounding. I am posting for collaborative information and reference for the use of expansion joints, or possibly double beam construction of which I am not experienced in.

 

[redeyedfly]

The old buildings in Hawaii are not very well constructed although they have held up through years of natural wear. The building is on post and peer and single ply framing. The Structural reviewer at the county is a hard ball and is insisting the addition must be free standing with 5.5" gap for seismic pounding.

The whole building is the size of a two car garage, it shouldn't be much work to retrofit it.

 

[e hilton]

The addition is designed to be free standing of the (E) building however we did not take into account the need for seismic pounding.

Your original post said the county wants a 6” gap. If that’s all he is asking for, and if it was designed to be free standing … I don’t see the problem. Just put a diamond plate “bridge” at the entrances and be done. Is he asking for the existing building to be upgraded?

 

[Yankee Chronicler]

Your original post said the county wants a 6” gap. If that’s all he is asking for, and if it was designed to be free standing … I don’t see the problem. Just put a diamond plate “bridge” at the entrances and be done. Is he asking for the existing building to be upgraded?

Looks like Hawaii adopted the 2021 IBC with no amendments, so we can look directly at the IBC, either the book or on the ICC web site, for the requirements. I would like to know where the 6-inch gap requirement comes from, because I don't see it in the structural provision of the IEBC for additions:

 

[Yankee Chronicler]

I'm the designer, working with a green architect. We brought the structural engineer in late in the game. The addition is designed to be free standing of the (E) building however we did not take into account the need for seismic pounding. I am posting for collaborative information and reference for the use of expansion joints, or possibly double beam construction of which I am not experienced in.

I'm not sure what you mean by "double beam construction" but, if the addition is free-standing (structurally independent of the existing building), how would there NOT be new beams parallel to the existing structure? The perspective view in your original post (the fourth image) does not appear to be structurally independent. It appears to have the roof rafters bearing on the existing building.

And I still wonder why the structural engineer is asking the designer how to fix the structure, which he/she should have designed for seismic effects from the git-go.

 

[Yikes]

If I understand correctly, based on YC’s code citation in post #8, the reason you are seismically isolating the deck rather than attaching it is because attaching it will trigger a seismic re-evaluation of the existing building according to current code. Is that correct?

If so, all he has to do is demonstrate that the addition won’t add more than 10% to the demand capacity ratio of the existing structure. Assuming that they are both wood structures, I would anticipate that the added deck tied into the existing building would only help its demand capacity ratio, not hurt it.

I recall from your earlier posts that the original building is quite old. It it considered historic, and if so, is there any other provisions in the historic building code?

 

[KassandraK+]

separation between Old and New required is 5 ½”. It means, need to have a separation gap between New and Old structure to be 6” to avoid Pounding effect.
You can refer to page 699 to 701 for more information in attached final calc file.

 

[redeyedfly]

So your engineer calculated the lateral movement but did nothing in the design to accommodate that movement?

The engineer should design it as one building. A plate over the movement joint at the floor is easy, tying in a roof with a movement joint is not.

 

[KassandraK+]

So your engineer calculated the lateral movement but did nothing in the design to accommodate that movement?

The engineer should design it as one building. A plate over the movement joint at the floor is easy, tying in a roof with a movement joint is not.

I agree, We have a height allowance to consider, I do not think we can add a (N) roof under the existing, it would bring us under 7'-6". to leave an open gap would not work. I am not sure flashing is acceptable. The architect has offered very minimum contribution to the point of letting me know I will need to make the necessary document changes.

 

[KassandraK+]

If I understand correctly, based on YC’s code citation in post #8, the reason you are seismically isolating the deck rather than attaching it is because attaching it will trigger a seismic re-evaluation of the existing building according to current code. Is that correct?

If so, all he has to do is demonstrate that the addition won’t add more than 10% to the demand capacity ratio of the existing structure. Assuming that they are both wood structures, I would anticipate that the added deck tied into the existing building would only help its demand capacity ratio, not hurt it.

I recall from your earlier posts that the original building is quite old. It it considered historic, and if so, is there any other provisions in the historic building code?

It is not on the historic buildings list. If we tie into the existing building. we would most have to bring the whole building up to code. 2018 IBC. The building is in HI. lots of seismic activity and wind.

 

[redeyedfly]

I agree, We have a height allowance to consider, I do not think we can add a (N) roof under the existing, it would bring us under 7'-6". to leave an open gap would not work. I am not sure flashing is acceptable. The architect has offered very minimum contribution to the point of letting me know I will need to make the necessary document changes.

Your architect and engineer should know all of this.

This is a code forum and I think you've exhausted your code questions. Yes, the city can require you to explain how you're dealing with differential movement (I do like the term "seismic pounding"!) . That's the answer to your code question. The rest are design questions which have many possible solutions but they are for your design team.

 

[Yikes]

Your calcs indicate "lateral" sway characteristics are at least equivalent" for the two structures. I assumes that means they will shake at roughly the same rate and will not fight each other or pound against each other. Is that what the engineer is saying?



If they are working with instead of against each other, then it seems to me that you only need to prove that the addition will not burden the existing building by more than a 10% increase in lateral forces.
On the other hand, if you engineer does not want to account for the existing building burdening the new deck with lateral forces, then yes you need the isolation.

 

[e hilton]

Maybe I’m missing the question and getting too far in the weeds. If you need/want to close the roof gap to prevent rain from coming through, but you don't want a rigid connection, maybe an expansion joint would work. This particular product is available to cover horizontal gaps of 4, 5, 6, 8, 10 or 12”.