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Heavy Timber (Type of Construction IV C) Used in Remodel of Existing Unlimited Area II B Building

archiyahoo

archiyahoo

Registered User
Joined
Mar 26, 2024
Messages
8
Location
Cincinnati, OH
The title summarizes the predicament. The existing building is a tilt-up concrete exterior, single story construction with appr. 40' x 50' unprotected type II B structure holding the roof by means of 4' deep truss joists with a few shear walls and open web joists running in the other direction. The bearing height of the joists is appr. 31' Its occupancy combines groups B and F-1, with some S-1 storage and an accessory A. It does have more than the required 60' of open space surrounding it on all sides.

We are trying to animate the interior by adding a partial floor (or mezzanine, not clarified yet) and raising a portion of the roof to achieve a clerestory over a portion of the building. The intent is to support the raised part of the roof with heavy timber elements. We'd like to feature the wood, and keep it exposed. We'd like to use CLT for the floor we are adding. To make it even more interesting we are considering removing a part of the exterior paneling and replacing it with curtain wall supported by large glulams.

The question is whether the building should remain type IIB as Type of Construction, using exception 603.1/19, which allows the use of heavy timber for roof construction, or do we have some benefits for classifying the whole building as type IV C. It appears that per section 507.5, we can have an unlimited area building, as it fits the profile the Occupancy Groups.

I appreciate any tips, tricks and Been-There-Done-That's.

Thanks for your time,
RT
 
A bit confused here, 20x50=2000sf, is this indeed the area? Is the building sprinklered? Depending on the A and other Occupancies you may be able to classify it as a Type V-B, sprinklered building. As a rule of thumb, ALWAYS use the least restrictive Type of Construction possible.
 
Thanks for the note, @khsmith55 . 40’ x 50’ is the average size of the structural grid inside the building. The whole thing is about 290 K sq.ft. It is and fully sprinklered and will be so after the renovation.
 
archiyahoo said:
We are trying to animate the interior by adding a partial floor (or mezzanine, not clarified yet) and raising a portion of the roof to achieve a clerestory over a portion of the building. The intent is to support the raised part of the roof with heavy timber elements. We'd like to feature the wood, and keep it exposed. We'd like to use CLT for the floor we are adding. To make it even more interesting we are considering removing a part of the exterior paneling and replacing it with curtain wall supported by large glulams.

The question is whether the building should remain type IIB as Type of Construction, using exception 603.1/19, which allows the use of heavy timber for roof construction, or do we have some benefits for classifying the whole building as type IV C. It appears that per section 507.5, we can have an unlimited area building, as it fits the profile the Occupancy Groups.
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The heavy timber roof exception for type IIb construction (foot note c) only applies to the roof framing and associated secondary structural members. You would not be able to support that heavy timber roof with HT columns or have any HT floor structure under IIB construction. You would need to reclasify the building to type IV
 
archiyahoo said:
Thank you much, @khsmith55 . Looking at similar threads, there appears to be a lot of support for that approach. Can you clue me in as to the benefits vs. pitfalls of such an approach?
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Type V-B permits any materials in the code, wood, concrete, steel, CLY, H-T, etc and it the LEAST restrictive of all Types of Construction. Being a B also requires no fire ratings of systems. This approach does not preclude you from using CLT, H-T or tilt-up concrete but does not require them.
 
khsmith55 said:
Type V-B permits any materials in the code, wood, concrete, steel, CLY, H-T, etc and it the LEAST restrictive of all Types of Construction. Being a B also requires no fire ratings of systems. This approach does not preclude you from using CLT, H-T or tilt-up concrete but does not require them.
Click to expand...
Thank you very much. So all pros and no cons?
 
archiyahoo said:
Thank you very much. So all pros and no cons?
Click to expand...
based on your description of the renovations I would consider the renovated building as 2 stories, once your project above the existing roof I don't see how you can classify it as a mezzanine. As a two story unlimited area building you might have some restrictions on use groups in the building.
 
Tim Mailloux said:
The heavy timber roof exception for type IIb construction (foot note c) only applies to the roof framing and associated secondary structural members. You would not be able to support that heavy timber roof with HT columns or have any HT floor structure under IIB construction. You would need to reclasify the building to type IV
Click to expand...
Thank you, @Tim Mailloux. I'm a bit lost on the nuance. So you say that footnote c. is does not allow heavy timber within other Types of Construction, but forces you to reclassify? I did not read it like that.
Any other opinions?
 
archiyahoo said:
Thank you, @Tim Mailloux. I'm a bit lost on the nuance. So you say that footnote c. is does not allow heavy timber within other Types of Construction, but forces you to reclassify? I did not read it like that.
Any other opinions?
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I don't see any nuance, foot note c allows for the roof framing in IIB construction to be HT. Columns supporting the HT roof framing are part of the primary structural frame which foot note c does not apply to. Foot note c also doesn't apply to floor framing. If you want HT column and floor framing you would need to reclassify as Type IV.
 
Tim Mailloux said:
Columns supporting the HT roof framing are part of the primary structural frame which foot note c does not apply to.
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Note c. in OBC reads verbatim: " In all occupancies, heavy timber complying with Section 2304.11 shall be allowed for roof construction, including primary structural frame members, where a 1-hour or less fire-resistance rating is required. "
How do you figure it does NOT apply to it?
Thanks,
 
Agee with Tim. All my comments were based on a 1 Story building with a mezzanine NOT a 2 Story building. The "A Accessory Occupancy" gives me pause, what class of A is it and is it indeed an accessory. I would nail down all the Occupancies, determine if its 1 or 2 stories and re-review Section 507 (2018). Good luck.
 
archiyahoo said:
Note c. in OBC reads verbatim: " In all occupancies, heavy timber complying with Section 2304.11 shall be allowed for roof construction, including primary structural frame members, where a 1-hour or less fire-resistance rating is required. "
How do you figure it does NOT apply to it?
Thanks,
Click to expand...
my interpretation is that the note is referring to the portions of the roof primary structural frame (beams and girder) having direct connections to the columns, but does not include the columns supporting the roof.
 
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code commentary:
Note c permits heavy timber to be utilized in the roof construction as an alternative to having a fire-resistance rating of 1 hour or less in buildings of Type IB, IIA, IIB, IIIA or VA construction. Note that heavy timber cannot be used in Type IA construction since the roof is required to have a rating greater than 1 hour. This allowance applies to both the primary and secondary members of the roof structure. The note is intended to allow the substitution of heavy timber for 1-hour roof construction; it is not intended to say that heavy timber is 1-hour-rated construction.
I would not consider the columns supporting the roof structure as part of the roofs primary structural frame as those columns are also most likely part of the buildings lateral system.
 
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