Gable End Truss, Florida Building Code 1

[StrEng007]

Per the 2023 Florid Building Code, Section 2121.2.4 requires that all masonry structures with gable end walls be constructed with a coping or tie-beam in which the masonry wall is full-height (implied). Similarly, Section 2318.1.8.2 requires that all wood walls have continuous balloon framing for gable end walls.

From these two sections, it's shown that gable end walls, located within the High Velocity Hurricane Zone, must be constructed full height to the underside of the roof. This would mean that gable end trusses are not allowed within the HVHZ. By definition, a gable end truss is not allowed because the wall framing must be continuous from foundation/floor to roof.

However, Section 2319.17.2.1.6 (which is located within the HVHZ section of the Code) randomly throws in this statement,

"Where gable end trusses are permitted in this code, they shall be designed for a minimum live load of 30 psf (1436 Pa) and a minimum dead load of 15 psf (718 Pa) on the top chord. The minimum load of 10 psf (479 Pa) on the bottom chord may be omitted where continuous support is provided. In addition, the gable end trusses shall be designed to sustain wind load as specified in Chapter 16 (High-Velocity Hurricane Zones) but not less than 30 psf (1436 Pa) perpendicular to the plane of the truss. Such trusses shall use a rationally designed system to resist lateral wind loads and be anchored to the substructure at intervals no greater than 4 feet (1219 mm) on center to resist the uplift forces and shall be designed to transfer the loads to the substructure. The design of the system used to resist the lateral loads imposed on the truss shall be prepared by the engineer or architect of record."

This is the ONLY reference to "gable end trusses" in the entire code, besides the definition provided for a gable end truss. I'm going around in circles trying to figure out "WHERE" gable end trusses are permitted by code, because they are certainly not permitted in the HVHZ. So why would HVHZ Section 2319.17.2.1.6 have this language?

I'm not sure what I'm missing here, but this appears to be an error in the 2023 Florida Building Code.

 

[XR250]

Maybe in a case where the masonry is designed to span horizontally so as not to be full height.

 

[Eng16080]

I'm not well-versed in Florida code requirements and probably can't answer your question, but this seems like an overreach of the code in requiring continuous balloon framing at all gable end walls. A responsible engineer could rely on a structural ceiling diaphragm or wind beam to resist the lateral load at the hinge point in the wall. I've done this before where I added structural sheathing on top of the ceiling framing to create a legit diaphragm (more than just the drywall). In terms of masonry, you could also design the wall to span horizontally rather than vertically if the end wall isn't too wide.

The code section that you included above in red seems more reasonable, in that it apparently gives the engineer the freedom to do these sorts of things. I agree that this seems like an error in the code as it contradicts the first section that you reference. They probably updated that and forgot to edit the red section.

While balloon framing in this situation might make sense, it's also worth noting that this creates a fire risk if blocking isn't added within the wall cavity.

 

[StrEng007]

but this seems like an overreach of the code in requiring continuous balloon framing at all gable end walls

You are absolutely correct. The FBC DOES allow for something like this by requiring that "Gable endwalls shall be structurally continuous between points of lateral support." So you're right, you could design the wall to be supported at a ceiling level.

However, this reference in the FBC is located in Section 2304.3.4.1, which is the 'general' section of the code.

Florida's building code has a unique history in combination of codes, one of which was the old 'South Florida Building Code' which has morphed into the 'High Velocity Hurricane Zone' section of the FBC. Essentially, you have a code that is unique to two counties (Broward and Miami-Dade) located within the state's code. It makes for a really fun time when you're doing projects in the HVHZ and a different set of rules apply.

So section 2304.3.4.1 which allows for that freedom you mentioned, is open to designs that fall outside of those two counties.

To illustrate, here is the language from the HVHZ section on masonry walls. Note the use of the phrase "only in accordance with this section"

2121.2.4 Gable End and Shed End Walls
All masonry structures with gable end and shed end (half gable) walls shall have such end walls constructed of masonry, only in accordance with this section. A horizontal tie beam shall be provided in line with the lower ends of the gables and sheds, except as permitted in Section 2121.2.3.6 above, and designed in accordance with Sections 2121.2.1.2 and 2121.2.1.3, and load requirements as set forth in Chapter 16 (High-Velocity Hurricane Zones). A concrete coping following the rake of the gable, not less than 64 square inches (0.04 m2) in area reinforced with two #5 bars shall be provided.

The code section that you included above in red seems more reasonable

While it seems more reasonable, it's placement within the body of the Code doesn't make sense. It's basically the HVHZ section saying "hey, if you're allowed to use a gable end truss, here's how you do it". While all the other sections of the HVHZ are saying "don't use gable end trusses here, thanks".

This is what the Florida Building Code looks like, take the Masonry chapter as an example. After you get through all the 'regular' sections you see adopted from the parent code "IBC", you move onto those sections specific to Miami-Dade and Broward counties (HVHZ):

 

[phamENG]

As a kid growing up in Florida, I still remember the news footage of Andrew's aftermath. Those images will be forever emblazoned in my mind's eye. So, I can forgive the code writers down there for their apparently overbearing prescription of hurricane resistant construction.

I think a similar issue (or, at least, a question brought about by confusing stemming from the same source) came up recently.

2121 is the "Construction Details" section of the masonry chapter of the FBC. Since nearly everything down there needs to be built with masonry, it's the Florida version of the 'conventional light frame construction' section of the wood chapter for everyone else. Prescriptive details that, when used on a 'typical' and 'simple' structure will provide a resilient structure capable of resisting the high velocity wind loads to be expected. This is where your full height masonry wall is required.

2122 is the "Go be an Engineer" section of the masonry chapter.

Section 2121 shall not apply where design and construction are in accordance with the provisions of this section.

So, if you're using TMS 402 (The Masonry Code) to design the structure according to the actual, anticipated loads...you can do as you please (more or less).

 

[StrEng007]

phamENG, what you're proposing goes back to the days when skeleton framing crept into the code. You might be onto something there. Connecting the dots, the detailing is a residual left over from when masonry was non load bearing and plugged into the tie-beam and tie-column openings. This is the thing about Florida codes, sometimes you really have to know your history to be able to recall the intent behind what they're saying.

You must have had some good mentors because I know you don't learn the ins and outs of the FBC without some old-timers who were around when the code was the size of a single textbook.

 

[phamENG]

Had and still have. The best ones didn't teach me what is in the code, but how to read a code. And code philosophy is generally the same all over the US - especially so since IBC subsumed the old regional codes.

I grew up in Florida, but left when I was 18 and have never practiced there (though I am licensed there...just haven't used it). My knowledge of the FBC is pretty basic. But standard code reading procedure - find what you need (or think you need), and then work your way backward and verify applicability. Then, look for anything that looks like it might be a duplicate section and check its applicability. Keep doing that until any apparent conflicts are resolved.

And, like I said, a similar question about the masonry section of the FBC came up recently so it's somewhat fresh in my mind.

 

[StrEng007]

Do you recall the thread of the recent discussion on 2121 vs 2122?

 

[phamENG]

thread507-518480

 

[StrEng007]

Maybe in a case where the masonry is designed to span horizontally so as not to be full height

Very intuitive. That seems to be the case going back to some old school design methodology.

 

[lexpatrie]

I haven't read the whole thread here, sorry.

The code officials are the people to talk to about this one. I've gone to a few M-D online seminars on codes lately (they are free) and those guys are pretty top notch. The structural one is done, but it was so much more detailed than I thought "plan review" would be. More of a standards update you'd see from ASCE, in terms of content.

That said, the gable end thing probably has multiple code-compliant (or alternative) solutions, and the main focus is a decent load path.

You could run wood studs all the way up to the roof deck and detail it correctly, using or not using the ceiling as a support, you could put a ladder truss and design studs and provide support at the base of the ladder truss for the wind loads, etc. If you have a flat ceiling the ladder truss approach seems potentially viable, you could put bracing in if the ceiling gypsum board isn't strong enough....

[link

https://www.structuremag.org/wp-content/uploads/2014/09/C-StructuralDesign-Gjinolli_Aug_071.pdf

]Wood Truss Gable end frames[/url], Gjinolli, Vogt, Structure Magazine, August 2007.

Regards,
Brian