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Will I Need to Use Double Trusses?

Will I Need to Use Double Trusses?

Reader DALE in TOMPKINSVILLE writes:

“On an open front 5 bay tool shed, roof length 60 feet, width 40 foot, height of 10 foot, 12foot, trusses on 6×6 columns, 12/4 pitch located in southern Kentucky zone 6 will I need to double the trusses?”

Need and want are two different things. In all likelihood, single trusses will support your climactic loads. Double trusses do have some distinct advantages however.

ASABE (American Society of Agricultural and Biological Engineers) published ANSI/ASABE S618 “Post Frame Building System Nomenclature” in December 2010. For those who are unfamiliar ANSI stands for American National Standards Institute (www.ansi.org). ANSI is a private non-profit organization overseeing development of voluntary consensus standards for United States products, services, systems and personnel.

In ANSI/ASABE S618, a Metal plated connected wood truss would be described as, “A truss composed of wood members joined with metal connector plates (also known as truss plates). Metal connector plates (MCP) are light-gauge, toothed steel plates. The most common type of light wood truss.” Ganged wood trusses are defined as, “A truss designed to be installed as an assembly of two or more individual light wood trusses fastened together to act as one.”

nailing trusses For Hansen Pole Buildings, any time we are using a “real” double (more specifically ganged) truss system, we specify top chords to be a minimum of 2×6, regardless of loads. I say “real” because placing a single truss along each side of a column is not a double truss. They are two single trusses, acting independently from each other. A true double truss system, such as used by Hansen Pole Buildings, features trusses physically attached face-to-face by means of mechanical connectors (e.g. nails, bolts, etc.). This allows for two members to actually load share, reducing probabilities of one weak single truss failing and pulling a roof system down with it.

True ganged trusses, due to their load sharing capabilities, often are able to utilize smaller steel connector plate, smaller or lesser dimension lumber for chords and/or webs, and require far less lateral bracing. Often utilization of two-ply ganged trusses results in a less expensive overall structural design solution, when all factors are taken into consideration.

Three sided buildings also pose their own unique set of structurally challenges, please read more here: https://www.hansenpolebuildings.com/2014/03/three-sided-building/

This entry was posted in Pole Barn Design, Pole Barn Structure, Trusses and tagged lateral bracing, ASABE, ANSI, ganged wood trusses, ANSI/ASBE S618, metal plated connected wood truss, ganged truss, truss plates, double trusses on May 11, 2023 by admin.

Working With a Building Official for Footing Design

Working With a Building Official for footing designs

Long time readers have read me opine on how Building Officials are our friends: https://www.hansenpolebuildings.com/2012/04/i-like-building-officials/

One of our clients recently received this email from his Building Official and shared it with me (red added by me):

“The Town of xxxx stopped plan review on your project because pole buildings with the type of foundation that was called out on your plans have a track record of failing in a short period of time in this area and the soil conditions that exist at your location won’t allow piers to be dug as called out in the plans The ground is full of rock. It was our intent to not have you try and build something that was not going to work and cost you a lot of money. With a frost depth of only 18” “T” foundations are the acceptable method of construction in that area and should be a lot less expensive.”

I responded asking for more explanation of wording in red, and heard back:

“We have a few pole barns in xxxx that are much larger in size and in areas that will allow piers to be dug without interference of large rocks below grade. The pier design that they used allows the wood post to sit on a gravel base so any water that might drain down along the side of post after the wood shrinks from age is allowed to drain away.

They also pour concrete up to 3” above the top of the pier and slope the concrete away from the post. Your design traps water at the bottom of post and allows the water to be wicked up by the end grain of the post and promote rot. Although the building codes does allow treated wood foundations to be buried we strongly discourage the use in our jurisdiction.

Thanks for your understanding.”

Thank you very much for your timely response.

I am probably remiss in not having offered a better introduction of myself. I studied architecture at the University of Idaho and have been Technical Director at Hansen Pole Buildings since 2002. I joined ASAE (now ASABE) and ICBO in the mid-1980s. The IBC references ASABE work for post frame buildings which was produced by the structural committee of which I was a member of. I am a frequent contributor as a writer for publications such as Structural Building Components, Frame Building News and Rural Builder magazines. I have also reached out to you on Linkedin, should you wish to know more about me.

We are currently working with Mr. Bxxx on a design solution to incorporate a continuous footing/foundation or thickened edge slab with bracket mounted columns. We and our engineer had not been advised by Mr. Bxxxr as to the soils/rock conditions at his site until quite late in the game. It is my expectation, with Mr. Bxxx’s continued assistance, to have an acceptable design solution arrived at shortly.

While an embedded column pier design on a gravel base sounds wonderful, Code does require a concrete or otherwise approved footing below isolated columns in order to properly distribute weight of building and applied loads. Actual testing of pressure preservative treated columns for over 60 years has proven there to be no decay of properly waterborne pressure preservative treated wood even in the most severe climates (this testing is ongoing in Mississippi). UC-4B rated pressure preservative treated wood is rated for structural use in fresh water, so a column being wet would not increase its chances of decay. In order for decay to occur there must also be oxygen, which is only present in the upper few inches of soils.

Please feel free to contact me with any questions or concerns regarding any post frame building structure. I would also invite you to subscribe to my blog, where there are currently over 1800 searchable articles regarding post frame construction.

“Thank you for your comments. The failures we have seen may have quite well been from pour constructions procedures done 10 to 15 years ago. No way to tell.

I will look into your articles and may have to change our policies”

Building Officials are not our adversaries and provided with accurate data policies can be crafted to create a winning solution for all parties involved!

This entry was posted in Fasteners, Pole Barn Questions, Pole Barn Design, About The Pole Barn Guru, Pole Barn Planning, Pole Barn Structure, Footings, Professional Engineer and tagged bedrock, pole barn piers, ASABE, piers, T foundation, ASAE on February 12, 2020 by admin.

True Double Trusses

ASABE (American Society of Agricultural and Biological Engineers) published ANSI/ASABE S618 “Post Frame Building System Nomenclature” in December 2010. For those who are unfamiliar ANSI stands for American National Standards Institute (www.ansi.org). ANSI is a private non-profit organization overseeing development of voluntary consensus standards for United States products, services, systems and personnel.

Reader RON in FORT BENTON writes: “I built a 24 x 36 x 10 pole building from a kit 30 yrs ago. And I have had a 20 x 28 x14 built by local professionals about 8 yrs ago. They each have 12 or 14 foot distances between posts. The trusses are the regular double 2 x 6 construction with 1 on opposite sides of the posts with blocking between them at the bottom. The side girts have been 2 2x6s configured in the L shape. I am not sure what you call that. Now looking at examples of kits, they seem to use (double?) 2×4 construction for the truss. Has there been a big change or am I just missing something. I like the 2×6 approach and am not sure how much difference it makes in final costs. I am looking into a 48x36x? monitor type building in, at times, very windy location. Thanks.”

Even though some very high grades of 2×4 lumber are available to metal plated connected wood truss manufacturers (such as 2850msr), only in very small spans and light loads would they work for top chords of double trusses spaced upon 12 or 14 foot centers. For bottom chords, it might be possible to get to 30 or 36 foot width spans, provided loads were light.

Learn about Machine Rated (MSR) lumber here: https://www.hansenpolebuildings.com/2012/12/machine-graded-lumber/.

For Hansen Pole Buildings, any time we are using a “real” double (more specifically ganged) truss system, we specify top chords to be a minimum of 2×6, regardless of loads. I say “real” because placing a single truss along each side of a column (as you have described) is not a double truss. They are two single trusses, acting independently from each other. A true double truss system, such as used by Hansen Pole Buildings, features trusses physically attached face-to-face by means of mechanical connectors (e.g. nails, bolts, etc.). This allows for two members to actually load share, reducing probabilities of one weak single truss failing and pulling a roof system down with it.