Tag Archives: floor joists

A Floor Raising Exercise: I Joists

For some obscure reason people planning new buildings tend to scrimp on height. In most instances, designing a new fully engineered post frame building – whether for a barndominium, shop house (shouse), garage, shop, etc., just a little bit taller is a relatively inexpensive proposition and can save many more dollars and mental anguish than having to alter after construction.

Reader CHRIS in SNOHOMISH writes:

“I have a pole barn with a center door for Rv, above is an additional living space, the width is 12’6” depth of 41’ height of 13’, I need to shorten the truss’s so I can gain 6” height , current truss are HY floor joists, question is can I put 2×6” spaced every 8”’s and have the same weight carrying capacity?”

Mike the Pole Barn Guru writes:

HY floor joists are wood prefabricated I joists. Let’s take a look at Chris’ proposed design solution (please keep in mind, any structural design solution should be reviewed by your building’s engineer to confirm structural adequacy):

FLOOR JOIST DESIGN

Assumptions:

Joist span 12.5-ft.
joist spacing = 8″ o.c.

joist  live load = 40 psf
joist_dead_load = 10 psf

Fb: allowable bending pressure
Fb‘ = Fb * CD * CM * Ct * CL * CF * Cfu * Ci * Cr
CD: load duration factor
CD = 1 NDS 2.3.2
CM: wet service factor
CM = 1 because floor joists are protected from moisture by roof
Ct: temperature factor
Ct = 1 NDS 2.3.3
CL: beam stability factor
CL = 1 NDS 4.4.1
CF: size factor
CF = 1.3 NDS Supplement table 4A
Cfu: flat use factor
Cfu = 1 NDS Supplement table 4A
Ci: incising factor
Ci = 1 NDS 4.3.8
Cr: repetitive member factor
Cr = 1.15 NDS 4.3.9
Fb = 900 psi NDS Supplement Table 4-A
Fb‘ = 900 psi * 1 * 1 * 1 * 1 * 1.3 * 1 * 1 * 1.15
Fb‘ = 1345.5 psi

fb: bending stress from live/dead loads
fb = live + dead load * joist spacing / 12 / 12 * (sf * 12 – 1.5)2 / 8 / (b * d2 / 6)
fb = 50 psf * 8″ / 12 in./ft. / 12 in./ft. * (12.5′ * 12 in./ft. – 1.5″)2 / 8 / (1.5″ * 5.5″2 / 6)
fb = 1012.5 psi <= 1345.5 psi so okay in bending

Δallow: allowable deflection
Δallow = sf * 12 / 360 IBC 1604.3
Δallow = 12.5′ * 12 in./ft. / 360
Δallow = 0.4167″

Δmax: maximum deflection
Δmax = 5 * live load * joist spacing / 12 / 12 * (sf * 12 – 1.5)4 / 384 / 1600000 / b / d3 * 12 from http://www.awc.org/pdf/DA6-BeamFormulas.pdf p.4
Δmax = 5 * 40 psf * 8″ / 12 in./ft. / 12 in./ft. * (12.5′ * 12 in./ft. – 1.5)4 / 384 / 1600000 / 1.5″ / 5.5″3 * 12
Δmax = 0.423″ > 0.4167:
2×6 #2 DougFir joists will not work at 8″ on center due to not meeting deflection criteria

Chris’ options are to buy #1 or Select Structural graded 2×6 DougFir or go to 6″ on center spacing.

Designing a Dream Barndominium Loft

Designing a Dream Barndominium Loft

Reader BRIAN in PETOSKY writes:

“ Hi Mike,

Mindi told me to email you my lofted floor question for our project.

To avoid messing with truss-support floors, we were planning to build a full 26×60 main barn with scissor trusses the full length. Then on one end, we would make a 20’x26′ loft. Have the floor joists run parallel to the barn, perpendicular to the trusses, so we’d have 20′ floor joists. These would be supported by the gable end wall and interior posts 20′ in.

We live in a barn home with this configuration and it works well. Allows consistent and uninterrupted ceiling space the length of the barn but still get a 2nd floor in where we want it.

The question is, I guess, what, if anything needs to be conveyed to the engineer for this design? Does it influence anything on the gable end wall? How far apart can the posts be on the interior end? Can stairs be free-standing next to this loft?

Thank you!”

When I used to call on Home Depots, Petoskey was one of my stops. Every time I was there the weather was gorgeous, making it difficult to get motivated to move on to my next appointment!

There are some challenges with running dimensional lumber floor joists to span 20′. Even using #2 & better 2×12 Douglas Fir joists, they would need to be 12 inches on center! Other popular specie of framing lumber has lower MOE (Modulus of Elasticity) values, so will not even begin to approach being able to span 20’. Chances are good there will be both a fair amount of spring to this floor, as well as a non-uniformity in deflection from joist to joist.

For extended reading on floor deflection, please read https://www.hansenpolebuildings.com/2015/12/wood-floors-deflection-and-vibration/

This would be my recommendation – we can use prefabricated wood floor trusses to span 26′. Doing so would allow there to be no interior supports within this 26′ x 20′ area. As long as stairs run perpendicular to the floor trusses, no columns would be needed where they attach. When you and Mindi have your building details finalized, she will relay this information forward on your Agreement with us, so everyone will be on the same page. Further, we send plans to you for a final once over prior to engineer sealing them, just in case.

Wood Floor Trusses

When I was first in the metal connector plated wood truss industry back in 1977, my employers – Dutch Andres and Tom Vincent at Spokane Truss, had just invested in a machine which would fabricate what would be called a 4×2 floor truss.

These trusses revolutionized the way floors could be constructed – freeing up areas below them from the need for load bearing walls and columns in all of the most inconvenient places!

Rick Ochs is new to the inside team at Hansen Pole Buildings, and earlier this week, he posed a question:

“Hey Mike,

No rush… I have been viewing tutorials from WTCA (Wood Truss Council of America) on trusses and structural building components.  I was wondering why we don’t spec floor trusses instead of the traditional 2×10 with hangers.  Cost I presume.

Thanks!”

Here is my response to Rick:

Floor trusses will be significantly more expensive.

Let’s say you have a 2×10 at .6285m (current price at The Home Depot®) so a 12′ would be $12.57.

(“m” happens to be lumber people’s secret code for 1000 board feet)

If they were even 16″ o.c., you are talking 0.79 per square foot for the cost of joists.

Floor trusses are going to run around $4.40 per lineal foot, spaced 2′ on center, this makes the cost per square foot for the joists at $2.20.

For a floor span of over 24′ trusses are certainly the way to go.

To which Rick responded:

“I’m thinking it would take a little more math on the builder/customer part to compare against labor cost savings of setting floor truss vs time required to set hangers, cut and nail joists.”

Personally, I have metal connector plated wood floor trusses in two of my personal buildings – in one case spanning 30 feet and the other 48 (yes a 48 foot clearspan floor).

Here are some of the benefits of using wood floor trusses:

  • Larger sheathing attachment, with 2×3 or (usually) 2×4 nailing surface,
  • Spacing up to 24” o.c. maximizes efficiency, decreasing installation time.
  • Each unique truss is engineered to proper codes and loading.
  • Speeds up mechanical installation (think heat ducts) with the open webbing thus saving dollars.
  • Span longer distances than conventional lumber or I-joists.
  • Special bearing, cantilever, and balcony details are easily built in.
  • Less pilferage, it is unlikely a 20’ truss is going to walk off the jobsite.
  • Faster jobsite build times, saving jobsite labor, construction loan interest, vandalism, and environmental damage.

Wood floor trusses can also be designed to limit the deflection and vibration, read more here:

https://www.hansenpolebuildings.com/2015/12/wood-floors-deflection-and-vibration/

In the global scope of life, having a wood truss supported floor is a fairly economical upgrade, which is certainly something worth investigating.

Scary Pole Barn Design

Scary Design

A one-time potential Hansen Pole Buildings’ client, who is a friend of mine on Facebook, didn’t invest in one of our engineered post frame buildings. Most likely it was due to price – people so easily believe they have gotten a great deal, when instead they set themselves up for nothing but potential grief.

He proudly posted the photo above on Facebook of the progress of his new building.

Disclaimer – in case you, gentle reader, were unsure – his new building is NOT a Hansen Pole Building.

I will let you in on a secret which truly frightens me about this building…….

loft floor framingIf a load approaching what the loft should be designed to support is placed upon it, I venture to wager it will fail. Do not stand underneath it, by any means.

As near as I can tell from the photo, the columns which support the second floor are spaced roughly 12 feet on center. It appears the floor joists are 2×12 spaced 16 inches on center and each end of the joists are supported by what seems to be another 2×12.

Building design and construction are only as good as the weakest link.

The building is in the deep south, so we will go with the premise the lumber being used is Southern Pine.

The floor joists are not a problem – they would easily support double the normal design floor live load of 40 pounds per square foot (for residential loading). The problem comes from the beams which support them at each end.

Here is the formula for design of the beams:

(Live plus dead loads) X ½ the distance to the next beam X Beam span^2  /  8 X 31.6406 (the Section Modulus of a 2×12) X Fb (for 2×12 Southern Pine 750) X 1.15 (Cr – repetitive member increase)

(40 + 10) X (72”) X 12’^2 / 8 X 31.6406 X 750 X 1.15 = 2.37 when it has to be less than or equal to one to work.

The floor, as built, is overstressed by 237%!! Or – think of it this way, it will only support 42% of what it should support by the BuildingCode!!

In either case, it is frightening.

Don’t construct (or have constructed for you) any post frame (pole) building which has not been designed by a Registered Design Professional (RDP – architect or engineer). To do so is scary pole barn design and nothing short of playing Russian Roulette.