Laterally supporting a 3 ply 2"x10" beam

[woodman1967]

Have a situation where a deck floor (with roof) is supported by a 3 ply 2"x10" beam (floor and roof loads). The floor joists bear on the top of the beam and the bottom of the beam isn't laterally supported. The beam doesn't pass without lateral support on the bottom edge (tension). If there is lateral support at the midpoint of the beam then the beam passes. What is a good way to provide lateral support for the bottom of this beam?

Thanks in advance

 

[driftLimiter]

The beam doesn't pass without lateral support on the bottom edge (tension)

What is the physical meaning of this statement? Bottom flanges buckles due to tension????

I suggest reviewing the NDS code requirements for beams etc. By hand to be sure you have input the arrangement properly into your design software.

You can create some kicker braces from the bottom flange up to the joists, could fashion some blocks into a lateral brace as well.

 

[woodman1967]

Sorry, I didn't clarify that we are located in Canada and probably not clear on the beam either. The beam is a lumber beam, 3 ply 2"x10". The loading is applied vertically on the top of the beam so I figured the top of the beam would be under compression and the bottom under tension, am I correct here?

Would kicker braces work here as well?

 

[Eng16080]

A diagram would be helpful. I agree with driftLimiter that it's confusing where you state that the bottom edge is in tension. If the bottom edge is entirely in tension, then I don't see why you should need lateral support along the bottom. So, is that statement accurate, or did you mean to say compression?

The typical scenario where you would need lateral bracing along the bottom of a beam would be where the beam is experiencing negative bending, in which case the bottom is in compression. This would most commonly occur where you have a continuous beam spanning over one or more interior supports.

 

[woodman1967]

The first pic (taken from the web) just shows the basic idea of what the beam is and how it is loaded. The second image shows the actual beam with the sonotubes (the drawing shows the supports as 6" Lumber column but it will be sonotubes. Does this make more sense? [URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1705517108/tips/ledger-and-joists-annotated_acrtid.webp[/url]

 

[Eng16080]

Given that you're using WoodWorks software, I assume you have selected "Lateral support spacing" as:
"Top" = "Continuous" and
"Bottom" = "At supports"

Assuming that's correct, then it would appear that the bottom of the beam does in fact require lateral bracing (as you initially indicated).

By the way, if you look at the Bending diagram provided by the software, where the curve is located above the horizontal axis, that indicates a positive bending moment (top of beam in compression and bottom in tension). Likewise, where the curve is below the axis, you have negative bending (top of beam in tension and bottom in compression).

If you can provide the woodworks calculation output, I can take a quick look at it to see if you're interpreting the output correctly. I would expect the CL value to be less than one for negative bending if buckling at the bottom of the beam is an issue.

To answer your original question, I would attach a kicker between the beam angled up to and lapped along the side of the nearest joist to the center of the beam. That should provide adequate bracing.

 

[woodman1967]

Yes, Top set at continuous and bottom at supports.

Please find attached the calc output.

Thanks

 

[driftLimiter]

You could also put a kicker from the columns back up to the framing. Might be an easier detail.

 

[XR250]

I doubt most would bother with bracing the bottom of this.
This is why I do things by hand

 

[Eng16080]

The analysis seems to generally be reasonable, although here are a few comments:
[ol ]
[li]If the beam is exposed to moisture (which I'm guessing it might be if this is an exterior deck) and you plan on using pressure treated lumber, I would use Southern Pine No. 2 for the lumber species and grade, not SPF No.2.[/li]
[li]If this is an exterior deck, you should verify that the live load is correct. Our (US) codes typically require 60 psf for an exterior deck (1.5 times the LL of the occupancy served).[/li]
[li]The issue is clearly the low KL factor for negative bending. I would either add the bracing (kickers) or use a deeper beam. Although it will sound ridiculous, I wonder if the design would work if you modeled it as 3 separate beams (discontinuous at the 2 interior supports). Not saying you should do that, but it's at least interesting to think about.[/li]
[li]One other thing, if you could somewhere get a single piece beam as opposed to multi-ply, that would likely work better. I believe the Canadian code conservatively requires that the KL calculation be based on the geometry of a single ply rather than the fully assembled (4-ply) beam. [/li]
[/ol]

 

[jayrod12]

I can comment on the SP versus SPF, in Canada where the OP practices SPF is the only available lumber unless you want to pay a premium for DFir.

 

[Eng16080]

That's fair regarding the SP lumber. My comment was based on my experience being in the Northeast US. Still, the lumber might need some form of resistance to moisture. If PT is being used, if it's incised, there may be a reduction factor for that (although I'm not familiar with the Canadian code).

 

[jayrod12]

Yes there is a reduction for incised lumber up here. Our code matches the NDS fairly closely.

 

[SE2607]

I think it would be appropriate to consider the unbraced length of the bottom face in compression as the distance between inflection points.

 

[jayrod12]

Does the beam need to be continuous to work? Or could it work as simple span? If so, just detail it to be simple span. Then the bottom flange bracing requirement would go away.

Inflection points were historically used as bracing points, however that is no longer an acceptable method.

 

[dik]

I think that PWF is nearly always incised...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[XR250]

Every deck around here is framed with beams cont. over at least 2 spans. Never seen a single one of them
having the bottom of the beam braced and have never seen an issue with it.
Nobody typically checks this in these situations.

 

[SE2607]

Inflection points were historically used as bracing points, however that is no longer an acceptable method.

I understand that, but, to me, it seems conservative to treat a beam loaded in this manner the same as a beam subjected to a net wind uplift with the entire compression face unbraced.

Before writing this, I did a quick review of NDS TR 14 and it seems one can use the same Cb modifier AISC uses for continuous beams. That seems like the ticket to me.

 

[jerseyshore]

I agree with XR, especially if the total span is 20 ft or less since that's what most lumberyards stock in dimensional lumber. Use a heavy duty Simpson post cap or kickers from the columns and that covers about 90% of decks that I see around here.

 

[woodman1967]

Thanks again to all,

Thought I'd just follow up with what we will be going with. We used the same spans as shown in the pic above. But, we used a 3 ply 2"x10" set at wet service and pressure treated SPF#2 (Southern Pine isn't readily available here in eastern Canada). The contractor wanted a continuous beam, wasn't interested in simple spans. Lateral support on the top of the beam was set at 16"OC, and on the bottom laterally supported at the supports.

That setup was passed by Woodworks.