DCA6-09 - Sizing Deck Beam

[miz0133]

Hi,

Did anyone noticed that table 3 (Deck Beam Span) of the DCA6-09 for sizing a deck beam is on the conservative side. I am wondering if I am missing anything here, but as an example it seems that for a 9ft joist span, a 4x12 DF no.2 could span 13ft instead of their specified 8' or 9'. Also I am also wondering why does table 3 specifies a doubler beam 2-2x12 instead of just a 4x12?

Any insights would be greatly appreciated.

 

[focuseng]

A few thoughts on this:

I never use a table for a beam. Joist maybe, Beam never. Decks take snow load like no-ones business (although this table does not take it into account).

1) Maybe consider preservative treatment modifiers for a DF beam. Any incising for this species group? Footnote #2
2) Wet service factors? Footnote #1
3) The Joist span accounted for includes cantilevered joists over a dropped beam. Figure 2 on previous page.
4) the beam is allowed to cantilever. See figure 3 on same page.
I would argue it is not conservative considering: typical decks and their high probability towards improper installations and decay failures. The table is written for non-engineers and thus it is very likely to be abused/misinterpreted.

Have you run the numbers to determine it is conservative? If you use the table then you use the table and all the baggage that goes with it and then who needs the engineer to second guess it. That is the value and the limit of a table in a prescriptive code reference publication. No engineer = over designed structure (ideally).

I have no comments on the DF market availability for you or rational for the member sizes in the table but in my table the 4x12 is included. Are you sure you are using the '09 publication? Tables are inherently limited by whom-ever wrote them and their bias for what to consider. For the record the table is small and meant to be a simple reference encompassing many factors. If you don't like the table make your own. As an engineer you can do that. Or just run the numbers and ignore the table on a case by case basis.

______________
MAP

 

[woodman88]

If you look at the joist table it has a "with cantilever". Since the joists could bear on top of the beam and cantilever over the beam. The reaction from the joists to check the beam spans should be the cantilever end of the joist reaction. If you do this you will come closer to the beam spans in the table.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[miz0133]

@ focuseng:

1- Incising factor is taken into consideration as per footnote 2
2- Wet factor is also taken into consideration. Fb=0.85
3- See figure 1b which goes along with table 1 (without overhang)
4- Same as item #4

I agree with you that this table is more often used for non-engineers (perhaps more for builders) and is expected to be on the conservative side. I usually run my own beam calcs but I thought to double check vs. the DCA table just to make sure. I did not really expect the difference to be that substantial.

 

[LorenAWC]

You should notice that Doug Fir-Larch, Hem-Fir, SPF, Redwood, Western Cedars, Ponderosa Pin, and Red Pine are all being thrown in together. That means that the design values for Doug Fir-Larch aren't being used, but the worst of that group is.

There is the issue of the assumed cantilever on both ends as well. The table assumes that there is an L/4 cantilever on both ends of the beam since that is allowed.

I don't understand your second question. The table has options for both 2-2x12 and a 4x12.