DCA6 (Deck) Questions

[medeek]

I just sent an email to AWC with the following (2) questions:

[pre]I am looking at Table 2 of the DCA6 (Maximum Joist Spans and Overhangs)

I am interested at how the overhang allowable length (Lo) was obtained.
Was the 220 lbs point load applied with all other distributed live
loads removed? I typically like to manually calculate my deck loads so
I would like to make sure that I am checking all load conditions
accurately and that they meet or exceed the DCA6 requirements.

Another thing I found counter-intuitive are the allowable overhang
lengths (Lo) as they vary by joist spacing. It would seem that joists
that are spaced closer together (ie. 12" o/c) would have greater
allowable overhang lengths. Based on Table 2 this does not appear to
be the case.[/pre]

The overhang of the joist is limited to the lesser of Lo or L/4 (1/4
the backspan) per this document. Where is the L/4 coming from?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[jayrod12]

to ensure there's no uplift on the rear connection of the backspan.

 

[medeek]

I get the no uplift idea or at least minimizing it. I'm just trying to figure out if there is some code somewhere in the IBC, IRC, ASCE7-10 or that mandates L/4 on the backspan or if there is a theoretical basis for this number?

I've got a deck with 12' of backspan and a 4' overhang (L/3) so technically it fails, the client is upset because the local bldg. dept. says his deck is a "no-go".

It has 2x8 HF No. 2 joists @ 16" o/c, according to Table 2 of the DCA6 the max. allowable (Lo) overhang is 1'-10", the backspan limit is L/4 or 3", so the Table 2 value governs in this case.

Technically it also fails on the main span @ 12', Table 2 gives an allowable span of 11'-1" for a 2x8 HF @ 16" o/c

I'm also trying to get a feel for what other engineers are doing with deck engineering. Is the DCA6 by the AWC the standard that everyone goes to?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[gte447f]

medeek,
I have always interpreted the DCA6 as a prescriptive code, similar in that regard to the IRC, meaning it's requirements and restrictions apply so long as the structure is not being designed by a licensed Professional Engineer, but if the structure is being designed by a licensed Professional Engineer then I would not expect the building department to overrule the PE's design to enforce a provision like the backspan/4 limit on cantilever length. They may generate a question/comment during their plan review, but then I would expect them to accept a response from you outlining that your structural analysis and design meet the requirements of the IBC as satisfactory.

 

[medeek]

Thank-you gte447f, that was pretty much the answer I was looking for. I think some uplift can be allowed but it just needs to be adequately countered with appropriate hangers or ties.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com