girder span question (yep another one)

[nathan.principe]

is there a span chart for girders and beams? not just joist?  Also I am interested in allowable girder/beam spans for decks that would have no load bearing walls, etc.  Anyone familiar with this?

[MountainDon]

A link to the...  Prescriptive Residential Wood Deck Construction Guide may be found in our Referral Links Board

Read thoroughly, ask specific questions if in doubt

[nathan.principe]

(Lj/4 = Maximum Overhang) does this mean a 1/4 of the length of the joist is the maximum cantilever allowed over a beam? Example: a 10' long joist can cantilever a beam 2'6"? thats confusing to me.  It looks like Lj refers to the length of joist

[MountainDon]

Could you reference the page # and approx position on the page? That would help narrow down where out of the 38 pages to read. Thanks

[nathan.principe]

You can see the reference on page 4 figure 1A.  It states the "optional overhang" to be Lj/4= Maximum.  It also tells you to reference table 2 (which I did) but still cant figure it out?  Specifically I am trying to find the max allowed cantilever for 2x8 joists 16" oc.  By the way, Thanks for your dedication in providing and seeking out answers for not only myself but everyone who you have ever helped on here

[John Raabe]

Yes, a rough rule of thumb is that a joist or beam can use about 1/4 of its span as a cantilever. That assumes the uniform loading is the same and not that a bearing wall ends up on the end of the cantilever.

[AdironDoc]

I was wondering the same thing about acceptable cantilevers. I had a gander at the table and was as confused as Nathan. At first glance at table 2, it would appear that allowable overhang is 1/4 of joist length.  I don't see any differences in the cantilever length between different 2 x's other than their overall length. This either means that allowable cantilever length was determined to be 1/4 span length, independent on whether its a 2 x 6 or 2 x 12, or that I'm missing something. Conclusion? I must be missing something.

Doc

[John Raabe]

Look at it this way... if you design a floor system using 2x6 joists and span say, 16' with them you are over the working span of the joist and can expect considerable bounce not only on the cantilever but also in the main span.

If that same floor system is made with 2x12 joists it will be nicely stiffer on both the main span and the cantilever. So you want to have the joist or beam sized for the proper span and load before the cantilever is attempted. If not, the rule of thumb will ensure that if you screw up on the main span it will be helpfully reflected in the cantilever.