floor joist span increase

huangyhg

floor joist span increase
greetings to all, my first post. i am a plans examiner for commercial buildings; however, the residential guys asked me to take a look at this situation.
a home builder, new to our area, is proposing to use 2 x 10 floor joists at 16" oc with spans of 16 ft to 17 ft (spf #1/#2, tables limit the span to 15'-4" w/ 40 psf ll + 10 psf dl).
the builder produced a set of engineer's calcs (dated several months ago for another city) which confirmed the table spans then added a statement that with every third 2 x 10 joist doubled and bridging at mid-span this would be equal to 2 x 10 at 12" oc. therefore, the maximum span for the 2 x 10's would be 17'-3" (limited by deflection, 17'-9" limited by bending). the sub floor is nailed (no adhesive) to the joists.
before i ask for his rationale and calculations, i wanted some opinions.
is there a rationale for this type of span increase? has anyone used or calculated this type of span increase?
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yes. we do it all the time. since more material is being added and the bridging is effectively distributing the load, the system is equivalent to having all the joists spaced at 12in. centres. we prefer the proposed system, because joists physically placed at 12in. centres do not allow much space for the hvac ducts.
the structural goal of a house is to safely transfer building loads (weights) through the foundation.  strength and stiffness are equally important. stiffness of structural   
i agree with the aforementioned threads...this is a common and accepted practice, at least for what i have seen in the se usa.
i would mention, that having been in your shoes before, ask that the engineering be site, plan, lot #, or otherwise specific to  your review.  i have seen alot of 'builders' pass off 'similar' engineering to save a buck, but it creates a liability problem for the inspections office.  where i worked, we had a policy that engineering must at least be labled by the engineer as specific to the site or plan.
also, we use to annotate our plans to let the field inspectors know to double-check these areas for plumbing penetrations, etc.  as at that span and configuration, the floor system is 'engineered' and the typcial code allowed penetrations and notches are no longer valid unless verified by the engineer.
boo1, i have been trying to find information on the influence area of sheathing.  where is the 40t referenced?
the sub floor plays a big role in this case. all uniform loads on sub floor shall be transferred safely to joists. if the sub floor can only span 16鈥?for the code specified loads, i doubt the system would work same as joist spaced 12鈥?o/c, because the double 2x10 joist is not able to take more loads than any other joists. in other words, the joist far adjacent to the double joist may still have same load on it as joist spaced 16鈥?o/c, and these joists are now critical for increased span. therefor, in order to make the system work, the strength and stiffness of the sub floor should be checked for transfer more loads to double joist 2x10.
boo1 mentioned gluing the subfloor to the joists.  i think you have to be careful about what glue you use, because most of the construction adhesives are not "rigid" glues.  they are more like a neoprene type (elastometeric?) glues that will deform under load (ie: not transfer your horizontal shear for composite action), and are generally used for sound attenuation (stop the squeaky floors).
pylko, i meant to imply (adding), we glue our decking as a supplement to the fasteners to prevent sqeeking and add additional ridgity.  the screws / nails should be specified per the loading.  
canadian wood council publishes a span book that gives span for dimensional lumber depending on, strapping, bridging, and attachment of the floor sheathing to the joist ie nailed and/or glued (elsatomeric) etc condition.  as the sheathing becomes more rigidly connected to the joist t beam action is emulated and the sheathing becomes part of the system re, stressed skin panel, the only structural glues permitted are phenol/rescorcinal glues requiring big time pressures at the glue line to get true composite/fixed condition between the sheathing/skin/flange and the joist/web of the t beam.  in between this connection and only nailed connections, are a variety of semirigid possibilities.  doubling every third joist to increase the span or spacing?  doesn't make sense from a cost perspective nor from a span perspective re load sharing and the variable stiffness across the floor system.  not sure of effect on vibration controlled spans.  should dampen though ie stiffness attracts.
i quess i'm stupid but it seems that adding the 2x10 at every third joist is the same number of joists at 12" o/c  so why not just put them at 12" o/c?
i agree with bylar.  decreasing the spacing would be more effective.  i have some doubts as to whether bridging would really be all that effective if the double joists are spaced at 4'-0" apart.  the down-side to 12" oc spacing, of course, is that mechanical ducts are more difficult to run.