Floor joist span increase
Floor joist span increase
(OP)
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?
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?






RE: Floor joist span increase
RE: Floor joist span increase
Maximum deflection limits are set by building codes. They are expressed as a fraction; clear span in inches (L) over a given number. For example: a floor joist appropriately selected to span 10 feet with an L/360 limit will deflect no more than 120"/360 = 1/3 inches under maximum design loads. Drywall attached to the underside of this system is not expected to crack when the floor joist system deflects 1/3". Typical deflection limits referenced in code books are L/360, L/240
Blocking is placed in-between longer lengths of floor joists to keep them from being able to twist and roll onto their sides, transfer shear and transfer vertical loads. Usually placed every eight feet for 2x10s. Blocking does not reduce bending stresses. The sheathing provides an area of influence (40t). Redhead indicated it is an acceptable practice, but I recommend adding a bearing check and glueing the decking to the joists.
Step by Step
Here is a checklist of steps to follow when using span tables
1) check plans to determine span and on-center spacing (design conditions)
2) check codes for allowable live load, snow load, dead load and deflection
3) select appropriate span table
4) match span in table to design condition and determine minimum Fb and E values listed in the span table
NOTE: you will have options for on-center spacing and size
5) select appropriate species and grade from values listed in design values table
NOTE: you will have options regarding species and grade providing you with an economic opportunity
6) determine required compression perpendicular to grain design value in table
7) verify that the compression perpendicular to grain design value for the species selected in step 5 meets the required design value determined in step 6
RE: Floor joist span increase
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.
RE: Floor joist span increase
RE: Floor joist span increase
RE: Floor joist span increase
RE: Floor joist span increase
See APA recommendation: http://www.apawood.org/pdfs/managed/R280C.pdf
RE: Floor joist span increase
RE: Floor joist span increase
RE: Floor joist span increase
RE: Floor joist span increase