Floor joist span increase 1

[archycad2003]

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?

 

[redhead]

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.

 

[boo1]

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 members is limited by maximum allowable deflection. Laboratory studies showed that gluing sub-floor to joists increased composite action between the two, therefore enhancing floor serviceability. Blocking/ bridging/strapping/strong-backs restrain joist torsion and increase floor stiffness.

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

 

[RG88]

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.

 

[emceditor]

boo1, I have been trying to find information on the influence area of sheathing. Where is the 40t referenced?

 

[0404]

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” for the code specified loads, I doubt the system would work same as joist spaced 12” 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” 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.

 

[LPPE]

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).

 

[boo1]

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.

See APA recommendation:

http://www.apawood.org/pdfs/managed/R280C.pdf

 

[steve111]

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.

 

[bylar]

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?

 

[frogit22]

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.

 

[m8589]

I think if you use load tables then engineer calcs of the subfloor must confirm that vertical load might be distributed according to joist flexibility(rigities) in lieu of the tributary area.