Does Sistering Joist Short of Supports Increase Design Value in Bending & Load Capacity? 1

[zink88]

I am in the middle of a bathroom renovation project.

Original floor joists from 1994 home build are 2x10 stamped as SPF #2 with a 13'-5" span. I have sistered them with 12' 2x10s of SYP #2 stopping ~ 8" short of the supports at each end. The sistered joists have been attached using Great Stuff construction adhesive and (3) 3" screws every 12". The joists are now noticeably stiffer.

Does sistering the joists in this manner increase the design value in bending and load capacity...or has the floor load capacity actually been decreased by the weight of the new joists?

 

[r13]

My opinion,

1. yes, it increase moment capacity if the bond (from adhesive) stays. The screws help to maintain the contact of the joist and the 2x10s, but I wouldn't count on them to provide strength.
2. the joist will have to checked for higher bearing/shear force at support end for every (non-original) additional load.

 

[zink88]

Is shear force the same as Compression Perpendicular to Grain referenced in Tables 9-1 & 9-2 in the Span Table for Joist and Rafters by the American Wood Council at the link below?

https://www.awc.org/pdf/codes-standards/publications/span-tables/AWC-SpanTables2015-1505.pdf

 

[JoshPlumSE]

It will increase the moment capacity. But, not the shear capacity near the support. Be sure to check the shear capacity of the original joist compared to the increased demand.

 

[KootK]

Does sistering the joists in this manner increase the design value in bending and load capacity

Yes but the critical design issue will be transferring all of the shear in the new piece back over to the original piece at the end of the new piece (8" from bearing). That creates significant, local demand for shear transfer between plies. You may well need a bunch of fasteners at the ends of the new piece.

 

[r13]

zink88,

Check out this document. Link

 

[zink88]

I will check out the link.

If I am reading tables 9.1 and 9.2 correctly in the document I referenced, a 14’ span requires compression strength of 207 to handle a 40 lb/ sq ft live load and 10 lb / sq ft dead load with load bearing surface of 1.5 inches. Increasing the live load to 60 lbs / sq ft and dead load to 20 lbs / sq foot requires compression strength of 1.6*207=331.

SPF is identified as having compressive strength of 425 in another table (Design Values for Joists and Rafters by the American Wood Council) and SPF (south) is 335. The load bearing distance is actually just under 2 inches at each end of the joist and my span is 13’-5”...so the original joist should be able to more than handle the compression requirements of 60 lbs live load and 20 lbs dead load...correct?

 

[r13]

Seems right, if is using ASD with all load factors equal 1.0.

 

[zink88]

Thanks for the confirmation.

In order to support 60 lbs/sq ft live load and 20 lbs/sq ft dead load, table F4 in the same document (link below) appears to indicate that 2x10s spaced 16" on-center with span of 13'-4" require a modulus of elasticity of 1,300,000 and design bending value of 1167.

A 13'-8" span requires moduus of elasiticity of 1,400,000 and design bending value of 1226.

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https://www.awc.org/pdf/codes-standards/publications/span-tables/AWC-SpanTables2015-1505.pdf

Table W1 in The Design Values document linked below indicates the original SPF #2 2x10 joists had design bending values of either 1105 or 980 depending on if they are SPF or SPF (south). Similarly they have modulus of elasticity of either 1,400,000 or 1,100,000.

https://www.awc.org/pdf/codes-standards/publications/span-tables/AWC-SpanTables2015-DVJR-1505.pdf

Would the sistering of the original joists with 12' 2x10 SYP #2 have increased the design bending value above 1167 or 1226?

Would the sistering have increased the modulus of elasticity above 1,300,000 or 1,400,000 if the original joists were SPF (south)?

 

[r13]

zink88,

When you use the Span Table Fxx, you shall read the design criteria on the left upper corner, and adjust the tabulate value accordingly. Also, read point 9 of Explanation of Tables on the same document, that the tabulate values of Table 9.1 is based on a total uniform load of 66.67 plf on the joist, so you may need to adjust the read off value by a scale factor corresponding to the design load.

I think it is proper to select bending value from Table W-1, but I don't know the difference on SPF and SPF (south), if uncertain, I would use the lower allowable.

 

[oldestguy]

I'd apply some bending to the new joist additions before securing to the old joists, if possible.

 

[r13]

Due to the confusion in directly picking design values from the tables, I suggest reverse engineering the existing joist to estimate Fb an E for comparison.

1. For one 2x10 SPF #2 @ 16" oc with 13'-5" span length, use the existing load w=60 plf, calculate bending moment, then calculate Fb from moment, how the result compared to the value from the table?
2. Let Δ=13'5"/360, then estimate E from deflection formula.

I hope the turned out results provide confidence to be used for the modified joist to check bending, compression and deflection under the new load.

 

[Tomfh]

Don’t skimp on screws, particularly at the ends where shear is highest. When I do these jobs at my house I just fill the beams up with screws. It doesn’t take long with a rattle gun.

 

[JoshPlumSE]

For what it's worth, I've had some projects where I didn't know the capacity of the connector of the original joist. In those cases, I sistered the joists full length and threw and A34 at the end of the new joist. It's more money for sure. But, when the new load is large, it's a better plan.