Deflection of Timber Floor Joist

[civeng80]

I designed floor joists for a commercial office. Joists span 6m but most are supported on interior walls, but I designed the joists for a simple span of 6m, so all are same size.

Joist are now installed, but I feel great support on supported joists and not as good but OK support on free spanning joists. The joists satisfy both strength and deflection criteria.

Is there any way to soften the difference in deflection ?

Any Practical suggestions would be appreciated.

 

[KootK]

Maybe some bridging? Or sister additional joists onto the free-spanning ones?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Triangled]

if I understand correctly, the deflections of some joists are being 'restricted' as they start to bear upon 'non-bearing' partitions below... and the deflection differential between these 'supported joists' and the other free span joists is the noticeable on the floor and is the concern....

1. I'd probably try to adjust the connection where the joist is bearing on the partition below. Perhaps notch out the top plate somehow.... to let the joists move freely as intended.
2. Bridging, at least the way I've done it, would tend to force adjacent joists to deflect similarly. In your case, since the problem joists are not deflecting enough, the bridging solution will tend to lift the adjacent joists, and that lift will add more load to the 'non-bearing' partitions... which may be a cure worse than the concern, which is something you'd have to evaluate.
3. Increasing the moment of Inertia to directly reduce the deflection of the 'ok' members can work too.

 

[civeng80]

This is an ammenties block in a warehouse. All walls timber framed and the top is being used as either a storage (mezzanine) or possibly office loading (3kPa).

So Im using all the partitions as load bearing walls. The joists are spanning 6m where there is no support. Where there is support the joist span 4.5m continuous support.

Deflection in free spanning joists about 12mm i.e Span/480 which is considered to be acceptable.

When I walk on the floor I slightly feel the difference between where the joists are supported and where they span freely.

The joists at this time are not blocked (bridged) maybe this will help a bit.

Kootk's idea sounds good.

Just wonder if anyone else has come across this and what they have done about it if anything, because everything complies with standards.

 

[KootK]

This won't be helpful now but, in a new build, I would:

1) Provide a deflection gap beneath the joists with walls below or;

2) Try to bridge the unsupported areas with supporting girders, even if those girders were designed to loose deflection requirements.

Differential stiffness / deflection is a common problem in wood: Link. At the end of the day, it doesn't much matter what criterion you designed to if you've got an unhappy owner.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[civeng80]

Thanks Kootk.

Sometimes its not possible to provide supporting girders because of headroom considerations.

I think in this case either solid blocking between joists with screwed fastenings or doubling up girders (which can still be done) will surely help, like what you suggested.

 

[EngineeringEric]

What i have done is double up a few of the longer span girders adjacent to shorter span to help reduce the deflection and mimic the shorter span. i think this is what SJI recommends as well...

 

[phuduhudu]

If you are happy for the internal wall to take a little more load, I would put some tight blocking in between these joists and the free spanning ones to ease the transition. At the moment you are getting a 12mm deflection over the spacing of a single joist. It will be half as noticeable if that is spread over 2 joists and perhaps you can get it to spread over 3 joists.

 

[civeng80]

Kootk a special thankyou for the link.

Gave me an idea, which solves the problem.

Just came back from site meeting where it is possible to place a bearer under the existing 6m joists to reduce span to 4.7 like the other spans.

Great outcome and economical.

 

[KootK]

You're most welcome cubeng80. I'm glad that things worked out. I'm curious though, what is this "bearer" that you used for your solution?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[civeng80]

Kootk

It is a supporting beam under the joists.

We also had a good walk around and jump around where the owner is about 110kg (Im about 75 kg) where remarked that the floor was as solid as a rock. Maybe it was my imagination but I felt a little difference in support between the 2 spans, he did not even though he weighed more than me. Still put supporting beam to reduce deflection since we have the opportunity.

 

[hokie66]

KootK,
"Bearer" is an Australian term for beam, mostly referring to wood beams in residential construction.

 

[KootK]

Thanks Hokie. Lig, cog, bearer... gradually, I'm learning to speak AU structural.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.