wood beams with partial steel flitch plate

While on an inspection I saw a 12" LVL beam with a steel flitch plate that terminated 1/4 of the span distance from the end bearings.It seems to me that this design would help with the initial bending moment but would cause 2 other BM at the end of the flitch and induce shears there also. The Engineer said he designed it that way to limit deflection. Any comments? Thanks Jeff in Fl. barnieo@attbi.com

 

[austim]

Jeff,

If the beam is not built in at the ends (ie can be treated as simply supported), then the BM and shear throughout the beam is independent of stiffness anyway.

After analysing a typical built-in beam with a uniformly distributed load, and a stiffer section over the centre halfspan, my conclusions are:

If it is built in at one end or at both ends, then the presence of the steel plate will reduce the maximum Bending Moment in the unreinforced timber sections. (Basically because the reduced deflection will reduce the curvature at the built in ends). The maximum shear will be reduced if only one end is built in, and unchanged if built in at both ends.

My guess is that there would be a similar effect for other load distributions.

I don't think that you need have any worries.

 

[inspectorjeff]

Thanks for the info. Doesn't a combination wood/steel beam usually neglect the wood as the 2 materials are so different in capacities?
So I was thinking that the ends of the steel plate would apply a bending and shear load to the wood beam, which if I remember has a higher moment(PL/3 than than evenly dist. load ( WL/8). I have never seen a beam like this in 28 years.If deflection was excessive would not the shears at the bearings be of concern and warrant the flitch plate to extend to the bearings?( This beam also had a concentrated load of about 10K near the end of the flitch.)
This job was a nightmare because the 2 flitch plates were put on the outside of the beam which then had to be drilled with 60 nail holes for a mis speced face hanger.

 

[austim]

inspectorjeff,

You ask several questions which I cannot really answer from my own experience. (I have never been close to a beam like yours). So I will limit myself to the basics.

I don't like your PL/3 at all (no offence meant;-)). The maximum BM with a point load P at the centre of a span L is PL/4. For a point load anywhere away from midspan the BM is less.

The "excessive deflection" would not necessarily be associated with high shear at the bearings. Any more meaningful answer to that would need a serious structural analysis of the beam (which none of us can do at a distance, particularly if we are not acquainted with your local codes of practice).

Perhaps you could get some more valuable answers to your legitimate queries if you tackle the original designer again?

 

[Ron]

Jeff...partial flitching is relatively common. I have run into a few and have designed a few (in your area). You are correct that there is a "change" at the termination of the flitch plate, but the purpose of selecting a place to terminate the plate (other than at the ends), is to select a point where the influence is minimal.

The plate, itself, does not "inflict" any additional BM into the wood. It does; however, create a "step function" in the shear and moment diagrams at the terminous. You indicated the intent of the design engineer was to reduce deflection. What the flitch plate does is flatten the deflection curve.

 

[sven]

Jeff,
Fitch plates are reasonably common in domestic construction where a beam span is to be increased without significantly changing the profile eg when an intermediate support is removed. Timber is often strong enough but not stiff enough. Steel being 10-20x stiffer that timber, a 5mm steel plate of comparable depth added to each side of a timber beam can be equivalent to doubling or more the beam thickness in timber.

 

[redhead]

If the beam is statically determinate, I don't see how there would be a step function in the shear and moment diagrams. There will be one in the M/EI diagram of course, that is where the stiffening effect comes from.
On another note, the effectiveness of a partial length flitch plate is limited by the horizontal shear transfers between the plate and the lumber. The original enquiry did not specify these connections. What approach do the other respondents use to analyze this condition.
Personally, I prefer to ensure that the flitch plate is the full length of the span and bears fully on the supports.

 

My PL/3 referred to the 2 spot loads on the wood beam coming from the ends of the flitch plate.
dist ld of 100plf
conc. ld of 10K
12' flitch !< 8'>
---'----------'---
^ 20' ^

2-12&quot; x 13/4 LVL + 2-3/8&quot; stl. flitch plates
ending 4' from each bearing point.

 

[Ron]

Redhead...thanks for the slap up side the head!! You are correct! I was thinking stress plots (bending and shear), as I use these a lot for determining ends of gussets, knee braces, flitch plates, etc., ....but shear and moment diagram came off the fingers!

Ron

 

[boo1]

Jeff, where was the 10,000 lb conc load applied at?
Are the ends ridgidly supported?
Is the top or bottom flange lateraly supported?

 

[redhead]

Ron:

You are welcome! We all have these moments. My excuse is they are &quot;senior moments&quot;.Cheers!

 

[inspectorjeff]

Sorry for the delayed answer.The 10K load was about 5' from the right bearing. Beam only supported on top surface by 2' of second flr hall. I thought that the steel would take the entire load and at the terminus dump it into the wood beam.
Jeff