Flitch Beam Analysis and Design

[sauce_man]

Hello Again Folks,

Looking for some insight again.

I would say I am a good wood designer, but not an expert. This may be a frustrating subject since it pops up often on the threads, but I do not seem to find anything addressing my current issues.

I am working on a heavy timber project and there are a few places we are considering using flitch beams. I have found a few links across the web (mostly from this site)and I think its got me going in the right direction.

I am looking at an enercalc image of their spreadsheet style calculation. I have essentially been following this template for my design as I move along. I can gather from this the percentage of wood flexural stiffness and steel flexural stiffness to distribute bending stresses. OK, that's great, but I do not see any examples go beyond this so let me fire off a few questions for the wood experts:

1. I assume its safe to say the wood should be design for the effective percentage of bending stress. But what I never see is adjusted strength values. My intention is to use (2) glulams and (1) steel plate typically. Would I calculate the adjusted ASD bending stress based on one individual glulam section? or would I use a section with an effective width of the two plys?

2. I did note someone say glulams can be fussy with bolts loaded parallel to the grain. interior bolts should be designed based on force parallel to the laminations. Can anyone expand on issues associated with this and any load reductions in the code?

3. It looks like the enercalc procedure is considering a maximum change in shear value (in k/ft) to design the interior bolts. Can someone ELI5 what they are doing and if this process is correct? The DeStefano example calculates the uniform load on steel plate and distributes bolts based on that, however I may not have uniform loads in all cases.

4. I am also looking a few situations where I may have unbraced beams picking up significant point loads. Similar to above, would I calculate adjustment factors for an individual section or an equivalent section of the full width of wood? Also, I can buy using the plate plastic capacity when the top flange is fully supported, but if the beam is unbraced can I still assume the plate is braced for LTB? or should I consider some Lb the length between top flange fasteners?

5. Finally, Is it enough to use the transformed moment of Inertia and wood modulus of elasticity for deflection analysis using general deflection equations?

link to reference:

https://enercalc.com/sel_help_20/flitch_plated_wood_beam.htm

Please feel free to chime in with any other input and experience!



Cheers,

 

[RWW0002]

You may have found a copy of this already, but there was a Structures Magazine article in 2007 that had some pretty good information on Flitch PL design methodology and some useful tables (see attached).

 

[ProgrammingPE]

1. If you are using 2 glulams when you determine the percentage of load going to the wood, then you get to use the capacity of 2 glulams when you determine the strength of the wood. (Keep in mind, though, that there are 2 separate beams and not one extra wide beam. This means the capacity is twice the capacity of an individual glulam which is not the same as have a single glulam that is twice as wide due to factors like Cv which are based on the width of the individual pieces.)

2. Maybe someone else is aware of issues here, but the wood specification includes values for Z,parallel for glulams, and I don't see any special factors that are required. The bolts are also designed to transfer load vertically from the wood to the plate along the beams length. At the ends they transfer load vertically from the plate to the wood, so you are checking Z,perpendicular.

3. Change in shear value is equal to the applied load so they are doing the same check. The assumption is that the load is applied to the beam so then the bolts must then transfer the percent of load that goes into the plate. This also applies for point loads so you would need additional bolts at these locations.

4. I would determine adjustment factors for stability using the width of a single glulam (similar to what I said in #1 since you have two beams, not one wide beam). The bolts will tie the glulams together some, but there's going to be a significant reduction in out of plane and torsional stiffness so I would not rely on that. In addition, the wood is also being used to prevent the steel plate from buckling so it will have that additional force that wants it to buckle, so I would be very conservative here. The enercalc example you shared is also using Z instead of S, but I would stick to using S because of the buckling issues, but also because once S is exceeded, a plastic hinge will begin to form and more load will be transferred to the wood than what was initially assumed.

5. The whole flitch plate design method works by splitting up the load so that the deflection of the wood and steel are the same. You can calculate the deflection for the wood using its percent of the load or you can calculate the deflection of the steel using its percent of the load and you will get the same result.

Structural Engineering Software:

http://www.structuralcentral.com

Structural Engineering Videos:

http://www.youtube.com/channel/UCOj2mXXf3ZbZtgxTc6BtkGg

 

[sauce_man]

Great answer, thanks for the input.

I will also note that the enercalc example does not convert their E to psi from ksi. when calculating for Cg which leads to a minor error.

Are you saying for Mn you would use FySx as your limit rather than 1.6My?

 

[ProgrammingPE]

You're right. It looks like enercalc is using ksi values when they should be using psi to determine Cg. Good catch.

I would try to get it to work using Mn = Fy*Sx for the plate. If this value is exceeded, then the load distribution assumption will not be correct since the plate will be more flexible where the plastic hinge begins to form. It also decreases the chance that the stability coefficient for wood is overestimated since the wood has to avoid buckling in addition to preventing the steel plate from buckling. This also matches the example that RWW0002 posted. (The LVL example is controlled by steel. They use Mn/Ω = 0.66*Fy*S instead of Mn/Ω = (1/1.67)*Fy*S since they are using the method from the 1989 ASD Specification.)

Structural Engineering Software:

http://www.structuralcentral.com

Structural Engineering Videos:

http://www.youtube.com/channel/UCOj2mXXf3ZbZtgxTc6BtkGg

 

[sauce_man]

I am also wondering about the effects of creep in wood. Where NDS typically says 0.5 times your sustained load, will a flitch beam actually tend to creep?

I saw someone note using a modified E to simulate this, and I am wondering if it makes sense to analyze flitch beams with a reduced E to account for final creep effects (and thus dumping more load into the flitch plate). Or am I thinking too much about this?

 

[ChorasDen]

Hi Sauceman,

For simplicity, in the past, I've often seen the flitch beam model designed in such a way as to assume the steel plate takes all the load. In that case, the wood on the outside of the beam is not used to carry vertical load, but rather, to provide resistance to buckling of the slender steel member. This will considerably simplify the design, and avoid your concerns regarding creep and differential stiffness between plies.

To directly address your question about creep, do not be concerned regarding creep deflection in a flitch beam. Creep deflection requires the entire system to deflect, when the wood begins to exhibit creep deflection, the load is then transferred to the steel to compensate and achieve equal deflection across the entire member. As such, your load distribution between members will act slightly preferential to the steel plate, but I don't think it will be significant enough to matter.

 

[CConrad]

Thanks for flagging the potential issue with the calculation of Cg. I'll check on that calc and report back.

Director of Technical Services
ENERCALC, Inc.
Web:

http://www.enercalc.com

 

[CConrad]

Thanks again for flagging the issue with the calculation of Cg. It has been correct in ENERCALC SEL Build 20.21.10.30.

Director of Technical Services
ENERCALC, Inc.
Web:

http://www.enercalc.com