Wood beam reinforcement, mixed sections LVL with built-up 1

[gasma1975]

Hi,

I have a wood beam built-up 4[2x10] (SPF), with 3 supports, the mid support is at 10 ft, and the total lenght of the beam is 17 ft. I want to remove the support in the middle. I would like to add an additional section for reinforcement (I guess it will be a ply of LVL) on each side. How do you determine the final resistance of the beam. How to determine the LVL depth needed, and when it is mixed with the 4[2x10] in sandwich, what will be the equivalent ?

Or, is it more common to determine the LVL needed and not to care of the 4[2x10] ?

thank you

 

[nhstruct]

I have always figured out the required capacity, and compare it to the actual capacity of the exist. beam. Then design the reinforcing to take the rest. Then design the connection of the knew to the old with the difference. Make sure you also check the bearing capacity of the beam, as you may need to add some additional bearing area.

 

[JAE]

That isn't exactly correct.

If the added members are stiffer they will take more load than just the amount that you are short. LVL's have a higher modulus of elasticity and thus will take a higher amount of load than the "softer" 2x10's.

To design these, you must first determine the applied loads on the beam and then determine your shear and moment values based on the longer span.

With that, most would simply "try" a couple of LVL members. Say two 1 3/4" x 14" members sistered onto the sides. These have a set stiffness (EI) that would be compared with the EI value of the (3)2x10 beam.

The beams will take their share of the load in proportion to the relative EI values.

For (3) 2x10's compared to (2) 1 3/4" x 14" the LVL takes about 80% of the load.

So you'd design the combined beam to share the load in that manner. Both should deflect the exact same amount by calculation (relative EI's create proportioned loads to result in the same deflections). This is also ensured by fastening them together.

 

[msquared48]

Yep...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[ToadJones]

But, if you don't install them correctly, your new LVL's wont take any existing dead load.

 

[Teguci]

You should give us a sketch of the proposed new section. If you mismatch 2x14s with 2x10s with the tops or bottoms flush, your section modulus will be different from top to bottom. I'd also suggest providing a sketch of your connections.

 

[JAE]

Good point, Teguci. The offset in their mid-points does make a difference too.

 

[woodman88]

With the materials given, the difference section modulus between the members being flush to the top or bottom will be so small that it is not a problem.
Personally, I would design the LVL's to support the total loading.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[ToadJones]

I agree with Woodman- I think I'd design the LVL's to take the full load.
Then, I'd jack the existing column say 1/2 of the anticipated dead load deflection as if the LVL's were carrying full load, then install the LVLs.

 

[spats]

Isn't there a limit on the number of plies in a built-up beam? How will the plies be connected to assure proper load sharing?

 

[ToadJones]

If you design the LVLs to take full load, they dont need to share load through fasteners. Its just two beams.

 

[MiketheEngineer]

I have done this in the past - I usually let the new beams take the full load - but if I am "low" by say 10% - I might let it ride and allow the "old" beams to help out.

 

[Splitrings]

A classic engineering problem. Calculate the properties of the transformed section using the ratio of the modulus of elasticities. Then be certain you have enough fastners and or adhesive to transfer the shear between the laminations.

 

[FishTheStructure]

Are any of the 4 existing 17' 2x10's spliced above the existing midspan column presently? I would discount any of the 4 2x10's that are spliced near the midspan of the 17' span as they will not contribute to moment resistance.

Great spirits have always encountered violent opposition from mediocre minds - Albert Einstein

 

[SilverBeam75]

Thank you all for the help. But finnaly the LVL had to much depth and it wasn't accepted. We have decided to put 2 steel channels C9x15 back to back and bolted through the wood beam.

 

[msquared48]

IK do not get why an LVL, a with a higher allowed stress and section, would have a greater depth than the original joists. Makes no sense to me.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[AELLC]

"IK do not get why an LVL, a with a higher allowed stress and section, would have a greater depth than the original joists. Makes no sense to me."

The original joists are 4 times 1.5" and the new LVLs are only 2 times 1.75", AND the new span is 17' vs old span = 10'

 

[MiketheEngineer]

Yeah - span went way up - so did bending - so did deflection.

I have often used C-Channels as you figured. Just make sure you have enough good bearing at each end!!

 

[msquared48]

Got it. Thanks...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com