×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Wood beam reinforcement, mixed sections LVL with built-up

Wood beam reinforcement, mixed sections LVL with built-up

Wood beam reinforcement, mixed sections LVL with built-up

(OP)
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

RE: Wood beam reinforcement, mixed sections LVL with built-up

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.

RE: Wood beam reinforcement, mixed sections LVL with built-up

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.

 

RE: Wood beam reinforcement, mixed sections LVL with built-up

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

RE: Wood beam reinforcement, mixed sections LVL with built-up

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.

RE: Wood beam reinforcement, mixed sections LVL with built-up

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

 

RE: Wood beam reinforcement, mixed sections LVL with built-up

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.

RE: Wood beam reinforcement, mixed sections LVL with built-up

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.

 

RE: Wood beam reinforcement, mixed sections LVL with built-up

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?

RE: Wood beam reinforcement, mixed sections LVL with built-up

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

RE: Wood beam reinforcement, mixed sections LVL with built-up

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.

RE: Wood beam reinforcement, mixed sections LVL with built-up

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.

RE: Wood beam reinforcement, mixed sections LVL with built-up

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

RE: Wood beam reinforcement, mixed sections LVL with built-up

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.

RE: Wood beam reinforcement, mixed sections LVL with built-up

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
 

RE: Wood beam reinforcement, mixed sections LVL with built-up


"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'

RE: Wood beam reinforcement, mixed sections LVL with built-up

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!!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources