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Glu Lam beam modification/suggestion and design calcs
3

Glu Lam beam modification/suggestion and design calcs

Glu Lam beam modification/suggestion and design calcs

(OP)
I am currently building a house and need to modify a glu lam beam.  I have a simple span of 16 feet over a garage opening.  The beam supports only the roof trusses/roof.  The support consists of 2 glu lam beams, each measuring 19 inches deep and about 5 inches wide.  I need to cut the bottom 5 to six inches out of the beam.

I am looking for advice and calculation software or tables  to make sure I can handle the loads.

Suggestions?

CP

RE: Glu Lam beam modification/suggestion and design calcs

  The butchered beams will have less than 1/3 of the original strength!  This loss can be made up in a variety of solutions.  Add a post, replace with steel beams, repair with steel plate at the bottom of the glulams, etc.

RE: Glu Lam beam modification/suggestion and design calcs

(OP)
I hear what you are saying, but it seams counter intuitive.  I would expect that the beam performance would be comparable to an equivalent lesser beam thickness with some reduction.  Do you have a basis or technical paper that addresses cutting abeam?

Thanks

CP

RE: Glu Lam beam modification/suggestion and design calcs

I think Glulams are more complicated than you think.  If I am not mistaken, the wood layers at the top and bottom are a lot stronger than the middle layers. So cutting 5" out of 19" deep doesnt necessarily mean it has the capacity of 14" Glulam.  I would use steel beam if I were you. OR design with 14" GL instead of butchering a deeper member.  Deeper members are more expensive anyway.

RE: Glu Lam beam modification/suggestion and design calcs

If true glulam, the ply strength is matched to the stress.  They have stronger plies on the outer edges where the stresses are higher.

Cutting the outer plies may significantly reduce the strength above just reducing the section modulus.

Also modifying the beam may void any warranty. Many fabricators specifically caution about modifying their products.

Dik

RE: Glu Lam beam modification/suggestion and design calcs

(OP)
Hey guys thanks for the input.  The beams are already installed and the roof in.  The builder set the garage door opening at 7'6" and I would like to take it up to 8'.  So cutting would be the only option.

The beams appears to be well over sized.  I have two 5" by 19 beams, side by side, supporting a 16 span with roof trusses resting on top.

I also may be using the term glu lam incorrectly.  The beam is composed of a series of what appear to be 9 =/- 2x5s stacked horizontally and glued to create the beam section.  The beams were made by weyerhauser. Is it correct to refer to this type of beam as a glue lam?  The exterior members appear to have the same properties as the interior wood.

RE: Glu Lam beam modification/suggestion and design calcs

(OP)
I am think through options here and could I not 1) bolt the beams together there by increasing the section modulus 2)place a steel plate at the cut base to increase/replace lost tensile capacity and there by increasing the section modulus?

  

RE: Glu Lam beam modification/suggestion and design calcs

That is a glulam.  I dont understand why someone would put GL in the wall.  Why not just use LVL (cheaper and strong).  I always spec GL only if it is exposed (looks nice).  You are the engineer captain.  Do what you think is necessary.

RE: Glu Lam beam modification/suggestion and design calcs

(OP)
The GLs were free so the price was just about perfect.  Otherwise, the beam would have been steel. I am an engineer, but the Geotech type.  Structural design using timber and specifically GL is well outside my comfort zone.  

Do you know anyone in the Denver area, that by chance, would have GL experience?

CP

RE: Glu Lam beam modification/suggestion and design calcs

I know someone smile.  See my name?  

RE: Glu Lam beam modification/suggestion and design calcs

(OP)
I figured.  Where are you in Denver?  I am out in east Parker...

RE: Glu Lam beam modification/suggestion and design calcs

I would suggest you calculate the bending stress based on your reduced section and compare it to the allowable stress in a #2 or #3 in what ever species the glulam is.  That means you need to calculate the load in the beam, ie span of the roof trusses and snow load.

RE: Glu Lam beam modification/suggestion and design calcs

Littleton.  Unfortunately, if you use our company, we would size a whole new beam (probably LVL or Steel depending on the load).  If you are building a house, didnt you have an engineer designed it for you?  

RE: Glu Lam beam modification/suggestion and design calcs

(OP)
I did the geotech investigation and the foundation design and used a structural for the ICF and beefed it up from there.  The beams, well you would have to see the beams, in it's present configuration it is obviously hell for stout.  

Now that I am considering cutting into the beam, I have a little pucker going and now I need to put paper to pencil and calculate the loads, bending stresses and deflections to see where we are in the grand scheme of things.  My gut says it's fine, however, given my inexperience, I need to look at it and get a peer review to make sure I calculated things right.  BTW, all the work performed on the house was independently peer reviewed.

I just haven't performed a beam design in 20 years.  Hence the questions.

Do you know of any freeware or tables I can check my calcs against?  I also will probably need a structural to look at it.  We are slammed in house so I may have to go outside my comapany.

RE: Glu Lam beam modification/suggestion and design calcs

http://www.anthonyforest.com/powersizer.shtml

Try this.  It is for a different company, but it should give you stresses (at least the older version did) based on the loads you input.  Note as discussed earlier in the thread your allowable stress will be significantly lower than the published values.

RE: Glu Lam beam modification/suggestion and design calcs

(OP)
What would you consider an appropriate reduction?  66% seems pretty high.  But, then again, this isn't dirt....

RE: Glu Lam beam modification/suggestion and design calcs

Like Dik and I said before, you are not suppose to modify glue lam.  Cutting the bottom of GL is almost like cutting the bottom of I-joist.  Tell your structural engineer to size you a header that will fit your door opening.  Calculating the reduction for cutting the bottom 6" of GL is way complicated.  Unless you work for the GL manufacturer, I dont think its a smart thing to do.  

RE: Glu Lam beam modification/suggestion and design calcs

A #2 S.P. 2x6 allowble bending stress is 1250 psi and a #3 is 750 psi.  D.F. will be similar.  There are a lot of factors that go into it, but if you get over 1000 psi, I would find some other solution for sure.  The glulam guys will just tell you to not cut the beam.  I would make sure your stresses are low (like well below 1000 psi) before thinking about cutting the beam.  Even then I am not sure if I would do it.  Keep in mind notches are really bad.

Are you sure there isn't another option, like raising the beam?   

RE: Glu Lam beam modification/suggestion and design calcs

(OP)
The beams in and the roof is on.  So it's either it's cut it or don't.  I hear what you are saying.  

Do you know any one in town that sells GL beams and can provide technical support? I am now curious how they are manufactured and how they create the stronger tensile members at the beam exterior.

CP

RE: Glu Lam beam modification/suggestion and design calcs

They just use higer grade lumber, even MSR lumber sometimes, in the outer laminations.  They see the most stress due to bending.

RE: Glu Lam beam modification/suggestion and design calcs

Sorry, and as far as tech support, forget it.  They will just tell you not to cut it.  I have been down that road bofore.  You won't find any manufacturer that will tell you what you are doing is ok.  

I had to build a model is RISA with plates some time back to look at what happens when you cut a hole larger than they allow.  It did work, but I bet I put 40 hours into the analysis.   

RE: Glu Lam beam modification/suggestion and design calcs

(OP)
Thanks for the info.  I had to jump a plane and I will be south for a day or so.  I will run the numbers once I get back and can run down my truss calcs.

I appreciate the input and feedback.

CP

RE: Glu Lam beam modification/suggestion and design calcs

TrusJoist has free software you can use but they mail you a CD (no download).  Enercalc is downloadable but much more complicanted for your need.  Either may give you the tool neccesary to calc the cut and see how it will vastly affect the flexural strength of the Glulam.  Remember, your strength of materials, flexural stress is inversely proportional to the moment of inertia and for a rectangular section, the moment of inertia is proportional to the cube of the depth.

Don Phillips
http://worthingtonengineering.com

RE: Glu Lam beam modification/suggestion and design calcs

(OP)
Thanks Don,

I appreciate the feed back.  I will be running numbers when I get back to Denver.......snow pending.  I will report back to the group what a find......

RE: Glu Lam beam modification/suggestion and design calcs

If you are cutting the entire beam by the same amount then you will need to reduce the allowabe bending stress by 50% to account for eliminating the higher strength lams. If you are notching the bottom the APA recommendations suggest no more than 10% of the depth and have stringent requirments for reinforcing the ends by installing lag screws up vertically thru the bottom of the beam to prevent the shear from splitting the beam horizontally.

If you want to pdf me a calc i could review

regards,

RE: Glu Lam beam modification/suggestion and design calcs

Is there actually design criteria for the lag bolts at a nocth?  It sounds like a good idea.  Maybe somewhere in the NDS or Timber Construction Manual?

RE: Glu Lam beam modification/suggestion and design calcs

yes there is a design doucment it is put out by American Plywood Assoication APA. To my knowledge NDS or TCM do not have it. I am off today but let me search the APA website for the document.

Bkgd

RE: Glu Lam beam modification/suggestion and design calcs

will do I did a search at APA and coukd not find it however I know that i have the paper at my office. I will dig it up Tuesday and PDF it to you.

Regards,

RE: Glu Lam beam modification/suggestion and design calcs

(OP)
I would like to take you up on your offer and would like a copy of the APA paper.  I was wondering how you send a PDF?  I posted my contact info and that was shut down pretty quick.  Do you have any idea how I would get calcs to you?

RE: Glu Lam beam modification/suggestion and design calcs

not sure i got side tracked this week, i will dig for the paper tomorrow and then i think we can post documents somewhere to this site, i recieved and email explaining the the need to not show addresses in order to keep spammers at bay which makes sense.

should be away to transmit data, let me find the paper first,  

regards,

RE: Glu Lam beam modification/suggestion and design calcs

I work in the glulam industry, and would not reccomend that you cut the glulams down, unless you have them analyzed by a structural engineer who has experience in glulam design.
Based on your description you have two 5" x 19" glulam beams as your garage door header.  

This tells me one of two things, either the beams are carrying a very large amount of load or who ever made the decision to use two beams does not have a good understanding of the load capacity of the glulams.  A single 5" x 19" glulam probably has a capacity of around 1500 bls/ft or more for a 16' span.

As indicated above glulams have higher strength laminations on the outside, so ripping them down can cause a large reduction in carrying capacity.  Neglecting the difficulty in ripping the members down in the field,  you would not want to do it unless the stresses were well below the allowable stress of the lowest strength lamination.

One final comment, the glulam supplier generally  will not give a person permission to field modify a glulam because they don't want to increase their liability exposure.   

RE: Glu Lam beam modification/suggestion and design calcs

(OP)
i appreciate your comments.  The beam were collected from a salvage.  They in the the original wrap and were give to me.  We are building with ICF and the only reason we used two was because 1) they were free 2) we didn't have any other use for them and 3) the combined 10" thickness worked well with 13" thick concrete walls.  The garage has a perimeter wall composed of ICF (Nudura 8" reinforced concrete, 5000 psi. #5 60 ksi rebar 8" oc vertical and 12" oc horizontal).  The header beam spans 16 feet with 15" bearing on each end.  The beam only supports the roof trusses.  The garage is 24 feet deep and the roof is a 4/12 pitch with composite shingles.  I just picked up the roof truss design and I need to look at the  loads, but I think they are pretty low.  Does this offer any perspective on why I am even thinking of cutting them?

I really appreciate all of the feedback you guys provide.  This has been a great learning experience!!!!!!

RE: Glu Lam beam modification/suggestion and design calcs

(OP)
I just ran through the numbers.....LL+DL is 80 psf.  So the linear load is 960 plf.

RE: Glu Lam beam modification/suggestion and design calcs

If you live in parker, I dont think your TL is 80psf, it will probably be 45 or 50 psf. Since you modified the GL I just annalyze it if the beam were heavy timber beam DF #1 5.5x13.5 and I am sorry to tell ya it is not strong enough with 50 psf (I already reduced your load to 600 plf).

RE: Glu Lam beam modification/suggestion and design calcs

(OP)
I have two GL beams side by side (5" by 19") total beam width is 10 inches.

From truss calculations provided by the structural engineer, the Top Cord Live Load is 30 psf, TC snow load is 30 psf and TC dead load is 10 psf.  I added the bottom cord load of 10 psf as well for a total of 80 psf.

I quickly just used the Anthony Glulam table for a 4F V3 1.8E Architectural Stock Depth with a LDF of 1.0 and have a n allowable load of 1932 plf.  I assume for two beams we are looking at an allowable load of 3864 plf.  My calculated load (span 16' x 1/2 garage depth 12') of 960 is about 25%.  The 24 feet is and external dimension and the span is closer to 22.5 or 900 plf.

What do you think now?

RE: Glu Lam beam modification/suggestion and design calcs

Well, you told me it is going to be cut 6 inches.  How could you use 2 5x GL header?  That means your beam size is 10" and your wall  is probably only 2x4 or 2x6 stud.

RE: Glu Lam beam modification/suggestion and design calcs

I use 13.5" deep because u were cutting it 6"

RE: Glu Lam beam modification/suggestion and design calcs

(OP)
My walls are constructed using ICF.  The walls are 8" think reinforced concrete with a top plate and a and a 3" by 19" GL beam (column), bolted to the concrete. The wall is Nudura 8" reinforced concrete, 5000 psi. #5 60 ksi rebar 8" oc vertical and 12" oc horizontal.

This not your regular house....The ICF wall in the main house include the basement and extend two stories above grade.

RE: Glu Lam beam modification/suggestion and design calcs

As a rough guide, you can determine the original stress level at each lamination interface and then use these values as the maximum new values.  Greater rational than just using 60% or whatever.

Dik

RE: Glu Lam beam modification/suggestion and design calcs

Since I don't have the plans and I can't verify any of what you are saying, and since I don't want to be liable for anything, I won't comment on whether I think it will work.  I will say that M=w*l^2/8 and Fb=M/S where S=b*d^2/6 (you should verify this with your strenghts of materials book).  Convert feet to inches.

If you get a stress (Fb) that is not well below 1000psi, Do not cut the beams.  What is well below?  I will leave that to you.  Deflection should be checked too.

If you do cut the beams, take care not to cut any notches in what is left.

If there is anyway you can the take load off of the beams, like adding another beam, even if it means cutting the trusses and doing truss repairs, that may be a better option.  Then the stresses in the cut beam won't really be an issue.

RE: Glu Lam beam modification/suggestion and design calcs

Boise Cascade just replied to my e-mail.  He said if you reduce the size it is conservative to analyze it as if it were D-F #2.  So analyzing it as if it were (2)6x14 is good enough.

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