Glu Lam beam modification/suggestion and design calcs 3

[captainplasma]

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

 

[structuralaggie]

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.

 

[captainplasma]

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

 

[DonPhillips]

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

 

[captainplasma]

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......

 

[bkgd]

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,

 

[structuralaggie]

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?

 

[bkgd]

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

 

[bkgd]

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,

 

[captainplasma]

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?

 

[bkgd]

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,

 

[RARWOOD]

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.

 

[captainplasma]

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!!!!!!

 

[captainplasma]

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

 

[COEngineeer]

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).

 

[captainplasma]

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?

 

[COEngineeer]

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.

 

[COEngineeer]

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

 

[captainplasma]

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.

 

[dik]

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

 

[structuralaggie]

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.