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

 

[civilperson]

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.

 

[captainplasma]

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

 

[COEngineeer]

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.

 

[dik]

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

 

[captainplasma]

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.

 

[captainplasma]

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?

 

[COEngineeer]

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.

 

[captainplasma]

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

 

[COEngineeer]

I know someone . See my name?

 

[captainplasma]

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

 

[structuralaggie]

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.

 

[COEngineeer]

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?

 

[captainplasma]

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.

 

[structuralaggie]

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.

 

[captainplasma]

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

 

[COEngineeer]

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.

 

[structuralaggie]

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?

 

[captainplasma]

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

 

[structuralaggie]

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