Ridge Beam design check

[sdavis123]

I am building a new structure. This is a Pole Barn style frame, but it will contain living quarters inside. It is a 30' x 64' building with 16' high walls. 4.5:12 pitch roof.
On one end, I desire a 20' long open room that is the entire height of the building. I also want vaulted ceilings in this room.
So.... I have elected a Ridge Beam / Rafter construction. My rafters are 2x10 #2 doug fir at 16" o.c. They will be supported by the exterior walls and the Ridge Beam. The Ridge Beam is supported by 8"x8" vertical posts. I desire the posts 20' apart in this portion of the building. So, the Ridge Beam would span 20'.

I hired an engineer, he has told me that the beam would have to be a 5.125" x 14" glulam. This seems to be quite large (and expensive).

Can someone give me an idea if this is a reasonable design?
I asked the engineer if it would be at all feasible to use the old-school method of 2x material and plywood glued and screwed to make a beam capable of supporting the load. Answer was no....

I guess what I need is the least expensive method to accomplish the vaulted 20 foot room. I am willing to add angled supports from the vertical posts to the ridge beam. I am actually willing to do anything to keep the vaulted ceiling. It just seems to me that there would be a more cost effective way than this massive engineered beam.
In fact, I have to wonder, what did they use before glulams?

The local code is 20PSF snow, 90mph wind.

 

[Ron]

The Glulam beam has a higher allowable stress than a dressed 2x member, so they are more efficient for this application. His sizing looks about right.

Other than price... the beam will be hidden anyway, plus you'll need collar ties for the rafters...what's the problem with the beam?

 

[Ron]

ps...you hired an engineer, now listen to him. His solution looks reasonable.

 

[MiketheEngineer]

Sounds about right to me also.

You might consider sticking with the post frame method but use scissor trusses in this area with sky lights. Using scissor trusses on 16' tall walls brings their own set of problems.

 

[sdavis123]

Well, you guys have given me exactly what I needed.... assurance that the engineer is not over-designing the roof support.

There is nothing wrong with the beam, other than cost. And cost IS my driving factor to look at other options.

I am also satisfied with the engineer that I hired, other than he just hasn't offered any other solutions.

I have attached an idea to reduce the span and thereby reduce the size of the beam... I think. Does the span reduce like I show in this sketch?

 

[FSS]

You need to also watch out for continuous spans with glulams. They typically come with camber and don't get along with multiple spans.

You would have to cut that span down about 8' or so to get 3-2x12's to work if I understand the situation.

You can also price lvl's or parallams.

 

[Ron]

One thing to consider in your cost is that for a 20' span of 2x lumber, you'll have to pay an arm and a leg, if you can even get them. That's why Glulam (LVL's and Parallams, also as FSS noted)is usually a better option. By the time you figure the cost of the lumber and bolting the 2x's together, you might not save any money.

As for your sketch, FSS is right...not worth it.

 

[sdavis123]

If I'm understanding the terminology correctly:
Continuous span would be a case where a single beam is supported at both ends and in one or more locations between the ends. right?

So, are you saying a glue lam is better used with ONLY supports at both ends, rather than multiple supports?

 

[sdavis123]

Ron,
I know in the old days, builders used to make laminated beams using pieces of 2x that were shorter than the span. The joints of each piece had to be staggered and plywood sheeting ripped to the beam width was used in between the 2x's. This would all be glued together and bolted as needed.
I guess that's what I was talking about when I said making my own beam out of 2x's..... It's probably the predecessor to the glulam.

 

[Ron]

sdavis123...yes, that was done routinely; however, when you do the math, the vertical plywood sections provide very little to the capacity of the beam. That's why the current practice of laminated horizontal sections provides much more bending resistance and efficiency.

This is one of those situations where the math doesn't necessarily coincide with the "gut feel". One would think that plywood on its side would provide a great deal of bending resistance. Not necessarily so....the moment of inertia calcs do work...in this case, they work a bit opposite to the "gut feel"....but they do work!!

 

[hokie66]

sdavis123,

The knee bracing shown in your sketch would just complicate things further. With that arrangement, the verticals on each end would have to take a lot of bending, and the connections would be complex. There is a lot more to building than just the material cost, so I think you should stick with the advice of your engineer.

 

[sdavis123]

I appreciate everyone's feedback.
I will be biting the bullet and purchasing the glulam recommended. You have all given me reassurance that the engineer I hired in fact knows what is best.
Thank you everyone.

 

[sdavis123]

Well guys,
I have an update....
I am quite pleased because I found a glulam that was purchased for a home, then the design changed and they had to purchase another beam. The beam has never been installed and is in great shape. I spoke with the engineer, he approved it.

This is a pressure treated 5-1/5" x 14" x 25' long beam and I got it for $150.

I'm ecstatic!! It looks pretty funny sitting on my 16' trailer though!
I guess it pays to look around (which I have been doing for 2 months).
And all of your help here was great.

Thank you!