Long Span Glulam Trusses

[CCox]

I am working in a church building with long span glulam trusses. The trusses span 70' and are 20' o.c. The profile of truss is shaped like the letter A. Pitch is 7:12. The dificult part about this problem is the thrust force. I have to model the truss as pinned-pinned in order to limit lateral deflections. The thrust force comes out to be 57k at each column. The best way I could figure of resolving this is to use large tube steel or wf columns cantilevered up from the footing. I think I could notch the glulam top chord member to make it a bucket connection. Part of it would sit on top of the column for the vertical component, part of it would bear against the inside face of the column to transfer the thrust force through direct bearing. Does anyone see any problems with this connection? I am wondering about stress concentrations in the glulam beam with such large forces.

The footing is going to be montrous to resist the overturning. the lateral thrust at the footing can be resolved with a grade beam or hairpins.

Thanks,

Cody

 

[Triangled]

How much does the "leg" of the A deflect before it reaches equilibrium?

 

[CCox]

Thanks for the response Triangled.

The max deflection is .5" when a GL 6.75x24, .9" when a GL 6.75x18. This member is continuous from column to apex, so the max deflection is about centered between the column and the collar tie. Max axial, shear, bending stresses are 600 psi, 115 psi, 1500 psi.

Cody

 

[Splitrings]

What happens if you replace one of the pins with a roller or lateral spring? How far does the leg deflect? A very slight lateral deflection may significantly reduce the lateral force.

 

[CCox]

Splitrings,
The lateral deflection is about 5.8" when one of the supports is turned into a roller. This also includes changing the leg to a 6.75x42. The problem is the relatively low roof pitch causing a large horizontal component.

You are right about the spring. There will be lateral deflection at the top of the cantilevered column, which would cause the support to behave like a spring. If the column deflection is around .75" at the top, the spring stiffness of the support would be 57k/.75" = 76k/in.

I think the best way to go is to increase the top chord member to a 6.75x42 or similar. This will limit deflections significantly. Then, check reaction at supports, design column for support reactions, determine deflection at the top of column, apply comp. spring at support in model and rerun to find new stresses and deflections for design of truss members.

In thinking about this problem, the top chord member is just going to have to increase in order to reduce deflections. This is one of the conditions with using wood since it is difficult to get fixity at column to beam connections.

Cody

 

[BAretired]

The geometry of your building may not be suitable to the selected framing system. Perhaps another system would be more suitable, such as a tied arch. Alternatively, perhaps the building geometry should be reviewed. A cross section might help.

BA

 

[dhengr]

Cody:
That isn’t a truss, that’s a couple of large rafters with a poorly located collar tie. To achieve some false/imaginary sex appeal, the Architect is trying to get you to do the ‘almost impossible’ by placing the collar ties up too high. Why not really make it look like a timber truss roof system, which it should be structurally, with a proper bottom tension chord, since you are also going to see 20' long purlins btwn. the trusses. The roof/ceiling system will still be plenty open and airy. Then your structural problems become manageable, and all the member sizes and connections would be more manageable. Make it a steel rigid frame and clad it in wooden plywood box beams. The (any significant) lateral deflection at the tops of the columns is going to give you fits, in terms of detailing the leaning/cracking side walls. Beyond the important structural purpose of the bottom chord on a truss, it is a wonderful place to hang lights, ceiling fans, church banners, etc. Make it a three hinged arch, all out of gluelam material; pinned at the ridge and the two bases.

The connection btwn. the stl. col. and the rafter gluelam end should be a horiz. seat cut for bearing (vert. reaction), and then a plumb end cut sufficient to transmit the 57k in compression (thrust) not parallel to the grain, maybe 6-8" high; all into a three sided shoe on top of the stl. col. cap pl. Then the top half of the rafter can cantilever on out for an eave detail or support. The way you propose cutting a birds mouth will split the timber parallel to the grain, starting at the inside reentrant corner of the birds mouth cut. Why not provide a simple sketch with some detail and dimensions on it, so we know what the column length is, how high the collar tie is, what the purlin and roof system is. What is the footing and tie rod detail and elevation? This is going to be difficult detail too. This would really help us understand the magnitude/proportions of your problem and likely get you better advice.

 

[Splitrings]

This is a church in a neighboring town. Sounds like what you are analyzing.

 

[ahmedhegazi]

Sketch would help , i have much doubts that this structural system is not suitable for your case

 

[CCox]

Thanks everyone for your responses. I will get sketches posted when I can to facilitate more discussion. I am working on other projects right now that have a higher priority. When those get wrapped up, I can jump back on this one.

I think a hinged tudor arch may be the way to go. Like in the picture Splitrings posted. I went to a church when I was younger that had these and I thought they were quite beautiful. Also, the idea from BAretired is an option. I was thinking something along that line might be aesthetically pleasing. Maybe a truss that looked something like the star trek symbol.

Thanks dhengr for the detailed comments. Giving me food for thought...

Been awhile since I logged in before this go around. Noticed no student posting allowed. Has there been a big problem with students trying to get freebie answers to homework problems? Just wondering if there are honest students who have used this website to actually understand eng. mechanics, design, etc.

Cody

 

[jayrod12]

In response to your no students allowed question. I used this website to research some stuff for university but never came on here and asked any questions.

I just found that sometimes reading the comments on here and some of the questions helped me understand the theory better which would help me with my school work.

That being said, at that time I was one of the few engineering students that had spent any time working at an engineering firm (I had 3 years as a tech before I went back to finish my degree) so I knew how to research stuff and understand how it applies across different problems.

Most of the students that were at school with me couldn't apply the theory in class to any problem they hadn't seen previously. When the professors would make exam questions that required someone to understand the theory and not just have done previous examples they failed, which caused uproars with the students. Many times they would argue with the teacher that it wasn't fair to present problems that we hadn't had an example that was exactly the same.

I think that students can peruse this website if they want, there is lots to learn! If they are able to post their "homework" problem so that the board helps answer it without anyone knowing it's "homework" then I would think that's fair. If they aren't able to word it correctly, then we generally call them out on it.