Horizontal Reaction from Scissor Truss 1

[ash060]

I am currently working on a project that requires steep scissor trusses to span over a open room (about 70 ft)

The horizontal reactions i am guessing at are about 1000 lb/ft along the top of the wall. I have a diaphragm, but I am not sure how much it helps to resist the horizontal thrust from the trusses.

Does anyone have any suggestion about what to do here?

 

[jike]

Do not try to resist the thrust. Let the walls tilt outward (slightly) but control the movement by specifying minimum structural properties for the truss.

 

[Stillerz]

What type of structure?
Wood frame?
House?

 

[ash060]

Church 70 ft span. I am worried about letting the whole truss slide becuase it may more a little too much.

I spoke with a truss engineer and it barely works for the span, so if I put anymore restrictions on movement I may not be able to get a truss that works.

 

[jike]

Instead of worrying, let's look at what you are dealing with.

How much movement is the truss guy calculating? How tall are the walls? What are the walls constructed of?

 

[ash060]

The walls are masonry and 10 ft tall.

The movement calculated by the engineer was 0.58" due to dead loads and 0.58" due to live loads.

 

[Stillerz]

Problem I see with letting it move too much is it may cause damage to your interior finishes.
I have seen this happen with small scissors trusses in stick framed homes. In the winter, cracks developed in the drywall where the wall meets the cathedral ceiling. In the summer, the cracks close back up, though not completely. The cracks I am talking about were not insignificant...i'd say at a minimum they were 1/4". Upon investigating, I found that the trusses were allowed to slide at one end...almost unheard of for a small wood framed house to my knowledge.

 

[jike]

Is this masonry veneer with wood backup or brick and block or just block?

 

[Stillerz]

I would think there will be significant movement dude to thermal effects as well. I have seen this with my own eyes.

 

[ash060]

Reinforced 8" masonry concrete block

 

[Stillerz]

use flying butresses....sorry, it seemed like a funny thing to say.

 

[Stillerz]

Are these typical trusses you'd see in light wood frames at 16" or 24" on center?
What is your diaphragm?

 

[ash060]

PLywood diaphragm, light gauge steel truss 2 ft on-center

 

[Stillerz]

So the diaphragm is just the roof sheathing?

 

[MiketheEngineer]

This has been discussed extensively in previous posts. You might want to search through them..

 

[jike]

Stillerz: The cracks at the joint between wall and ceiling in the house you describe are most likely due to truss uplift due to changes in temperature and humidity. This is a well understood phenonmeon in the wood truss industry.

http://www.askthebuilder.com/169_Truss_Uplift_and_Ceiling_Cracks.shtml

Personally, I think the best solution is not the ceiling clips but adding crown moding to hide the cracks.

 

[Stillerz]

I have built many a house over the years....i'd say at least 75. I am well aware of the truss uplift...espcailly in the midwest U.S. where we can have pretty drastic temp changes in just a day's time. This can be a really bad problem when a ceiling drywall joint lines up very close to the splice in the bottom chord of the truss. Cracks almost every time. Opens and closes with seasonal changes.
The cracks I was investing were substancial and I only ever saw them so great on the scissors truss that was literally seated in a slider bracket on one end.

As for the Crown moulding...as a former carpenter, I can tell you, putting crown moulding on a cathedral ceiling is quite a trick.

 

[ron9876]

In the end the walls will tilt unless you provide a very substantial wall/foundation system. Probably won't be a problem except where the bearing wall has perpendicular walls which would provide a large resistance. You could disconnect the perpendicular walls from the main walls and use a small footing that will make it easier for the wall to tilt. Architect would need to deal with the joint at perpendicular walls.

Is the movement that you are talking about at both ends or is that total movement? Also the weight of the trusses and probably most of the roofing will be in place before the interior finishes are applied. Biggest problem would come with wind loads.

 

[ash060]

If the wall tilts won't that put a moment in the wall? I can't pin the wall at the top and bottom and than put a horizontal force at the top or the wall would fall over.

I am think of breaking up the trusses and using a ridge truss and two mono-scissors to make the roof. That way I won't have a thrust from gravity.

 

[BAretired]

You can use a ridge truss if you wish, but when it deflects under gravity load, the mono-scissors trusses will still push the walls outward by a variable amount, proportional to the deflection of the ridge truss.

BA