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?

 

[ron9876]

You would normally assume a pin at the base of the wall when designing the walls. The walls will tilt to relieve the lateral loads and the diaphragm will transfer wind loading.

A ridge member would have the advantage of not shipping such long trusses if you have somewhere to hide columns.

Depending on the area that you are spanning a joist girder at the arch with sloped steel joists is another option.

 

[msquared48]

Another option here is to lower the bottom chord of the truss a foot or two or three or four until the lateral deflection is better controlled.

Mike McCann
MMC Engineering

 

[ash060]

I all ready tried that, but the architect balked. He wants to keep the steep bottom chord slope for the look.

So are you saying that I don't design the walls for any horizontal thrust from the gravity loads? Just assume the whole masonry wall rotates like a rigid body, so no forces are induced.

 

[ron9876]

It will either rotate or it will resist the load. It will reach an equilibrium which I would guess is closer to rotating. I would add a little more reinforcement in the walls.

Can't use steel joists and girder? Maybe with bottom chord bearing.

 

[ash060]

I don't think I could use steel joists, the chord slopes are 7/12 on the top and 5/12 on the bottom. There is not alot of room for the web members

i thought of using joist girder with a wood truss infill, but I would need panel points at 2' c/c. The roof is plywood and standing seam, so I can't really space the joists too far apart.

2/3 of the building is wood trusses with flat bottom chord, I didn't really want to switch roof systems for a 1/3 of the building.

The reason I am worried about the scissor truss is that the layout pushes metal trusses to the limit and if there is just a little more load on them the metal trusses won't work.

 

[ron9876]

Already switching with the lt gage trusses. You can order steel joists with various configurations. Place at 5' c/c with steel deck with plywood over the steel deck.

That is a long lt. gage truss that will likely require field splices.

 

[paddingtongreen]

Surely, the wall must be designed as a cantilever from the ground. Besides the horizontal kickers from the vertical load, there is wind load. Pins at the base of the wall would cause collapse.

The trusses could be detailed 0.58" short, with slotted holes because not all of the dead load is there immediately. Clamp down on one wall and allow the base to slide to slide on the other for the live load case. I don't know the relative loads and friction resistance though. then the clamped side would take the wind reactions.

The Architect could use details that hid the end of the truss.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[BAretired]

How about 8/12 and 4/12 for the top and bottom chords? That still provides a fairly steep ceiling slope and provides a much stiffer truss.

BA

 

[Stillerz]

Rule of thumb says the interior slope should be no more than half of the exterior slope.
Helps mitigate your very problem.
7/12 outside should have no more than 3.5/12 inside.
This is why architects are so irritating.

 

[Stillerz]

i sketched your 70' span truss with 7/12 outside & 5/12 inside.
It flat-out looks assinine. I don't think I owuld even attempt it.
Sorry to say it that way, but there is a reason you dont see trusses like this.

 

[BAretired]

I sketched it too and came to the same conclusion as Stillerz. The top one attached is as the architect wants it. The bottom one is my suggestion.

BA

 

[msquared48]

This is a perfect example of why an Architct should not dictate structure.

Mike McCann
MMC Engineering

 

[paddingtongreen]

Mike, I have, on occasion, asked the Architects to provide a solution since they provided the problem. When they see those truss members beginning to look like W14s, they will back off. Make them as intrusive as necessary.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[ash060]

I tried to get him to change his mind, but he thinks that the ceiling is to flat if you use 3.5 or 4/12.

i am going to do the mono scissor and a ridge truss down the middle. I have all ready spoken to a truss engineer and he says that he can get a 7/12 5/12 mono scissor to work in wood

 

[Stillerz]

I have to chime in here one more time.
If the architect wants a steeper ceiling inside, make the truss steeper outside. There really is no way around that.
I like the idea of a girder ridge truss + mono trusses.
If the architect is still wanting to use a scissors truss with the 7/12 & 5/12 configuration, well, that is plain stupid. It really is no diffent than spanning a W8 beam 40 feet. Trusses, too, have depth:span ratios.
Architect: "Here, make my pipe dream happen"
Engineer: "It won't work"
Architect: "Waa, waa, yes it will. See how pretty it looks. You have to make it work".

 

[Stillerz]

The 7/12 - 5/12 truss looks like it is in danger of a snap-through buckling collapse.

 

[dhengr]

ash060:
Almost anything is possible if the arch. and congregation have enough money. A number of the concerns expressed above are legit. And, the boss (your boss?) should go on record with the arch. that there are potential problems and potential consequences with what the arch. wants to do, and there are some finishing details reqr’d. to hide the cracks which are certain to form. This could be a certified letter to the arch. to keep your whatcha-call-it out of the wringer a few years down the line. Tell the arch. that defying the laws of physics involves and extra engineering charge at your office.
As I understand midwest truss uplift, it happens to the greatest extent in the middle of the of the bldg. with a continuous bot. chord trusses, and that won’t be your problem, here. Since crown molding conjures night mares in carpenters, let’s say that is a bad choice of words: but you will want some form of trim, maybe only a 1x4 attached to the correct surface, to hide some movement cracking in the ceiling sht. rock. The two 5/8" horiz. deflections the truss manuf’r. gave you are probably reasonable numbers; have him send you a set of his computer printouts and I think you will find they assume one end pinned vert. & horiz. and the other end free to slip in the horiz. direction. It’s mighty tough to make this movement zero, but that thrust was what flying buttresses were intended to accommodate. The problem is that at that time the arch/engr/
GC were one in the same guy, called a master builder. And, he tended not to let his arch. hand, tell his engr. hand to try to do the impossible (thus flying buttresses came into being) because he knew another appendage near his middle would end up in the wringer. Maybe another bad choice of words, since wringers weren’t invented yet, lets go with ‘an appendage between two massive masonry bearing wall units.’
The ridge truss is a fine idea assuming you can find a bearing point for it at each end. Have the truss manuf’r. design the ridge truss with a ledger attached to each side for the mono trusses, a horiz. brg. Then, the mono trusses and side walls can be handled the same way and total horiz. movement at the mono truss brg. on the blk. wall is a function of the ridge truss deflection +/-, not really the mono truss deflection any more, but this is partly where the +/- comes in. Bear the mono truss on the ledger, tie it parallel to its length to the ridge truss/ ledger and leave a slight gap at the top btwn. the mono truss end and the ridge truss side. Most of the movement will now happen at the top of the mono truss. Assuming the ridge truss is deeper than the mono trusses, I would put a sht. rock soffit around the bot. of the ridge truss and a piece of trim on the side of the soffit to hide the crack (maybe caulked, not taped and mudded) where the ceiling sht. rock meets the side of the soffit.
The mono truss brg. at the conc. blk. wall must be held down with clip angles on each side, but slotted to allow the horiz. movement. This is a one piece brg. fab’ed. for each truss, attached to a top bond bm. on the wall, and it has a brg. seat with low coef. of friction w.r.t. truss brg. surface on the mono truss. This area must be detailed to allow the horiz. slip, but also be weather tight in and out. Again, the trim could be something as simple as a 1x4 on the wall, with the ceiling sht. rock passing over it, or it could be a continuous lighting trough.
An 8" reinf. CMU wall may not be thick enough, and while it may kinda act like it’s pinned t&b, you will be caulking a crack in a horiz. blk. joint a few course above the slab in short order. There will tend to be movement at the top and you ignore this at your own risk. This is a reinf. CMU cantilever from the footing or possibly tied into a strip of floor slab. The lateral load at the top, in either direction, is (coef. of friction)(total load/ft.) and the helping gravity load is only dead load. Maybe not so bad, but maybe not with 8" blk.
Finally, wind loads are taken to the end walls, ridge truss brg. walls, through the roof diaphragm. Now, the question becomes, do you hold the last mono truss 24" away from the end walls, span the diaphragm material to the end wall which probably doesn’t deflect like a mono truss, and finally make your shear flow connection to the end wall. This shear flow connection will also tend to inhibit the mono truss brg. movement at the conc. blk. wall, less so as you move away from the end walls to the center of the bldg. This is another tough detail, the ceiling sht. rock spans and flexes to a 2x4 ledger on the end wall and the 1x4 trim goes from the ridge truss trim to the side wall/clg. trim. This piece of trim actually holds the sht. rock to the ledger as the sht. rock flexes downward with the mono trusses. And, if the arch. doesn’t like that tell him to take his Michelangelo butt up there and shade the trim so it blends in.

 

[paddingtongreen]

I can't see relying on the roof membrane with that steep a slope. The horizontal component of the load is more perpendicular to the membrane than it is in it's plane.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Stillerz]

dhengr:
I agree with a lot of what you have to say.
But, I have to say, that is by far the longest reply I have ever seen on this forum.
Wow!

 

[BAretired]

I am not sure there is any "right" answer to this problem. Should the roof trusses be tied to the top of wall or allowed to slide? Hard to say, but I would bet my money on tying them together and reinforcing the wall accordingly.

If the architect is unwilling to accept a more reasonable geometry for the structure, I would be inclined to suggest that he find another engineer.

BA