Scissor truss thrust on to supporting beams

[Cruezr]

I have a project calling for scissor trusses to be supported directly on beams spanning between columns (part of an outdoor entertaining area).
The previous threads all talk about supporting these types of trusses on a wall (the material of the wall being somewhat irrelevant).
The trusses are usually design as pin-roller supports to get the most conservative truss design.
But the actual connection, if using proprietary framing anchors or hold-down straps) is more akin to a pin-pin situation.
Most previous advice simply suggests let the top of the wall move under the lateral force.
So what happens when the truss support is a beam spanning between columns ?

Do you:
- design a beam that resists the horizontal thrust based on a pin-pin support condition at end of the truss, the reactions at each of the beam are then transferred to the columns which can be designed for the total horizontal thrust (most likely being a fixed-based column)? The maximum lateral deflection of the beam then being limited to the horizontal spread of the trusses?

And advice or comments would be appreciated.

 

[CELinOttawa]

I think you're spot on. Unless you can get the client to agree to a tension tie, you have to design for the thrusting forces to get to the top of the columns and then design the column to handle the thrust.

Be careful here: The trust of a large scissor truss can cause visible deflections on walls that are not particularly stiff. In your case that would mean that the beams and columns might be visibly deformed by the trust. Consider designing this to a strict deflection criteria and check the forces as a final review. Most likely what will work for deflection and appearance is simply going to fly by stress, the other way around and you could have problems.

 

[Cruezr]

Tension tie definitely not an option.
I did intend to design the supporting beams for a stringent lateral deflection control...beam span is about 4m (13') so thinking a lateral deflection limit no more than 10-15mm in order to reduce the chances of cracking in the plaster linings - possibly using steel C-channel or large timber beams. The scissor truss span is about 7.5m (24.8') at 900mm (35") centres.

 

[KootK]

- on walls, the pin/roller decision has less to do with the anchorage to the wall and more to do with our inability to keep the walls from displacing laterally except, perhaps, at corners.

- Assuming pin/pin may be conservative for the truss in some respects but not necessarily all. You would expect axial compression in your bottom chord and an atypical demand for bracing there that field personnel may not pick up on.

- I would allow your beam and column system to spread just as we do with the wall case. Achieving plausible thrust restraint in a beam and cantilevered column system will be difficult. The stiffness just isn't there.

- If the lateral system for the structure is cantilevered columns, I'd recommend treating the truss spread as an imposed displacement on those columns.

For what it's worth, I was a truss designer myself for a few years back in the 90's. What are your proposed top and bottom chord pitches?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[XR250]

Spend some extra doh on the trusses to make them stiffer (larger chords etc)

 

[EngineeringEric]

Specify maximum horizontal deflection, after talking to a truss manufacture to see what is realistic. And note that the DL deflection likely will be 80% complete before it is ever restrained... so you have that going for you

 

[Cruezr]

KootK - 35 deg, 25 deg are the top/bottom chord pitches

For interest, the calculated maximum horizontal deflection is only 6.2mm (1/4").
Top chord has battens for roofing at 900 spacing, bottom chord has battens for plaster ceiling at 450 spacing...so lateral restraint of the compression chords isn't a problem for any load case.

It just seems to go against my intuition to "let the walls / beam bow out" under the horizontal thrust generated by pinning both ends of the truss.
But for reference, what is the recommended range of maximum lateral deflection at the top of the wall (both absolute and span/X) for different wall types (a timber stud wall with flexible cladding can displace more than a brick/CMU wall or a glass infill steel-framed wall without causing a problem). It isn't very common to provide an expansion joint in the plaster to accommodate the wall spread either, so I would think that about 15mm would be absolute maximum horizontal displacement before the plaster starts cracking.

 

[KootK]

6.2 mm is nothing if that's accurate. With a bottom chord that steep, I would have expected more. Short span I guess. I try to keep it under 25-38 mm acknowledging that the movement will be split between the two supports.

I don't understand the concern about the plaster joints. Your bottom chord elongations ought not be much greater than a flat bottom truss. Maybe less. The spreading phenomenon is really more about the curvatures developed over the truss as hole. And you'll have a natural gypsum joint at the apex of the vault where you probably need it most anyhow.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Cruezr]

Again for interest, the attached technical note is worth a read.
I don't believe it is proprietary information, just some old guidelines from a truss manufacturing company in Australia.

 

[KootK]

Yeah, that's basically XR's recommendation which I wholeheartedly support.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Cruezr]

Ok, so I understand the wall lateral deflection issue...let it bow but keep it within some prescribed limit (0.5-0.75") depending on the wall type.

If the building were a column/beam frame with infill wall panels of anything (straw, glass, timber, air), I would have thought the lateral stiffness of the supporting beams & columns would have an effect on the behavior of the truss...with each truss horizontal force acting at a discrete point along the beams, all horizontal loads then being transferred back to the supporting columns and thence down to the footings/foundation.

 

[KootK]

No argument here. Imposed displacement all around.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Cruezr]

I just can't get my head around that concept.
It seems to imply that a supporting beam of near infinite stiffness (say a very large I-beam) would provide no more resistance to lateral movement than a small timber beam, spanning the a very long or very short distance.

 

[KootK]

You'll have to tell me a good deal more about your system but I'm pretty sceptical of "infinite stiffness" in this scenario. If base fixed columns are your final destination for the thrust, you'd probably see 3mm of displacement at the top of each column before you even draw the slack out of your base connections.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dhengr]

Cruezr:
When clients/Architects/designers want sexy, stupid, design details and conditions, it is incumbent on us, the Engineer, to lay out/explain the difficulties and potential problems and the pros and cons of the situation to them. Then let them live with what they decide to do. Almost anything is possible today with enough money and complex detailing, but we as Engineers must understand what’s going on and likely to happen, and design for these conditions, in the best possible way. Don’t forget the other potential lateral loads and uplift, etc. The MiTek paper pretty well sums things up, if you read btwn. the lines. Scissors trusses are sexy, but difficult to deal with because of the large lateral trust reactions they induce, if restrained; or alternatively the need to allow a reaction point to move laterally to eliminate the thrust.

You can’t get away from it, it is the nature of the beast. You can design a truss much stiffer than loading alone would call for, just to reduce the lateral movement/deflection at the bearing points, but you can’t eliminate it. The easiest solution to the problem would be to provide lateral ties at/btwn. each column pair. Then design the beams to transmit the truss thrusts over their 13' spans, to the columns. Then use these ties as supports for part of the lighting system in the bldg. These ties, when handled properly, are surprisingly unintrusive in terms of still showing the vaulted ceiling space, but they solve a real messy and expensive structural problem. Detail the ceiling sht. rock or plaster with metal expansion/movement joints at the ceiling ridge and at the transition btwn. the ceiling and vert. wall/beam plane.

 

[EngineeringEric]

I like the concept of "All things bend". And in scissor trusses, when they bend they thrust outward. if we have an infinitely rigid (almost case, say a concrete slab on grade we put the trusses on the SOG then the truss will 'expand' side to side. if we bolt it down before letting it load it self (self weight) then we imposed the horizontal restraint. now the same concept applies to this now rigid truss when we add Roof lice load, the truss would have expanded more so therefore we added additional imposed horizontal restraint/deflection.

Now in the non-SOG condition, everything bends, including steel beams supported by columns. we can either accommodate the lateral deflection or resist it or combo. Both cases require attention and detailing and making sure all of the elements can handle the loads (direct load or indirect due to restraint of movement).

Now this is a cartoon of reality but it may help or confuse... sorry if I did the latter!