Siccors Truss Connection
Siccors Truss Connection
(OP)
With A Siccors truss being design with one end as pinned and the other as a roller to eliminate the horizontal force onto the wall, how is the connection made to the wall as not to put the horizonal force into the wall. I understand that a slip joint can be made from the truss to the wall on one end of the truss. Doesn't this allow movment at the top of the wall due to wind forces? What type connection should be made. Are there any example or illustration on the internet.
Thanks
Jason
Thanks
Jason






RE: Siccors Truss Connection
It is also common to enter some minimum lateral force even at the gliding support, especially if friction, since some always exist.
RE: Siccors Truss Connection
With the actual fixed connection (and idealized pinned connection), you can calculate what the lateral deflection is at the top of the wall. You need to make sure that the wall can take this distortion without undue distress in finish surfaces, etc.
I've never seen teflon, rollers, etc. used on this kind of construction. If you have a more significant structure, such as a concrete or steel building, then, if you do use some kind of detailed slip connection, the wall itself must be laterally stable on its own. It (the wall) cannot be designed as though the truss/roof is bracing it, but rather, designed as a cantilevered wall or a wall with some other type of independant bracing.
RE: Siccors Truss Connection
RE: Siccors Truss Connection
Of course there is an interesting exception in that trusses over or near end walls or cross walls are likely to be totally restrained and should not be modelled as having one sliding support. This is not likely to be a problem for the wall but you need to check that the truss can take the different set of forces. You also need to check that the difference in deflection of adjacent trusses with different support conditions is acceptable.
Carl Bauer
www.bauerconsultbotswana.com
RE: Siccors Truss Connection
The truss is designed as an individual structural element for the loads specified by 'others', with the requirement for review of the drawings and field execution by 'others'. The pinned end, roller end is 'economically' a convenient model for them, with the only TPIC requirement being to limit the roller end horizontal deflection to 1". Pinned, pinned would cause their software to abort the job, making dimensional lumber and their plates unmarketable for this type of application.
What's the jobsite reality of constructing a pinned, roller connection? None that we have found so far. We did find one cold formed steel connector that was designed to allow horizontal movement but issues of corrosion between the sliding plates, friction, and uplift seemed to be lost on the manufacturer. Issues of serviceability limit states, ie deflections and finishes, seem to be lost on the truss plate industry. Using pinned, pinned connections of course negates any liability of the truss plate supplier and their manufacturer as you have completely changed the loading on their truss design. It is truly a convienent model on their part.
The Truss Plate industry really needs to develope 3 dimensional models that account for roof diaphragm action, load sharing systems, bearing connections and wall (both exterior and interior) stiffness and interaction, etc. I suspect that the BELIEF of many is in the rundundancy of structural wood members at two foot or less centers (read Ultimate Limit States) collapse won't occur, and that between normal shrinkage of wood members and cracking of finishes (read Serviceability Limit States), nothing a little 'mud' and paint won't fix, and the Truss Plate Industries knowledge that most of their designs are never reviewed by anybody remotely qualified as required by their shop drawings, that this is all 'someone elses' problem (read Engineer of Record).
It's a little early in the morning here, and re-reading this 'rant' one can tell i've had a few to many coffees. I would be interested in hearing from others with similiar experiences and 'solutions' they have found to the scissor truss pinned/roller connection.
RE: Siccors Truss Connection
Steve111 - while I agree with you about modeling trusses to match reality, I don't think you can "turn" this industry any time soon. Most of those "designing" trusses are not engineers and those that actually do the sealing of truss designs cover a large sales area. It would just be way too complex for this industry to respond to a 3D system of designing.
ishvaaag - no need to apologize for your English....I work with only one language and I always respect those who can learn more than one, like yourself.
RE: Siccors Truss Connection
Your opinion is all deflected about truss + pin + design.
Scissor (and all other configurations for that matter) are typically designed Pinned-HzRoller because that yields a more conservative design. If you were to take a 38' span scissor and analyze it Pinned-Pinned at the reaction points you would notice a significant decrease in material requirements: Say 2x6 chords --> 2x4 chords, and significant metal gusset plate (steel) savings. Truss fabricators would love to build your EOR projects that specifiy Pin-Pin and let you handle the horizontal thrust load at the walls.
Fact from friction be that a quality truss fabricator will run the design internally as multiple load cases and supply a component that meets both reaction conditions. An EOR such as yourself might even REVIEW the truss calculation and potentially request such a design criteria if absent.
I would respond to the initial post with three categorical cases;
1. Design snow load = 385 psf (such as Tahoe)
2. Design Live load = 16 psf to 25 psf (90% of US)
3. Conventional framing - Interior support - Steel truss
1. High snow load area engineers expect trusses to 'breathe' with seasonal changes. Usually spans are shorter and interior bearings more frequent with OC spacings of 16" and 12" and 2x8 chords common. The increased truss mass still responds to the design limits a truss engineer may specify (say 1" Hz max) as in case 2, but the expected movements do indeed happen with each winter storm. Special attention is needed for this expected movement.
2. In lighter live load areas this problem occurs much less frequently. Most vertical and Hz deflections occur during the first 7 days of construction due to dead weight materials being applied. Truss design engineers typically flag designs with special Hz movement notes in an effort to get the EOR to think. I don't know of a TPI supplier that would not supply an EOR additional load/reaction cases gratis.
3. Limit the potential for deflection problems by practicing the following. If possible
-Make the truss-plate attachment with dead load in place.
-Use a horizontal-slotted framing clip at the attachment.
-Make sure slot-nail is placed to allow anticipated movement
-Specify flexible blocking between trusses.
-Install ceiling rock held back 1/2" and wall rock flexible
-Design double top plates for horizontal thrust.
-Design home with limited truss run between collar-tie walls
-Add a beam at scissor apex and specify two mono-scissors.
-Expect walls to plumb-out at each truss heel
-Specify strongbacks at start and end of truss run
-Install a metered heat-strip on the bottom chord so as to reduce moisture content and increase shrinkage in proportion to deflecton during heavy snows (pat-pending...
RE: Siccors Truss Connection
Carl Bauer
www.bauerconsultbotswana.com
RE: Siccors Truss Connection
On the issue of sliding details, we have to be careful not to let our models steal our common sense away. We do model one end of a truss as a sliding restraint normally because the wall cannot provide the required lateral restraint for it to be a pin. I have seen some very unconvincing attempts to produce a sliding restraint dreamed up by an engineer who is thinking of text book diagrams and not reality.
Carl Bauer
www.bauerconsultbotswana.com
RE: Siccors Truss Connection
well yes we do review the shop drawings as submitted. with regards to your example one to the initial query in this thread ...i believe it is this ' special attention is needed for this expected movement' that is of interest to myself anyways.
deflection in light wood framed structures is purely a serviceability live load (snow and/or wind) driven calculation. i will dig through my project notes we were involved in several agricultural facilities with 60' and 70' clear span scissor trusses @ 4' o.c. DL 12psf., LL(snow + rain) 46psf. the buildings were post/beam framing with wood posts @ 16' centers and open side walls. will look for the truss manufacturers drawings but a believe the hz reactions per truss were in the neighbourhood of 2500 lbs (specified loads, unfactored reaction).
Not sure what your metered heat strip is all about but we had a winter ice storm here several years ago in which the electricity was off in a significant geographic area for between 1 and 3 weeks. significant number of structural collapses, significant amount of damage deflection related, and piles of people unloading their roofs with shovels (fairly low tech but very effective). calculated roof loads about 80 to 90% of design load. Oddly enough as an aside to this all the roofs (cold formed steel sheet metal roof cladding)had been designed as "slippery roofs" with a slope factor of less than 1. we had a combination of rain and snow over three days and the whole load (snow, rain and ice) was frozen (stuck) to the heads of the steel screws holding the roof cladding to the purlins. not very "slippery" as it turned out.
carlbauer: yes agreed, a little harsh for sure.
RE: Siccors Truss Connection
http://www.alpeng.com/Publications/GoodCon/Summer_1997/gcsu9704/gcsu9704.htm
RE: Siccors Truss Connection
Building practice is almost pinned and almost pinned.
Pinned and roller as a model is conservative relative to building practices.
Pinned and pinned as a model is dangerous relative to building practices - it is impossible to build pinned and pinned with wood construction.
RE: Siccors Truss Connection
conservative to (i.e.) design truss for both 'pin/pin-slide' and 'pin/pin' conditions. 'slide' governs top & bottom chord design.
check location of walls intersecting => rigid spots
dont forget to get the architect to detail for stress and lat'l movement if there are beams subjected to weak axis bending with glass or similar walls below. diaphragm action helps. quantify it w/ defl compatibility/stiffness....
you need to look at all of your conditions & design/detail accordingly..