Truss support

[qase]

For a steel transfer truss with top and bottom chords connecting to columns, should the boundary condition be assumed fix-fix? Any way to take care of this big chord force? other than using a slide-bearing at the end to allow for horizontal movement?

 

[Ron]

Your connection will define the restraint condition. It can be either fixed, partial moment transfer, or pinned. Since you are connecting to the column at two locations, you will cause rotation on the column in essentially any case, so the first approach might be a pinned connection then see how your member forces/column moments shake out.

 

[BigInch]

Chord forces from truss loads are not transmitted to the column. Example: In a horizontal truss, the top chord axial load opposes the bottom chord load plus the horizontal component of the strut. If vertical loads on the truss and truss geometry are such that the following axial forces are generated,

1000 1500
compr compr
+-----+-----+-----
|\ |\ |\
| \ | \ | \
a b c d e f
| \ | \ | \
| \| \|
+--0--+-----+----
1000 1500
ten ten

The horizontal component of the tensile force in member "b" will = 1000 tension, and all horizontal forces are balanced, so there is no horizontal reaction at the column at all.

If the truss is rigidly connected to the columns, it is possible to transfer truss end moments to the columns, which is usually not the desired action. The truss should just rest on the column in order to insure that no moment is transmitted from truss to column. If the top and bottom of the truss is welded or bolted to the column, it will transmit moment from the truss, so truss-column connections should be as similar as possible to a classic roller support. If a slide plate is used, there is a potential to transmit a small amount of moment to the columns through friction forces.

I would let the top chord of the truss bare on a plate welded to the top of the column and hard bolt or weld that. I would not let the bottom chord bear on the column at all, and only make a minimal connection there for stability. Like attaching the truss to the column by placing a couple of bolts in a slotted connection, the slots being parallel to the direction of the bottom chord of the truss. If the truss attempted to transmit moment there, the bolt would slide in the slot and no moment would be generated.

bolt|
=]| [==+----
+---|+|\
| || \ |
|col || truss
| || \ |
| || \|
| (===o=)+--+--
| |bolt in slot
| |
| |

 

[Lutfi]

Modeling truss supports as fix-fix is a tricky boundary condition. It will cause lots of complications for the support connection.

I always model my trusses as pinned on one end and a roller on the other end. If pin-pin supports on trusses you will get differing end reactions and different axial loads and force distribution. These end reactions has to be considered in the design on the columns or walls which ever the case may be.

BigInch, if you model the support as pinned, you will have a horizontal reaction to maintain static and equilibrium. Your statement would be true if you model as roller.


Regards,
Lutfi

 

[JAE]

qase,

Well - here's my two cents:

In your original question you say "with top and bottom chords connecting to columns..."

If that is the case, then modeling pinned-pinned (the column is a wet noodle) OR modeling fixed-fixed (the column is an immovable object) are both wrong.

In reality, if the chords are fixed to the columns, then the true behavior is somewhere in between.

Whenever you model anything, your model should approach reality to such an extent that you are conservative (for safety). This is especially true if your modeling capabilities, software limitations, etc. will prohibit a more accurate model.

But for most 2D programs including the columns in the model, and any other stiffening elements attached to the columns other sides, is what you ought to do.

Since the truss chords are connected, the column WILL recieve stress in proportion to its relative stiffness to the truss itself.

 

[BigInch]

Modeling a roller and making a roller connection in the practical sense (without using ball bearings) are two different things. Fortunately its usually not necessary to have a true roller bearing and neither is it usually of practical advantage (unless friction loads are very high in proportion), as it will not give any inherent lateral stability when resisting horizontal loads as you mention, nor will it provide secondary restoration force to hold the columns ends in place and keep the column from swaying, if there are any Horiz forces generated by either horizontal loads or column buckling tendencies.

Slots don't transmit axial load, at least until the bolt hits the end of the slots, so no moment is transferred and its the closest thing to using rollers I know of without including ball bearings.

In my other life, I have literally thousands of trusses in mostly in multistory process buildings, pipe bridges for river and road crossings and wood chip conveying systems using the connection I described which have been installed in areas of highest hurricane loads. No problems I ever heard about.

 

[Ron]

qase...as JAE noted, you'll probably be somewhere in between a fixed, pinned, or roller condition depending on how you design the connections.

If you pin the top and bottom chords, it will reduce the member forces in the truss, but transfer a moment to the column through a couple created by the opposing forces in the top and bottom chords. If you release the bottom chord by providing a slot (which is rarely constructed properly, so you'll get some restraint there!), then you will reduce the moment transferred to the column, but increase your truss member forces.

To reiterate another of JAE's points, the model does not define the action/reaction of the system in the field....its construction will.

 

[WillisV]

For large transfer type trusses, if it is possible I have generally tried to minimize the moment transferred into the support columns by the couple that a full "pin" (top and bottom chords on both sides) condition would result in.

I usually first model the truss as one side of the top chord pinned with the far end of the top cord as a roller and then both ends of the bottom "free" or as rollers. This will give you the maximum forces within the truss for internal member and connection design as well as the maximum expected deflection.

Then I model the truss as top chord pinned-pinned and both bottom chord ends on rollers and see how far the bottom chords displace outward horizontally. I detail the bottom chord connection with long slots capable of handling this deflection using hand-tightened bolts with marred threads. Though of course not a perfect pin this has seemed to perform well "in the field" as long as a close eye is kept on the construction. I also detail the top connections to handle their horizontal forces or if possible tie them into a diaphraghm (e.g. with studs along the top chord) so that the diaphraghm can act as a tie to counteract the thrust.