Bottom Chord Bearing Truss End Reactions Support Conditions

[Jbennett93]

I am currently designing a bottom chord bearing steel truss to pick up an existing building column. The bottom chord will sit on a stiffened angle seat and the top of the truss will be attached for stability with a angle with slots to stabilize the ends against rotation, while allowing the load to transfer properly to the bottom seat.

My question is - In the bottom chord truss seat, if I put slots in the direction of the truss (parallel to the chords), is that enough for me to neglect the axial load at the column connection? Am I overthinking this? If I were to use standard bolts, is the axial internal to the members and need not be considered? (I am envisioning standard steel joists with welded connections where the "pull-in" force is not necessarily considered.)

In my analysis model, the horizontal deflection is minimal with pin/roller-ping supports. (0.1") This would allow for the movement to be within long slots.

As mentioned, these are existing columns and I am trying to minimize the added forces. SIDE NOTE: This is a 40' truss, picking up a crane girder. If I analyze as pin-pin, axial reaction is about 70K.

I appreciate any and all feedback.

 

[BAretired]

If the bottom chord is free to move parallel to its axis, a slotted connection will allow it to move; if the bottom chord is restrained by friction and the supports are capable of acting like pins, a horizontal force will be generated. Proper detailing can ensure that the tension in the bottom chord is zero.

 

[Jbennett93]

@BAretired The bottom chord would be bearing on the beam seat, causing some frictional resistance. Without running the numbers, I can't imagine that the friction resistance would be enough to transfer the full axial load to the connection (70K).

Do you have any suggestions on how to ensure it has proper movement? -- Outside of a bearing pad; similar to that of a bridge girder.

 

[BAretired]

Why is it bearing on the beam seat? Can't you connect to the end vertical with a slotted connection? Show a detail of your connection. ESP is not my strong suit.

If you must use the bottom chord in bearing, there will be friction. Whether or not it develops the full 70 kips depends on the ability of the bottom chord to slide and the ability of the supports to act as pins. Usually, the supports are not capable of acting as pins, but I don't know your situation.

 

[Jbennett93]

It's a bit of a long story, but essentially it's tied to the erector and some requests for installation. We are removing a column in the building, which has a haunch the supports the crane girder. We are supporting the haunch independently of the building column they want removed (which will be supported on it's own truss). The erector wants to be able jack the truss up from below, and weld it once they have the haunch fully supported and the truss in place. They will then proceed to shore and cut the building column and do the same as previously stated. They specifically requested this method and connection type.

In hindsight, they will be welding the seat to the column anyway, so I may be able to change their mind and go with a standard shear connection with slots.

I appreciate the time and responses. Sometimes talking it out leads once to the obvious solution hidden in plain sight.