A pin truss support

[Nasreb]

Please can anyone help me regarding how to design a pin truss support!?
I think that I should let the both ends of the top chord of the truss bare on a plate hard bolted (8xM27) to the top of the column and the bottom chord should not bear on the column at all, and only make a minimal connection there for lateral stability.
Is this a way how I should treat a truss pin pin support?
Thanks in advance!

https://files.engineering.com/getfi...le=IMG-987955239219bc87142b91c9b9e31368-V.jpg

 

[r13]

How the column cap plate was bolted? Usually it is welded to the column to provide a stiff bearing for the truss. Is that you called pin (plate to column) pin (truss to plate) condition? It really goes back to my question at the beginning.

 

[Nasreb]

@retired13, my mistake, I mean a truss pin support!!!
Thank you for your reply!

 

[r13]

Yes, simply bolt the truss to the cap plate constitute a pin connection. Note for long span truss, and truss subjects to significant thermal effect, you might want to provide slot holes on one end of truss support to allow axial movement, similar to a roller support.

 

[Nasreb]

@retired13 thank you once more for your reply!
My span truss is 34m, is it nesseserly to provide slot holes??? And my question is, does it works as a pin support, if I design connection with 8 bolts???
Thnaks!

 

[r13]

The reason I suggest to provide slotted holes is to avoid the axial pull force, unless you include the effect in the design. For 34m long span, you might also want to investigate the deflection, with both ends tightly pinned, you can reduce the deflection, however, IMO, I rather to control the deflection by selecting a stiffer/deeper truss, if I can. Because the high pull force will have a negative effect on the column, or the support wall.

Unless both top and bottom chords are bolted to the support, there is no bending moment in a truss. The problem is the connecting truss chord inevitably will experience localized bending stress, which requires design. Look into joist catalog for stiffened end support, usually used for long span joist, to get some idea for design.

 

[KootK]

1) Accept the usual disclaimer that nothing is ever truly fixed or pinned.

2) In my opinion, you can call the truss end pinned even if you run the bottom chord through the column and add a vertical web.

3) I would consider the truss pinned, using your detail, regardless of the nature of the top chord connection to the column so long as horizontally slotted holes were used at the bottom chord. Without the bottom chord slotted holes, you'd have a pretty effective moment connection between the truss and the column which you likely seek to avoid.

In practical terms, most trusses will be pinned unless both chords connect to axial resistance at the support.

 

[Nasreb]

@KootK, thank you for your reply.
My span is L=34m, and in my model I didn't include thermal effect. So my question is should I provide slot holes in one end of the top chord, cause in my model top chord in both ends are pin support???
Thanks in advance

 

[BAretired]

How do you expect to achieve a pin support at both ends of the truss? If you model it that way, your computer will assume a zero change in length of the top chord. That is not what you will find in a truss supported on columns. The columns will bend to permit the top chord to change length. A horizontal slot at one end of the truss will make no difference in behaviour because friction from the truss reaction will not permit sliding.

EDIT: If by pin, you mean hinge, that is a different matter. You can have a hinge between truss and column at both ends without significantly affecting stress developing in the top chord. For that condition, your model must include the two columns in the analysis. The usual practice is to consider the truss supported on pin at one end and roller at the other, so that no horizontal resistance can be developed at the supports. Alternatively, if you prefer, you can put a roller at both ends and a horizontal only support at some other point, usually at midspan.

BA

 

[KootK]

My span is L=34m, and in my model I didn't include thermal effect. So my question is should I provide slot holes in one end of the top chord, cause in my model top chord in both ends are pin support???

We'd need to know more about your structure to say for sure but, in the overwhelming majority of these situations, the top of the column will be allowed to move laterally with the truss top chord in order to for the pin-roller condition that we assume in analysis. In reality, most of these things are actually roller-roller with the roof diaphragm providing translational restraint along the length of the truss.

BA is certainly correct about your bolted connection likely not working well as a slip connection though. To do that for real, you'd probably need to get into Teflon bearing pads etc.

 

[Nasreb]

@BAretired, @KootK, thnkyou very much for your replies.

In my model I assume both ends pin support.
Length from thermal effect (for L=34m span) is 1.63 cm, it means both columns will take that lateral movement (1.63/2=0.81cm per column)!!! from ULS all secstiona are ok, even from SLS all section are ok, so I think that thermal effect column can effort.
Thanks in advance

 

[BAretired]

I hate to keep harping on a theme, but you cannot use pin supports each end for the computer model for reasons already given. And if you did, it would be 0.0 cm per column for thermal (or any) effect.

BA

 

[Nasreb]

@BAretired sorry I didn't mention, thermal effect I calculeted manually without computer!
You are right about modeling, so I change model on my PC, in one and is pin and in far end is roller, but i don't know if it will work as a roller in practise, if I constitute a roller support, with 4 bolts, and a slot hole. Any advice??? Thanks

 

[r13]

Nasreb,

As pointed out before, for long span truss with significant support reaction, shear friction may affect the performance of simply bolting in slotted hole (prevents movement). I suggest to use double bearing plates with sliding enhance material in between. The top plate should welded to the truss with slotted holes, and sitting on the column cap with standard holes. The other end to have standard holes on both plate to make a pin support. Hope this helps.

 

[BAretired]

@BAretired sorry I didn't mention, thermal effect I calculeted manually without computer!
You are right about modeling, so I change model on my PC, in one and is pin and in far end is roller, but i don't know if it will work as a roller in practise, if I constitute a roller support, with 4 bolts, and a slot hole. Any advice??? Thanks

Good that you changed your model. If it bears on flexible columns, it won't work exactly as a roller in practice. If the end supports are immovable, such as a bridge, you would need to make it work as a roller by using a sliding connection at one end. In your case, end supports are columns which bend as required to accommodate thermal and other effects. The roller effect comes from bending of the columns, as you said earlier, 0.81 cm per column for thermal movement.

You could analyze a frame consisting of the truss and two columns, but unless you are relying on the frame to resist lateral forces, which I don't believe you are, that would be an unusual and unnecessary step.

BA