Fundamental Truss Question 1

[faromic80]

I have a fundamental question about trusses that I always think about..maybe I'm just overthinking it.
I attached a file of a truss I'm going to be analyzing. I know that trusses transfer axial loads only through their chords and web members. This occurs because the joints are pin connections, or at least that's how I understand it. In the file attached, and in general, trusses have gussets and the members framing into the gussets are welded. Isn't a welded connection represent a fixed connection? The gusset is welded to the top/bottom chords to transfer the force into the chords. Is this a fixed connection? Or is the welded (fixed or semi-fixed connection)not important and the only thing that is important to "create" the pin connection is that the center lines of their axes intersect at the center line of the chord?

This leads to a second question I had about trusses. I had a situation one time where there were existing 24" deep bar joists spanning approx. 40' bearing (top chord bearing) on masonry at their ends. about a 20 foot span of masonry was being removed and they wanted a scheme of how to shore up the bar joists during the process. I immediately thought "you can shore underneath the bottom chord, but you can at the top chord". This was a one story building with decking on the bar joists. Anyways, I had that thought, but didn't exactly know why I couldn't shore underneath the bar joists, I guess it was my intuition. After thinking about it, it's because you would create a moment in the bottom chord, right? Could you shore under the bottom chord at the intersections of the webs and bottom chords?

Third related question is: Although the joists were top chord bearing, some of the bottom chords were welded to an angle bolted to the masonry wall. What is the purpose of this, bracing? Wouldn't this create a moment in the truss? Not sure about this one after a lot of thought.

Thanks all

 

[msquared48]

Most truss top and bottom chords tend to be continuous, except at the splice points. Many connectionsare boltes, many welded. IN GENERAL, most bolted joints can be treated as pinned, most welded joints as fixed. However, the level of fixity assumed, particularly in steel trusses, depends on the detailing and design of the joint.

The joints intersecting a continuous top or bottom chord may have a mix of fixed and pinned joints at the intersection, particularly in wood trusses, the chord being fixed due to continuity, but the webs and verticals pinned.

Regarding the shoring, yes, shore under a panel point, but if you shore the bottom, I would also block to the top chord above the shoring, particularly if the truss is to be modified at this point - a vertical will have to be added in all likelihood.

Regarding the connection of the treuss bottom chord to the wall, that's a standard detail for open web trusses - could be for either a ceiling extension parallel to the truss, or bridging considerations normal to the truss.

Mike McCann
MMC Engineering

 

[faromic80]

The truss I'm looking at has a continuous chord at some intersection points and is spliced at some intersection points. This is an existing building and they was to remove the columns from the center of the building.

So regarding the fixity of the truss connections: When analyzing a truss at the joints, how should I approach it? Typically, and in statics books they assume pinned joints. If a welded joint between webs and chords is fixed or even semi-fixed, can I still analyze it for axial loads only and not take into account moments and loads due to fixity?

The shoring was going to be temporary, so the vertical member between the shoring and top chord would have been temporary. Could I alternatively just have shored under the top chord between panel points? But then I would have to take into account the moment induced by the point load on teh top chord from the shoring, correct?

 

[Lion06]

The extension to the wall could also be to brace the bottom chord. This is something often overlooked, but just because the chord is in tension doesn't mean it doesn't need bracing. It has to be able to brace the compression diagonals via its own bending stiffness. if it isn't supported anywhere then it has no bending stiffness.

 

[fasboater]

If you are bracing the bottom chord you need to make sure you check/account for stress reversals in the web members.

There should be a tag at one end of the joists with the manufacturers information, you can always look the manufacturer up and give them a call.

 

[Castigliano]

If you are only loading a truss at the joints, your assumption of connection fixity is relatively unimportant. The deflections of the truss will tend to induce frame-action moments in the truss members, but because the members are so stiff axially, the moments tend to be small enough to neglect.
If you're loading the truss along the chords, and not just at the panel points, then all bets are off, and you need to accurately account for connection fixity and the transfer of moments along the chord and into the web members.

One bridge engineers take, anyway
Castigliano

 

[jike]

Shore only to panels points to prevent bending in the chord. An additional consideration when shoring to the bottom chord would be that the bottom chord might want to kick one way or another since it is not braced laterally whereas the top chord would be most likely braced by the deck.

Bottom chord extension can be used for several reasons:
brace bottom chord
ceiling extension
brace wall

 

[civeng80]

On the question of welded joints as opposed to pinned joints, the following comments are from the famous book "Theory of Structures" by our all time favorite Timoshenko and Young.

"Bending of the bars of a truss due to rigidity of the joints superimposes additional bending stresses called secondary stresses. If the bars are carefully arranged so that their centre lines meet at one point at each joint we find that the presence of secondary stresses due to the rigidity of the joints can be ignored and pinned joints assumed."

If Timoshenko says it its good enough for me.


Hope that helps because that question bugged me to for quite a while!

 

[Dinosaur]

A long time ago, I modeled a truss as an exercise just to learn the significance of the secondary stresses. I used coincident joints (all C/Ls meeting at one point). The secondary stresses were small compared to normal truss behavior. I didn't save the file so I can't tell you how small is small.

 

[SteelPE]

On question #3

I was always told not to rigidly attach the bottom chord of the joist to its supporting member without notifying the joist manufacturer. Attaching the bottom chord induces a moment in the joist (axial compression in the bottom chord), which the joist manufacturer has to account for.

Rigidly attaching the bottom chord to its supporting member is a typical detail that was done a number of years ago but has been phased out over time.

If you need to brace the bottom chord this can be done with uplift bridging placed at the first bottom chord panel point.

 

[civeng80]

Hello Dinosaur

Thats an exercise Ive wanted to do for a long time but never got around to it.

I never will now that Im convinced that secondary stresses are small if truss members meet at a point.

Cheers !