Truss modeling and connection question..another one

[faromic80]

I have some more questions about trusses and decided to start another thread because the one I started the other day is a little off topic. I have attached a scan of some questions I have on a truss I am analyzing. I got these shop drawings and have to verify everything works.
From what I understand, truss members have only axial loads, but this is not the case here. In the attachment I show the model i used for hand calcs: simply supported truss. Then I show the model with the existing columns coming up the sides. The top and bottom chords are attached to the existing columns at the sides. I also attached the RISA output for the top and bottom chords. The bottom chord has moment and NO axial loads?? The (2) C10x15.3 channels can't handle that moment. I'm thinking this is because of the connection to the existing columns? the top chord has both moment and large axial forces in it and I can't understand why? because of the distributed load on the top chord?
Also, the two center diagonal M15 and 16 have no axial force in them?
I don't exactly understand the splice connection at the top chord. I don't understand why the the two plates are welded on the the larger splice plate...is this for out of plane buckling reinforcement?

You can also see in the RISA model at the intersection of M8 and M9 the members don't coincide with the center of the top chord (see truss shop dwg) how can I model this in RISA?

As for the model I think it's pretty accurate because my hand calcs for the joint forces is within +-10 kips (attached)

 

[frv]

I'll admit I only glanced over your drawings, but the main issue here is the distributed load over the top chord. A truss should always be loaded at the joints. What you are showing is more of a "truss-shaped frame".

As to whether or not it is a moment connection at the joints really depends on your detailing. If the axial forces are all coincidental (or very close), then you really don't have any moment (assuming no distributed load) because there is no eccentricity at the workpoint.

You cannot say definitively that a welded connection can be considered "fixed". It depends on what is permitted to rotate and the relative stiffness of the members being connected.

As an example, think of a huge beam, framing into a very small column (what for? who knows; I ask for some creative license to illustrate my point). From the beam's "perspective", the column offers little rotational restraint- essentially pinned. From the column's "perspective" it is wholly fixed.

Hope that helps.

 

[rb1957]

i don't know RISA ... but that's never stopped me from "whaling in" ... truss members are "assumed" to react only axial load because that's their stiffest loadpath; they react a small amount of the applied load in shear (causing bending) but generally the assumption is good. if you're doing a hand calc you know this and set out the problem this way. in RISA are you modelling the lower chord as "rod" type elements (only capable of reacting endload) or as beams. i agree that you can remove the columns from the model, so long as you analyze them later (of course). then the pinned reaction points should have lateral fixity. i would have modelled each truss member as it looks in the truss ... you've combined several bays together ... i think you have 3 elements across the top chord, i think you need 9, but the results "seems" to understand that there are transverse elements.

 

[LowLax]

Did you model your truss with 2 pin connections? You should place a roller at one end.

I would model the whole thing as hinged connections eventhough in reality you will have some continuous members, and some stiff connections. The way I look at is, I don't care if there is plastic hinging at the joints, the truss will still be stable because I designed it to work with hinges at all the joints.

You do have to consider the combined axial and bending in the top chord because of the uniform loadng, but everything else I would treat as pure axial and try to detail it that way.

 

[csd72]

A few comments:

1. I would model the whole thing as a portal frame to check that you are not inducing too much bending in the end columns.

2. The details that you give are shop built details, not appropriate for a site built job. I would suggest that you use more bolted connections in this design with double angle diagonals.

3. Make it clear on the drawings when the middle 2 columns can be cut back.

 

[faromic80]

csd,
I was searching in the forums and I saw your post on gusset plate design a while back. I have some questions regarding gusset plate design. When looking at the welds connecting the gusset plate to the chords and diagonal members, it's the resultant of the diagonal forces that you need to design the welds for right? You don't have to take into account the axial force in the chord correct?
My coworker told me today when I asked him about the connection at the top right corner to design the gusset welds for the resultant of the compression force in the top chord and diagonal force. When I take the resultant of these forces I get a horiz and vert load of about 90 kips. this seems pretty high and I have to really increase the plate size to account for this. I'm thinking i should just be designing the welds diagonal force??

Also, going back to a topic on my first post:
It doesn't matter what the fixity of the connections is if the connection is concentric. I'm talking about the stiffeners at the bottom chord where the diagonals intersect. I didn't do the orignal shop drawings, but I'm assuming the stiffeners are there for local stress effects.

 

[IFRs]

I agree with one of the previous posters. Your support conditions for the RISA analysis need to be changed. Both supports should have only one degree of fixity - in the vertical direction. But the system will be unstable. To make the system stable, add fixity in and out of the paper on both ends and along the truss on one end only.

 

[faromic80]

I did change my model to what you say, and I did get reasonable results. I have a distirubted load of 1.36 kips across a 73.416' span. The tension/compression in the chords is:

T=C=1.36*73.416^2/8=225 kips which is about the same I get in RISA. However, my model is still stable so I can't get the reactions in the direction of the truss, but I don't think they would be significant.

 

[csd72]

Faromic80,

Method of sections is also a good way of understanding what forces apply to what in a connection. Put your cut line at the lines of weld and balance your forces then it will become clear.

csd

 

[faromic80]

Does anyone have an example calculation for gusset plate design. I'm looking at the weld at the top right of the truss where the top chord and column meet. I went through the examples in the AISC connections volume II for gusset plate design. It was example 16 in chapter 7. I could not follow the example: it seems like they pull numbers out of the sky and I can't follow where they are coming from. What I did was take the horizontal and vertical resultants at the joint and apply them to the L shaped weld at the corner. This results in a horizontal, veritcal, and torsional stresses on the weld at the corner. Doesn this sound reasonable. But I get such a large weld size req'd (about 8 kips/in) and I can't get it to work. Do I take torsion into account even though a diagonal is coming into the joint and preventing rotation of the gusset?

 

[csd72]

That torsion doesnt sound good - can you eliminate it by realigning members?

 

[IFRs]

8 kips per inch is not really that much. 1/4" welds on both sides should do it based on the old rule of thumb that 1/16" holds 1 kip per inch. There should not be torsion - where does it come from? There are no out-of-plane forces are there?

 

[faromic80]

I might be looking at it wrong. I have a horizontal and vertical resultant at the corner . The torsion is coming from the fact "I think" that the resultants are at the intersection of the centroids of the members (concentric connection). I am multiplying these resultants by the distance from the intersection of the members to the centroid of the L shaped weld at the top corner. Or do I not take this into account since the diagonal member is restraining the gusset plate/weld?

 

[csd72]

I think I understand the connections, but I am not sure. Can you post a sketch in the morning?

 

[faromic80]

Here is a sketch should I take ex and ey into account for weld. they will induce torsion on the weld

 

[RARWOOD]

You indicated that you are checking truss shop drawings and want to analyze the truss. As a starting point I would contact the truss supplier and request that they send the analysis and design calculations for the truss. After reviewing that information if you still have questions I would talk to the truss suppliers engineers.

Another thing you should do is talk to the EOR and find out what braces the truss against lateral load and to find out if the truss and columns are only intended to carry vertical load. When I look at the truss profile, the size of the truss chords compared to the size of the columns, I would think that the columns are intended to take vertical load only.

Generally I would model a truss like this as a top bearing truss. I would not include the bottom chord between the column and first joint on each end. In the field then the columns would run up to the underside of the top chord. Then truss would have a seat similair to a bar joist and would be set down on the columns top plate. At the connection between the bottom chord and the column, I would design the connection as a slip connection.

 

[lkjh345]

FWIW: If you have access to the connections manual from the red (8th Edition) of the AISC Steel Manual, IMHO, this edition seems to go over this type of connection much better than the later edition for the green manual.

Just for my understanding of the problem: In the sketch attached to your first post, the height of the column (from the bottom of the truss) is shown as 25 inches. Is that correct? The proportions of the sketch are throwing me off a bit.

 

[csd72]

faromic80,

A few points:

Is the existing beam cinnection a pinned bolted connection? If so, then all axial load will transfer to the brace via the stiffest path(i.e. the weld along the top of the gusset) So this weld needs to be designed for this as well as the stress induced by the brace.


The distances ey and ex that you have shown are not relevant for the gusset welds. The only eccentricity that matters for them is the distance of the applied load from the centroid of the weld. You then apply P/A + My/I same as a beam(but use width =1 to get k/in).

As I said above, apply method of sections and these things become obvious.

 

[faromic80]

the column is 25 feet tall. I can bear on the columns because the W14 beams (top chord) are exising as are the columns. so the only load the horiz weld gets is from the horizontal portion of the diagonal force, that's more reasonable. Since the connection is concentric, no moment is induced on the column, right?

csd, i'm going to analyze this with method of sections tonight. I'm asking these questions because I'm on a tight schedule to get this finished.

 

[rb1957]

i might "fuss" the end support conditions. i'd start assuming the ends are fully fixed (as though the columns are rigid). then apply the moments onto the columns and see what sort of rotation you have at the ends. allowing this rotation for the beam ends would relieve the end moment slightly, and i think you'd quickly converge on the final rotation.

if you have only horizontal load, what stopping the frame from swaying ... cantilevered base ?