Truss modeling and connection question..another one
Truss modeling and connection question..another one
(OP)
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)
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)






RE: Truss modeling and connection question..another one
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.
RE: Truss modeling and connection question..another one
RE: Truss modeling and connection question..another one
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.
RE: Truss modeling and connection question..another one
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.
RE: Truss modeling and connection question..another one
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.
RE: Truss modeling and connection question..another one
RE: Truss modeling and connection question..another one
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.
RE: Truss modeling and connection question..another one
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
RE: Truss modeling and connection question..another one
RE: Truss modeling and connection question..another one
RE: Truss modeling and connection question..another one
RE: Truss modeling and connection question..another one
RE: Truss modeling and connection question..another one
RE: Truss modeling and connection question..another one
RE: Truss modeling and connection question..another one
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.
RE: Truss modeling and connection question..another one
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.
RE: Truss modeling and connection question..another one
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.
RE: Truss modeling and connection question..another one
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.
RE: Truss modeling and connection question..another one
if you have only horizontal load, what stopping the frame from swaying ... cantilevered base ?
RE: Truss modeling and connection question..another one
RE: Truss modeling and connection question..another one
RE: Truss modeling and connection question..another one
RE: Truss modeling and connection question..another one
lower chord only would be a pinned joint
RE: Truss modeling and connection question..another one
Am I reading something wrong: Your taking the reaction into an existing 25' tall TS 4x4 column? Is the column braced in both axis somewhere along the 25' height?
FWIW: I disagree with connecting the bottom chord at all. This connection will attempt to 'fix' the truss end until it fails. With a very small column size (TS4x4) any moment introduced into the column could be too much. JMHO, but I would not attach the bottom chord at all.
RE: Truss modeling and connection question..another one
RE: Truss modeling and connection question..another one
It is my interpretation that a 'roller' assumption should not be assumed in the frame design unless the horizontal deflection determined in the analysis can actually be achieved. Unless slotted holes are detailed, the truss's rotation will be prevented by the column fixings inducing moment into the column - all a function of the relative stiffnesses of the elements.
Note that even in a true 'pinned' connection, the load is still eccentric to the column CL and some moment will result - P delta effects need to be considered with any horizontal deflections.
Cheers
RE: Truss modeling and connection question..another one
RE: Truss modeling and connection question..another one
as the other posters are noting, there are many "issues" to be addressed with the columns.
RE: Truss modeling and connection question..another one
RE: Truss modeling and connection question..another one
RE: Truss modeling and connection question..another one
I just keep asking myself if the top chord connection to the column is fixed or pinned after I add the gusset plate at the intersection of the existing column and existing top chord. When you introduce that brace and gusset, it should still act as a pinned connection since the member centroids are concentric, right? I guess my biggest concern is my unfamiliarity of the detailing of the connections to ensure they behave like "pinned" connections and not fixed connections, especially at the ends.
RE: Truss modeling and connection question..another one
RE: Truss modeling and connection question..another one
RE: Truss modeling and connection question..another one
I was asked to take a look at the end connection as well as the splice connection at the top chord to make sure the load could be transferred. I've attached a picture of my detail of the end condition, showing the MC7x22.7 reinforcing members. Is the vertical reaction of 49 kips transferred to the column through the gusset plate. I've designed the weld to transfer this load. Should I add a stiffener? It's a 3/4" plate! The reason for the 3/4" plate is that it's being built entirely in the field and I wanted to account for construction tolerances and any eccentricities being introduced.
RE: Truss modeling and connection question..another one
Your analysis software should provide the horizontal deflection and associated column moments. If you want to determine the force from a pre determined horizontal movement simply work backwards by applying a deflection instead of a force to the appropriate deflection formula. ie the stiffer the member the larger force.
Can't say i'm a fan of not connecting the bottom chord. I think fixing it and slotting the holes horizontally will provide a more stable arrangment. I assume there can be no lateral loading on the bottom chord and that it doesn't go into compression (no upward loading possible)?
RE: Truss modeling and connection question..another one
RE: Truss modeling and connection question..another one
Regarding the load eccentricity you have to run a second order analysis to account for p-delta effects. What you have done with the nominal offset seem adequate though.
Cheers
RE: Truss modeling and connection question..another one
RE: Truss modeling and connection question..another one
RE: Truss modeling and connection question..another one
- No release on the top & the intermediat column joints.
- Release vertical & moment constraints on the bottom chord members immediate adjacent to the columns (member end on column side).
- Release the rotational restraint on the remaining truss members.
RE: Truss modeling and connection question..another one
Sorry missed your sketch shown loading, the uniform load will at least causing moment on the top chord members. A truss usually takes concentrate load at the panel joints, not uniform load, through some arrangement, such as purlins/girts...etc. For your system, if you have continuous top & bottom chords, it acts as a deep beam rather than a typical truss.
RE: Truss modeling and connection question..another one
when I model the truss with dummy members and then model it with the actual members I get approximately the same reactions in the members. Actually the chord axial forces decrease by 15 kips with the acutal members being modeled..?
RE: Truss modeling and connection question..another one
how are you accounting ofor beam column effects on the upper chord ? consider the middle span/element ... it's in compression and has a transverse distributed load
RE: Truss modeling and connection question..another one
RE: Truss modeling and connection question..another one