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Truss modeling and connection question..another one

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)

RE: Truss modeling and connection question..another one

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.

RE: Truss modeling and connection question..another one

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.

RE: Truss modeling and connection question..another one

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.

RE: Truss modeling and connection question..another one

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.

 

RE: Truss modeling and connection question..another one

(OP)
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.

RE: Truss modeling and connection question..another one

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.   

RE: Truss modeling and connection question..another one

(OP)
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.

RE: Truss modeling and connection question..another one

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

RE: Truss modeling and connection question..another one

(OP)
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?

RE: Truss modeling and connection question..another one

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

RE: Truss modeling and connection question..another one

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?

RE: Truss modeling and connection question..another one

(OP)
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?

RE: Truss modeling and connection question..another one

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

RE: Truss modeling and connection question..another one

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.   

RE: Truss modeling and connection question..another one

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.  

RE: Truss modeling and connection question..another one

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.
 

RE: Truss modeling and connection question..another one

(OP)
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.

RE: Truss modeling and connection question..another one

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 ?

RE: Truss modeling and connection question..another one

(OP)
as of right now, i assumed vertical load only and no moment transfer in the column.  The connection is a shear tab with 3 bolts.  the gusset will be added when the truss is fit up in the field. i don't think this will induce moment at the connection so it should act as a pinned connection.  If I fix the end connections, I will get a moment which I don't want and is not how I modeled it.  The top chord is connected to an existing metal deck roof which acts as a diaphram.  However, the lateral load is dumped into masonry shear walls, so the truss frames aren't assumed to resist any lateral loads.  I'm going to reinforce the existing columns with 2 MC7 channels for the increased vertical loads.  I'll run them up to the top to of the existing column and cut them off as necessary to clear the existing W14 beam (top chord).

RE: Truss modeling and connection question..another one

but aren't the upper and lower chords connected to the column ? ... that'd provide moment stiffness to the end of the beam.  if one chord isn't connected to the column, this'd reduce the moment stiffness, but i think this'd drive the chord load into the nearest load path, presumably not really designed for this load.

RE: Truss modeling and connection question..another one

(OP)
The bottom chord is connected to the column with L3x3 at top and bottom of the channel.  it's a pretty flexible connection just for the sake of continuing the bottom chord.  it should act as a pinned connection? what do you think?

RE: Truss modeling and connection question..another one

the top chord isn't connected to the column ?

lower chord only would be a pinned joint

RE: Truss modeling and connection question..another one

My 2 cents:

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

(OP)
I am reinforcing the TS4x4 with 2 MC7x19.1 at it's sides.  It is unbraced along it's height.  the top chord is currently connected to the existing TS4x4 by a 3 bolt shear tab connection.  So it's pinned I am reinforcing the TS col below the existing top chord for the increased vertical load.  so I'm assuming a pin connection because of the concentric connection and no moment at the connection....

RE: Truss modeling and connection question..another one

I really think it is worth while modeling the frame in its entirety to get insight into the moment the column will attract. Two 'pinned' connections still achieve fixity to the column due to the distance/lever arm between the connections - the connection does not permit rotation.

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

At a minimum you should apply the load at the face of the column.  

RE: Truss modeling and connection question..another one

if both chords are pinned, then the overall truss beam is effectively fixed at the ends.  if the top chord is already attached to the column, then leaving the bttm chord unattached (as ljkh posted) would make the truss pinned.

as the other posters are noting, there are many "issues" to be addressed with the columns.

RE: Truss modeling and connection question..another one

(OP)
I agree.  It may be beneficial for me to model the columns to see the moment transfer I get at the column.  I knew that some moment transfer had to occur just because of the shear tab connection at the ends.  By the way, this connection is transferring the vertical reaction at the ends, right?  I decided to not take into account the existing connection and see if the 7.5" vertical length of the gusset at the existing end columns could resist the 50 kip reaction.  Just for curiosity, I found that the 3 bolted connection can handle much more than 50 kips from the AISC manual tables.  Do you think it's sufficient to check the existing column for the moment due to the reaction at the face of the column?

RE: Truss modeling and connection question..another one

Look up the design rules for shear tabs, taking the reaction at the face is only applicable for a beam bearing on a cap plate.

RE: Truss modeling and connection question..another one

(OP)
My biggest concern is the end connections at the top chord.  I am not going to connect the bottom of the truss to the column.  I will extended it past the last diagonal but stop it 2" short of the column.  Then I'll weld some plates to the sides of the existing column for torsional considerations since it's going to be field built.  
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

You are to be congratulated for being concerned about the connections.  This is too often overlooked!  There will be some degree of local fixity.  Analyzing the truss with pinned ends will be conservative for the truss members.  Some moment will be transferred into the column.  You could try using the vertical end reaction of the truss times the width of the gusset to get a moment into the column.  There will also be some moment due to the tendency of the top chord and/or truss to deflect under load and transfer that deflection into the column via the "fixed" gusset.  So, you have an induced moment from deflection and a moment from the gusset connection.  Add them together and see what that does to your column.  Good work!!

RE: Truss modeling and connection question..another one

(OP)
How would I determine the approximate load transferred into the column due to the deflection of the top chord imparting load to the column through the "fixed" gusset plate.  I could figure out the horizontal movement due to vertical deflection, but how to translate that to a force?

RE: Truss modeling and connection question..another one

(OP)
One more question:
  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

faromic

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

(OP)
The load is not reversible.  I was not a fan of it either, but did not want a moment induced.  I modeled the truss with fixed connections at the top and bottom chords at the ends and still didn't get a moment in the column.  There were only vertical loads acting on the column, but still, a moment should be present.  What I did was calculate the moment by multiplying the vertical reaction by 4" eccentricity.

RE: Truss modeling and connection question..another one

The moment that is transferred into the column is a function of the stiffness of the connecting elements. So if you have a stiff beam (little rotation) and an unstiff column (pin based?) you will get little to no moment transfer through the connection and into the column. ie the column offers little resistance to the rotation of the beam.

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

Your model is correct. But have you removed the rotational constraint at the ends of the truss members?

RE: Truss modeling and connection question..another one

(OP)
I have.  I'm getting an instability error in RISA, but the results are reasonable so I can ignore the error.  This is according to the help file.  The results are close to what I get by hand.

RE: Truss modeling and connection question..another one

Just for fun, try the following if you haven't done it yet:

- 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

Final words:

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

(OP)
Something interesting also....
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

that sounds odd to me ... the truss-beam doesn't look that redundant, implying the internal loads should be reasonably independent of the element areas ...

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

(OP)
I am designing the top chord as a beam column element.  I check the spans between the splices and then the splices for combined loading.  I've attached a sketch of my configuration.  Speaking of the splice connection.  I'm concerned about how the load from the diagonals is transferred through the gussets.  Is there a moment (in plane torsion) induced on the gusset plates or is there not because the connection is symmetrical???  I just can't seem to visualize this connection.   

RE: Truss modeling and connection question..another one

Suggest to draw a free body diagram at the notes with results from your model analysis, it provides better idea.

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