russ modeling and connection question..another one

huangyhg

truss modeling and connection question..another one
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)
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   
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   
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   
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.

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   
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.   
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.
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
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?
that torsion doesnt sound good - can you eliminate it by realigning