ube frame with sheet metal skins

huangyhg

tube frame with sheet metal skins
hello,
i have a tube frame consisting of 1 x 1 x .120 wall tubing with 14 ga sheet metal skins on the top and bottom of the frame.  the frame is 48" long x 24" wide.  the frame also has two more 1 x 1 x .120 tubes running along the 48" length.  the frame has two point loads of 440lbs each 10" from each end of the 48" length and 3" in from the ends of the 24" length.  this is a fairly simple structure but some questions arose about specifying the number of welds to attach the skins to the tubing.  we are planning on using 1/4" dia plug welds.
for simplicity, we are considering the structure to be a simply supported beam and running the calculations on the 48" length and 24" length separately.
the question arose about calculating the shear stresses at the interface between the tubes and the skins which the plug welds must resist.  i've seen a shear flow calculation method that uses the following calculation:
q=vq/i
q=shear force per inch of length
v=vertical shear force
q=static moment of the area or ay (y=distance from netural axis)
i=moment of inertia of section
in this structure (48" length section)
i=1.216 in^4
y=.5375 in
a=1.80 in^2
based on the above calulation, i'm seeing a shear load of 350lb/in which seems high since shear loads are typically small as the outer fibers.  the 350 lb/in will require a larger number of welds and i don't think the fabricator will be too happy.
this type of structure is common especially in the aircraft industry and i want to get some input as to whether this calculatoin method is typically used.  aircraft designers must use something similar to specify riveted joints when attaching skins to wing spars and ribs etc.
the above information is not to get someone to do the work for me, but to provide enough information to uncover any flaws in the method.
thanks,
kyle

i think your definition of q is a little astray, i think it is the 1st moment of area with respect to a section (a cut throught the cross-section), not necessarily the neutral axis.  i think you've seen the neutral axis referred to in a standard beam, 'cause the shear flow is maximum at the na.  
aways, in your problem you've got two modest forces that the sheet will shear into the edge   
rb1957
thank you for your response.  the equation i was following was an adpatation of the shear stress formula whereby they are analyzing box beams or t shaped beams constructed from wood and in the example they are looking for the quantity of nails based on their shear load capability. i assume you could also build up sections from stuctural shapes and determine loads on rivets as well.
the definition of q is where some confusion lies.  the examples of the built up sections take the distance of the centroid of the section from the neutral axis.
if i use the standard stress formula and incorporate the width of the cut, "b", should i then consider the shear at the surface of each tube where the width is small, and then divide that up among the four tubes?
t=vq/ib
i'm glad to see that your shear loads are smaller.  the calculations i ran lead me to believe that something was off since the final shear loads appeared excessively high relative to the loads imposed on the structure.
thanks,
kyle
yes, i think it is simply the shear force over the side length
rb1957,
i'm still a little confused then.  if i were to build up this section and use 1/8 aluminum pop rivets that have a 140lb shear strength, with 4 rivets holding the skin to the tube per space, how would i arrive at the spacing needed between rivets?  the shear flow examples calculate the shear flow, then divide the strength of the fasteners by the shear flow to obtain the spacing.  in my case i'm looking for the spacing between the welds.
kyle