Show Calculated Rod Effective Area for CSHEAR

[brianabreugomes]

Hi guys...need a little help in understanding something here.

I am using CSHEAR's to model some spar & rib webs for a typical wing structure. I have rib caps and spar caps modeled which run in the X & Y direction. But I have no stiffeners in the Z direction so I am putting in F1 or F2 values to take the vertical direction loads.

I am using F2 = 30.0.
Thickness of Webs (spars & ribs) are 0.040".
Vertical Lengths (Z Direction) of panels are 1.063" and Longitudinal Lengths (X or Y Direction depending on rib or spar panel) are 5".

Nastran is stating that my effective area for F2 is larger than my actual area. I want to know what the effective area nastran is calculating. Isn't my rod area going to be A = (0.5)(30.0)(0.040)^2 = 0.024 in^2

I don't get it.

Thanks in advance.
-B

 

[40818]

Hi,

I have never used CShears, but do have at work a document by Ian Taig (do a search...) in it he describes the use of these elements and the F1 and F2. If you dont get a better answer by monday lunchtime (UK) then i will have a re-read and repost. I wouldnt want to state anything that was incorrect or misleading. Your aircraft is mighty small though....

Why dont you just use rods though to carry the endloads??

 

[40818]

Heres the excerpt from my notes on CShears:

The CSHEAR element normally carries only shear loads and thus should be surrounded by ROD elements to carry endloads in the equivalent areas. if this extensional stiffness is not present the panel will be singular and would probably lead to a fatal error or excessive displacements.
A facility to give the shear element extensional stiffness in either direction exists using the F1 and/or F2 factors. (See PSHEAR description).... and it goes on.

Now with reference to Nastran (the PSHEAR definition), the effective extensional area is defined by means equivalent rods on the perimeter of the element. If F1<1.01, the areas of the rods on edges 1-2 and 3-4 are set equal to (F1 x T x PA)/(L12+L34) where PA is the panel surface area - half the vector cross product area of the diagonals - and L12 and L34 are the lengths of sides 12 and 34. Thus, if F1 = 1.0, the panel is fully effective for extension in the 1-2 direction. If F1>1.01, the area of the rods on edges 1-2 and 3-4 are each set equal to 0.5 x F1 x T^2
Thus, if F1=30, the effective width of skin contributed by the panel to the flanges on edges 1-2 and 3-4 is equal to 15T.

Now looking at your numbers, your panel is only 1.063" wide and is 0.040" thick. So your thickness to width ratio = 1.063/0.04 = 26.575 (or W = 26.575t) So by setting your F1 to be 30, your trying to say that your going to take an effective width of 15T of your panel from the opposing side of your element. But you only have a maximum of 26.575/2 = 13.2875T available before the element becomes fully effective. And thats the reason for the statement about the value being larger than the area.

Hope this helps.

 

[brianabreugomes]

Sorry it took soooo long to say thanks. Been in Italy on special projects for a while.

That makes sense...haha...that really helps a lot...much appreciated.

I am doing a little experiment to see what are the different results using different methodology when FEM of a basic rectangular wing. Then I'll do the same for a swept wing. I'm new to this so I'm in a learning mode at the moment. Unfortunately for me, there is no-one at my company to ask because we do not have any experienced people to give me straight up FEM/Femap/Nastran advice/answers.

Cheers brother,
Brian