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RE: FEM and Cables-Reply
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: FEM and Cables-Reply
- From: Waterman Drinkwater <Drinkwater(--nospam--at)EQUATION.COM>
- Date: Tue, 6 Apr 1999 11:12:49 -0400
Real cable is geometric nonlinear, and has a 6X6 stiffness matrix for 3D cables. The stiffness matrix also can be formulated into 6x7 matrix with a constraint for advanced applications. Standard input for elastic catenary cable includes the following: (1) coordinates of two joints (2) unit weight (3) unstrained length (it can be < joint distance.) (4) material constants (E and thermal expansion coefficients) Standard procedure does not ask users to assume a pretension. A good cable program can analyze cable nets, cable-stayed roof and bridge, cable truss, or other type of cable-supported structures. Dynamic analysis will be more complex. ---------- From: James_F_Fulton(--nospam--at)RohmHaas.Com [SMTP:James_F_Fulton(--nospam--at)RohmHaas.Com] Sent: Tuesday, April 06, 1999 9:21 AM To: seaint(--nospam--at)seaint.org Subject: FEM and Cables-Reply STAAD has pretensioned cable members which are invoked with the MEMBER CABLE command. You specify the cable pretension, but these forces must be statically admissible, i.e. the forces specified must correspond to a real, achievable equilibrium state in the system of cables. If they are not, the final forces in the cable results will be significantly different than those input in order for the cable system to be in static equilibrium. For symmetrical cable layouts, *equal* pretensioning forces in all the cables is statically admissible and hence final cable forces will all be the same, but somewhat less than specified if the cables are, for example, connected to a stack where elastic shortening takes place under the cable forces. This is one little glitch in the program since in the field the pretensioning force is measured (?) as the stack shortens and hence elastic shortening of stack is accounted for as pretensioning takes place. In STAAD, you would have to specify a somewhat larger cable pretension and adjust by trial and error to end up with a specific final pretensioning force. In the situation where the cables are not symmetrical, it is (usually) practically impossible to predict beforehand what a set of statically admissible cable forces will be. To determine this, I first run the problem of an equal, unit temperature drop on all the cables. However, MEMBER TRUSS members have to be used to represent the cable response rather than MEMBER CABLES. This then gives the linear force dependence among the truss members (cables) for the particular geometry. Then, I run the lateral load (e.g. wind pressures) together with the MEMBER CABLES. This requires several runs, incrementing the cable pretensions, but maintaining their relative, statically admissible set of forces as determined from the temperature drop analysis, until a desired design criterion is achieved. One such criterion might be that no cables go slack under the design lateral load. As is usually the case with the STAAD folks, none of this necessary procedure is even touched on in the STAAD user manual. My approach I discovered on my first unsymmetrical cable situation. Maybe there are other approaches. If so I would like to hear about them. Also, in the field, I would like to know how the erector goes about measuring a specified cable pretension, particularly a relatively small 1500 lb. to 2000 lb. required pretensioning force for small (3/8' - 1/2" diam) cables, which I encounter with exhaust stacks.
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