• 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: Thu, 8 Apr 1999 17:47:37 -0400
Catenary cable is geometrical nonlinear. Nonlinear geometry is not
identical to "large displacement". It would be better to verify if the
cable member programmed in Robot 97 are real elastic catenary cable or
some sort of approximation.

Elastic cable uses "linear stress-strain relation" and the "first-order
displacement". First-order displacement does NOT formulate "large
displacements". Because of geometrical nonlinearity, the temperature
change and thermal coefficient appear in the stiffness matrix. For more
introduction of elastic cables, there is a book "cable structures" by
Max Irvine.

In fact, the nonlinearity in cable equation is not from "displacement".
Many books in engineering mechanics introduce catenary cables. Those
cables (assumed to be undeformable in fundamental engineering mechanics)
are formulated without a consideration of "displacement", but the cable
equation is always nonlinear. This is a simple illustration the
nonlinearity in cable is not from "large displacement".

----------
From:  jk [SMTP:jk(--nospam--at)issstanford.com]
Sent:  Thursday, April 08, 1999 4:12 PM
To:  seaint(--nospam--at)seaint.org

Robot 97, structural analysis and design software,  is
specializing in cable
elements. They very realistically implemented. They are treated
as
- non-linear elements with large displacements,
- have coordinates of two joints
- on which you can impose the nodal load
- you can inlclude the self-weight of a cable,
- you can include a temperature load
- you can impose the initial shortening/elengation or prestress
- you can load the deformed shape of the cable with concentrated
or
- you can specify the E and thermal expansion.

Robot 97 can  analyze all the structure types you mentioned. Our
clients
have used Robot successfully to model very complex structures
bridges or nets. Also, given that Robot 97 has the best user
interface and
postprocessor among structural engineering software to model
cable
structures, I think you should take a look at this program.

BTW, original message mentioned also tension-only and
compression-only
elements: Robot 97 allows for such application.