Regarding the Zero-stiffness problem, Basically, at any node of the structure you can have six degrees of freedom viz. Translation in X direction, Translation in Y direction, Translation in Z direction, Rotation about X axis, Rotation about Y axis, Rotation about Z axis and they are named as degrees of freedom 1,2,3,4,5,6 respectively. Since you have specified those members to be TRUSS members, that type of specification leads to all the members having only axial degree of freedom and having no capacity in shear, bending and torsion. If you look into the warnings, then you will see that the instability occurs in FZ, MY and MZ directions. Obviously, those members do not have the capacity to resist the translation in Z direction and the rotation about Y and Z axis (i.e. third, fifth & sixth degree of freedom) i.e. the specification leads those degrees of freedom to be zero. When the program runs the analysis, it calculates the displacement vector through matrix inversion (shown as triangular factorization during Run Analysis) and if it encounters zero value of matrix, it becomes infinite during matrix inversion. So, the zero stiffness in the corresponding direction comes up. I would request you to have a look at the following links for details on this warning on Zero stiffness. These links would lead you the specific FAQ in our web page which is a pop-up box. So, please ensure to disable your pop-up blocker in order to view the contents of the above links, or press "Ctrl" when you click on the link. http://www.reiworld.com/Search.asp?id=SP-1496 http://www.reiworld.com/Search.asp?id=SP-8118 http://www.reiworld.com/Search.asp?id=SP-1794 copy and paste these links in the address bar of your internet browser Instability conditions caused by the presence of Trusses in the model _____________________________________________________________________ An instability is a condition where a load applied on the structure is not able to make its way into the supports because no paths exist for the load to flow through, and may result in a lack of equilibrium between the applied load and the support reaction. There is some explanation available in Section 1.18.1 of the STAAD.Pro Technical Reference Manual for the typical cause of instabilities. You will find it under the heading "Modeling and Numerical Instability Problems". When you declare all members connecting at specific nodes to be truss members, the alignment of the members must be such that the axial force from each member must be able to make its way through the common node to the other members. For example, if you have 3 members meeting at a point, one of them is purely vertical and the other 2 are purely horizontal, and they are all truss members, the axial force from the vertical member cannot be transmitted into the horizontal members. On the other hand, if they are frame members, the load will be transmitted into the horizontals in the form of shear. This is an inherent weak point of trusses, and a potential cause of instability. How to avoid instabilities __________________________ There is a rather simple way to eliminate instabilities, especially if truss members are present or when MEMBER RELEASE commands are used and certain degrees of freedom are subjected to a 100% release. That is through the use of the PARTIAL MOMENT RELEASE option. This is a mechanism by which you can declare that, at the start node or end node of a member, rather than fully eliminating the stiffness for a certain moment degree of freedom (d.o.f), you are willing to allow the member to have a small amount of stiffness for that d.o.f. The advantage of this command is that the extent of the release is controlled by you. For example, if member 5, has a pinned connection at its start node, if you specify 5 START MY MZ it means MY and MZ are 100% released at the start node. But if you say, 5 START MP 0.99 you are saying that the bending and torsional stiffnesses are 99% less than what they would be for a fully moment resistant connection. Thus, the 1% available stiffness might be adequate to allow the load to pass through the node from one member to the other. So, this is what I suggest you do : a) Change the declaration of the truss members in your model from MEMBER TRUSS to MEMBER RELEASE memb-list START MP 0.99 memb-list END MP 0.99 b) Run the analysis. Check to make sure the instability warnings no longer appear. Then check you nodal displacements. c) If the displacements are large, reduce the extent of the release from 0.99 to say 0.98. Repeat steps (b) and (c) by progressively reducing the extent of the release until the displacements are satisfactory. When they look reasonable, check the magnitude of the moments and shear at the nodes of those members and make sure that the connection will be able to handle those forces and moments.