Actually, wrinkling is a buckling problem of membrane structures( I mean structural wrinkles not material wrinkles here). Since the bending stiffness of fabric is very small, when the minor principal stress is zero or negative, i.e. compressive stress, the wrinkle occurs.
There are many factors can induce wrinkles not installation only. There are a few methods for analysis wrinkling problem.
If the major principal stress is also non-positive, i.e. both two directions without tension, as you said, the membrane is not wrinkled but slack. A slack membrane or a slack region of membrane can not carry any load. In this case, the membrane is just a cover but not a structural member. Such a structure, in view of mine, is not a membrane structure. Membrane structures are tension structures, how can we call a membrane without tension as a structure? it's just a fabric, a piece of cloth only!
Lets concentrate my topic on wrinkling problem here. Wrinkles are the out-of-plane deformation. There are two types of wrinkles, i.e., structural wrinkles & material wrinkles. Structural wrinkles due to localised buckling/compressive stress. Material wrinkles due to material, fabrication and folded packaging. The wrinkles that you mentioned in No.2 are material wrinkles, the others and rockmad mentioned are structural wrinkles. Material wrinkles are permanent deformations. Structural wrinkles are not permanent, they may be disappear when change the prestress or boundary conditions or applied load, in other words, they may be eliminated through careful design and installation. However, the analysis model and design software need further study and development.
To make the wrinkles disappear is tough, but find a girl without wrinkles is possible. rockmad xiong, form-finding. ;)
The previous posts in this topic are mainly about the wrinkling criterion and the types of wrinkles.
It seems no more response. I put one more post here and welcome everyone join to discuss. I am sorry for my English input again, I can read but can not type Chinese by my computer.
The drawbacks of the wrinkled membrane are the wrinkles not only affected the look but also affected the structural behaviour. Once the wrinkles occur, that means the minor principal stress of the membrane was zero or negative; in the regions nearby the wrinkles, the prestress also did not reach the design value; the force in the surface of the membrane was re-distributed. Hence, the stress in somewhere was decreased, while in otherwhere increased.
In the regions that the stress decreased, the stiffness of the structure also lower, the membrane will vibrate under the wind easily. In the regions or direction that the stress increased, the overloaded stress will induce large strain and large time-depandent deformation, moreover, it may make the membrane torn.
The reasons of wrinkling have mentioned in previous posts but not in particular. Which friend can analysis in detail?.
Thanks to sillyboy and zhaoyu. I talk a little more about the wrinkling problem.
For analysis wrinkling problem of membrane structures, there are three types of theory, i.e.
1. Tension Field Theory;
2. Wrinkling Theory; and
3. Critical Compressive Stress Theory.
No compressive stress to be allowed for the membrane in the tension field theory and the wrinkling theory, but in the critical compressive stress theory, a small compressive strss is allowed.
For finite element analysis of wrinkling problem, there are two types of element available, i.e.
1. base on tension field theory and wrinkling theory, use membrane element with no bending stiffness;
2. base on critical compressive stress theory and bifurcation buckling theory, use shell element, treat the wrinkling problem as a buckling prolem.
Why I have interesting in this topic?
When perform load analysis, a few researchers use a factor for reduce the stress of wrinkled element, but in view of mine, wrinkling means the stiffness of the membrane is not adequate, or in other words, the initial geometric surface is unresonable, in such case, the analysis should be return to the formfinding and modify the initial curved surface.
The formfinding, load state analysis(include wrinkling analysis) and cutting pattern generation should be a whole-process.
Current software, such as Surface and Patterner, do not include the load analysis module; others such as Easy, Forten32, WinFabric etc. can perform formfinding, load analysis and cutting pattern generation, but can not perform wrinkling analysis.
What you have written is reasonable. But it is similar to doing some optimise if it returns to the step of formfinding. I am not sure whether I have got it or not. However, it is inconvenient unless the software is intelligent. Is there an alternative? How about to modify the stiffness of wrinkled membrane element?
Maybe we can call the modification as a shape optimization. There is a paper about the shape optimization in this forum, but I can not open it ( I can not open CAJ file since there is no Chinese Word in my computer). I met the author when he was a MEng candidate. I have interesting in knowing that whether the shape optimization is based on the results of load state analysis.
I am not very clear the meaning of " to modify the stiffness of the wrinkled membrane element". As a numerical method, we can modify the stiffness of wrinkled membrane element ( to reduce or ignore the stiffness ), but what about the result? Maybe we can say that we have consider the affect of the wrinkled membrane element in our analysis ( as I read in a few papers ), but if we do not modify the structure ( to modify the B.C or prestress ), the structural stiffness does not changed.
Which one should be modified, the stiffness matrix or the structural stiffness ?
This is an interesting topic; I have these additional issues to be considered.
a. More often than not the structure is different from the computer model. One example of this is typically we have cable and fabric connected together in the model but separated in reality. In order to re-distribute the imbalance force from the cable (cable now wants to have constant force in the entire length), the stress in the membrane can no longer maintain uniform. It is not a problem for an open fabric area, but it will cause wrinkles at corners and edges.
b. When there is not enough definition in the patterning model to represent the curvature in the membrane, wrinkles will happen. The mast top is a good candidate.
c. The sequence of stressing the membrane system can influence the outcome of the look of the membrane as well. Extra care is warranted when you have two catenaries cables merged at a narrow angle to a connection point. The friction between cable and fabric can cause wrinkles.
This is a joined effort among architect, engineer, fabricator and installer. Do not take this issue lightly because first impression is so crucial for our business.
