地点：宜兴
时间：2005年3月4日
前去宜兴办事，路过此膜结构。当天风很大，在车上看到这个膜随风振动，位移比较大，大得离谱。我的直觉告诉我可能会出事，于是下车前去观察，拍摄到以下照片。膜结构的事故也见得比较多了，但都是事后到现场或者是通过照片了解，象这次亲眼目睹膜的破坏全过程倒是第一次。印象深刻。贴上来与各位分享一下Bz的感觉！
  
1、破坏前

2、成了充气膜

3、另一角度看气球

4、撕裂

5、撕裂近景，我冒着生命危险拍摄

6、柱被拉倒

7、背拉杆断了，螺纹处断裂

膜面刚度严重不足，强风作用下膜面曲率发生突变，膜面翻转，成了兜风的降落伞。
  
这种现象类似于传统结构的失稳，乃张力结构设计之大忌。

也许是因为支承结构的刚度不足，导致了膜面的应力松弛，刚度下降。如果是动力原因，膜应该会不停地振‘啊振啊，然后才坏。。。

偶觉得问题就在最后一张图上，看起来好像是脆性破坏，说明这个螺杆材料有问题，要么是劣质产品，要么是疲劳损伤。
  
这个点破坏之后膜面没有足够的支撑，导致整个膜瞬间向上鼓起（图2，支柱已经歪斜，膜这时候至少还完整）。这又使另一个角突然承受很大拉力，不但使这个角的柱子破坏，而且膜与膜角板脱离，在大风持续作用下产生“脱裤子”现象，然后风还是刮个不停，最后膜全部撕裂。
  
随便猜猜的，这个风还真大啊。

那天，风是大但肯定不是50年OR 30年一遇。旁边不远也有一个膜结构还不是好好的没事。

非常好的教材，值得认真研究，风载下的膜是膜设计的难点也是研究的热点！

请大家从此好贴中吸取经验，最近膜破裂的例子太多，今天看到曹溪北路附近的一个小品已经破了，前不久路过常熟北路收费站也是冒险过去的（已经积成池塘），听说义乌体育场最近也破裂倒塌。
在此呼吁大家一起来保护膜结构市场，只有将膜蛋糕作大，大家才有饭吃。

感觉首先是拉杆设计有问题，图3那个未破坏膜拉杆的显示来看，两根索和杆的夹角很小，这就象一个三角架，如果夹角太小，容易倾倒。从图3显示表明，沿这根杆的对角线方向，未破坏的膜也有大面积的松弛和一定的上鼓，估计这根杆有了一定程度的摆动。
因此我觉得这个膜的破坏，关键是拉杆设计有问题，主要是拉杆的破坏才造成了膜材的破坏。

These are priceless pictures. This is not a criticism to Goodidea, I only wish there are more.
  
Picture 1 shows 2 sails properly connected to the supporting elements. From Pictures 2 & 3, we may see that fabric had slid down the catenary cables from the top connection to the building center tower. Picture 2 also indicated that one of the posts (on the far left but not shown in the picture) was leaning to the right and the post right to it leaning to the right as well. It is also evident that fabric was in contact with the roof top railing, the railing cut the fabric while fabric/cable dented the railing.
  
Though it is not clear where does this broken clevis belong to and not knowing what type of membrane plate design for all the corners, I can see there is a broken cable in Picture 6, looks like a Guy cable, I come up with this scenario of the event.
  
1. A Guy cable attached to the left post in Picture 6 broke.
2. This post lost equilibrium and began to tilt to the right.
3. Catenary cables attached to this post lost tension.
4. Fabric began to slide down the Catenary cables.
5. Loose fabric caught a lot of wind, the rest is history.
  
May be there are lessons learned here:
  
1. Any structural system should have certain redundancy build in. If a structure like this one is counting on the fabric to maintain stability, other security measurement should be put in place such as overhead tie cables.
2. Quality control of all essential items such as cable clevis. A qualified clevis should be able to survive at least 200,000 times cyclic dynamic loading test.
3. The cause of the broken clevis seems to be fatigue. One problem would be the quality of the clevis; the other would be the tension in the system was not enough. When pre-tension is low, too much vibration will damage the clevis.
4. Corner fabric was not held in position with clamping bar. Without the restraining device, fabric could slide down the cables creating the bloom.
  
Luckily no one was hurt. We need to pay attention to small details even for small structures. Just like a tailor, it requires the same process to make a shirt for a giant or a new born. The difference is only in quantity.

感觉膜的找形迎风面积太大,像船帆一样,

也许不是设计的问题。  我认为这个工程的破坏更多的原因该是施工！施工时膜基本没张紧，导致刮风时膜轻易被鼓起来。
如楼主所示图片，整个结构刚性太差，估计施工后膜体内部预应力最多仅能达到0.2~0.5。这相对于设计一般所取的值1~3kN/m2相差太多。
“膜张紧后可以在上面开汽车！”――――这是经常用来告诫施工队长的一句话！！

膜结构的市场啊，被这种老鼠屎的工程败坏殆尽。

前几天我看到的一个工程（另部分膜前段时间曾有DX发过贴子），我用手拉拉膜的钢索，能不费力地拉动膜面。我曾经在车上远远地看到过他们安装，这是在安装完几天后的事情。

如楼上图片所示工程，膜太松了。还是那个问题，施工中膜没张紧，没给膜本身施加预应力！简单点说，膜只是披上去而没经过张拉。这种工程遇上大风估计还是要出问题。 。  另外，压膜夹板与钢构之间的连接好像是用索，个人感觉这个节点还是不要用索，可调节的范围太小，很难对膜材施加足够的预应力。

请问各位高手，实际施工中如何确定膜面中预应力的大小？除了评经验以外，有没有真正的科学的定量的测量膜面预应力的方法与技术？据我所知，目前国内还没有这种技术，这才是问题的所在！

如楼上图，索是机械式自锁的，其索头与索的连接全靠现场工人的责任心，有的在张拉施工的时候就会断开，即使安装好了，但在风荷载的作用下随着索的振动也会慢慢松动，前几天看见一个工程就是刚作好半年，脊索就断开了，膜非常松。其实很多膜结构破坏的工程都是因为索断开，使施加在膜上的预应力消失导致破坏的。所以要想保证质量就得所以公司决策人重视，不要偷工减料，共同来维护膜结构的声誉。

This is in reference to T_HOOPER's picture.
  
A few things I would like to point out for discussion:
  
a. It appears that the only tension still remaining in the fabric comes from self-weight gravity force. It requires re-tensioning in both WARP & FILL directions immediately. Other than Fiberglass material fabric, it is essential to have re-tensioning mechanism designed into structural elements. Without proper tension in the system, this structure will be easily damaged by either wind or snow.
b. Some adjustability in cables can re-tension the system. Instead of cables, steel bars with threaded ends can easily achieve what we need. Turnbuckles may be expensive for this application.
c. I hope the fabric corner is reinforced with bias patches.
d. I also hope the cables are galvanized.
e. Design of cable anchor is also not without questions. 1. Why anchor bolts are not parallel to the cable similar to the post anchor? 2. Why the line of action is not through the center of the anchor? 3. Why anchor bolts spaced so far apart? 4. Do we know the base plate is thick enough for bending?
f. The concrete work for the anchors is suspect. If the whole anchor is only the concrete block we see in the picture, the weight of it is simply not enough to resist the load from the fabric. I do not believe this is the case. But the other possibility is also structurally wrong. It appears that the top block is cast separately from the underground portion. This means the top block can move independently and bend the anchor bolts. I don’t believe these bolts are designed for bending.
  
This is alarming. When designers don’t pay attention to seemingly innocent details, we are in for trouble.

1、应该说，在风荷载不大的情况下。两片膜之前已经丧失了预应力，并且边界点（膜角）已经没有刚度了（可能膜布已从膜角板处脱开，随着边索滑动，或支撑结构-落地拉索已经破坏）。如图1。
2、在风荷载继续作用下，膜布逐渐从膜角向两边滑落（slide down） ，逐渐被撕开。同时远端的支撑杆倾斜，一根拉索破坏，柱脚耳板弯曲屈服，丧失刚度。如图4。
3、davidchen分析得非常有道理！When pre-tension is low, too much vibration will damage the clevis（预应力的不足，结构振颤会摧毁整个结构）。从受损的配件看到，某些销轴截面已经减小了近1/3！
4、破坏的直接原因很难讲得清楚。很不幸，因为它有很多缺陷！

1、当初完工时的情景

2、可能破坏的节点

3、可能破坏的节点2

4、曾经出现积水的区域

据我个人了解的情况：
这个工程实在是不幸！
1、在安装过程中，连接膜角和钢构的U型件，比设计低了一个级别，如图3。
2、找形设计时，膜面有些过于平坦；施工阶段，预应力施加不足。留下了积水的隐患。
3、巴氏合金浇铸质量出现问题――起因。
     安装完毕后的大约半年，落地拉索一端的巴氏合金松动，丧失预应力，膜面出现积水。
     在安装脊索方案失败后（又出现积水）。将膜布拆除，拉回工厂对膜布进行改动（如何改动，不详）。
    经过多次修补（期间，设计人员已经离开该公司），返工，膜布已有很大损伤！尤其膜角范围处的边索套，几乎已经磨穿。
4、如goodidea抓拍的照片，在风荷载不大的情况下，可能膜布从膜角板脱落，受力状况发生很大的变化，使落地拉索屈服拉断（从上图看似乎有颈缩的现象）。从而导致整个结构翻转！
  
可惜！大家引以为戒！

ayamm:
    "听说义乌体育场最近也破裂倒塌。"
   确有这事吗？