Structural support detail结构支撑详情分析
In this part, I will suggest a structural support detail about our truss. 在这一部分中，我将提出关于我们的桁架的结构支撑详情。The report is including potential differential thermal movement, how the load transferred, future maintenance and construction tolerance. 该报告包括电位差热运动，如何进行负荷转移，日后的维修和建设能力。
First of all, the definition of thermal movement is the movement of a material resulting from temperature changes. 首先，热运动的定义是由温度变化引起的物质的运动。Building materials move by changing temperature when it heat up or cool down. 建筑材料通过升热或降温的温度变化来运动。This is because of different coefficient of thermal expansion1. The change of temperature leads to the change of the thermal vibration of an atom of materials. It measures the fractional change of the material when the temperature changes one degree at a constant pressure. The linear coefficient of thermal expansion is how much a material is able to expand when the temperature increases one degree. Mathematically, there is a formula for it:
dl: the changing of length in material has been measured
l: the total length of material has been measured
dt: the changing of temperature in material over dl
The value of the coefficient of thermal expansion depends on the structure of materials.
On the other hand, stress builds up where movement is not allowed, or is restricted. 在另一方面，在不允许动的，或者被限制动的地方建立压力。 And also, there is a temperature modulation process in metal truss, such as there are large thermal changes due to solar gain followed by radiation on cool night. 此外，有金属桁架中有一个温度调制过程，比如由于辐射对夜晚凉爽后跟到太阳能增益有大的燠变化。This is called thermal cycling. It has been applied mainly in metal truss nowadays. But there are many buildings still have failed for thermal stresses caused by differential thermal movement. The risk can be reduced by these four methods. At first, materials should be selected carefully. Different material has different coefficient of thermal expansion, and the thermal movement will be different. Next, the surface should be chose with low emissivity. There might be large thermal changes for solar gain followed by radiation on cool night. Consequently, it might increase the effect of thermal movement. Then, applying solar reflective paint is the third way to reduce the risk of thermal movement. The solar reflective paints prevent the temperature of materials heating up efficiently. And the last method is careful detailing to allow some movement. The movement cannot remove absolutely, it is allowed. So the movement should be calculated carefully2.
The difference of material leads to the difference of thermal movement. 材料的差异导致热运动的差异。 For example, timber is one of the most general materials of truss. 例如，木材是桁架最一般的材料之一。And movement accounts for most joints of timber truss have failed. Maybe the building is well protected from the buffeting of the weather. It will be affected by movement from the changing forces of the wind, rain and snow on its roof structure3. For steel and concrete, it is significantly important that the magnitude and direction of design movement be predicted with accuracy. The force may lead to severe damage if movement is not properly accommodated through joints and bearing. Figure 1 has shown a typical damage that may occur if there is no allowed movement3.
For our truss, the cost is especially huge, but the maximum load is also big. The figure 2 has shown the length of supports and how the load is act on the inner of the truss by compressions and tensions. And the red arrows have shown how the load transferred between the truss and the foundation.
On the other hand, future maintenance of the truss is very important and necessary. 另一方面，桁架将来的维护是非常重要和必要的。 There are some potential problems of the supports. 有一些潜在的支撑问题。First and foremost, the inclement weather is a major drawback. The inclement weather leads to fluctuate of temperature. Thereby the magnitude of thermal movement might become bigger. It is considerable dangerous if the movement exceed the allowable value. The heavy snow and rain might add the load of the truss. Furthermore, the steel truss is exposed to outdoor weather condition. Although there is an organic coating system in order to prevent the steel truss from corrosion, the coating system itself is also subject to deterioration. And then the steel would lose its coating system and start corroding.
The first sign of a structural failure may be severe bowing or splits. 结构失效的第一个迹象可能是严重的弯曲或分裂。Through the observation, people got the result that most failure in truss occurs in the bottom supports or in the connections where the edge and end distance from the bolts is insufficient. Therefore, the bottom part and edge should be reinforced. The maintenance actions for bowing or split are repair or replace. If the support is entirely failed, it will be replaced a new one.
The maintenance of coating system can be done to improve the condition of coating system and extend the lifetime of the steel. Typical maintenance methods of coating system are spot repair, repainting and replacement. Spot repair is to repaint the corroding part of steel until it is as well as the normal part of the steel without replacement. In the replacement action, the totally corrosion and old coating have to be removed and change to a new coating. The spot repair and repainting are not able to remove all of the corrosion, so there are still some disadvantages of this method. The replacement is the best way of maintenance of coating. But obviously, the spot repair and repainting is significantly cheaper than replacement action5.
Problems with construction tolerance are common cause of impaired performance and unacceptable appearance in construction, and are potential sources of liability to both building designers and contractors. 建设耐受性的问题是受损性能和不可接受的建筑外观的常见原因，也是建筑设计师和承包商赔偿责任的潜在来源。There are some potential differences in construction tolerances that may occur when comparing structural elements manufactured on site and elements that are prefabricated in a factory. The steel which manufactured on site has to cast from blast furnaces into billets, and then be hot rolled into standard sections or sheets. The steel which pre-fabricated in a factory needs specialist heavy tools and cranes to handle.
They will be cut to size and drilled for bolts in factory. 它们将在工厂中被切割成一定的尺寸并且钻螺栓。And the size has to small enough to transport. 尺寸要足够小以便运输They are erected quickly on site with cranes. Therefore, the construction tolerance of structural elements manufactured on site may bigger than the elements that are prefabricated in a factory. The prefabricated element must have a same size to make sure they can be transport, but the manufactured elements would have bigger tolerance when they are made on site.
References:参考文献
1. Martin Tarr (2007) “Coefficient of Thermal Expansion” (online)
2. Richard Twinch (2007) “Differential Thermal Movement” (online)
3. Barry Hillman (2006) “Historic Ironwork Repairs in Timber-Framed Buildings” (Accessed 16th March 2010)
4. Charles W. Roeder (2002) “Thermal Movement Design Procedure for Steel and Concrete Bridge” (online) (Accessed 16th March 2010)
5. R.P. Nicolai, J.B.G. Frenk and R. Dekker (2009) “Modelling and optimizing imperfect maintenance of coatings on steel structures” (online) (Accessed 16th March 2010)