By Mueed Rasool
Highly qualified researchers after conducting thorough studies have authenticated that Kashmir valley falls in Zone-V in terms of earthquakes. The state faces disquieting earthquakes time and again. And researchers have predicted that future earthquakes could be more devastating for the Valley. To prevent loss of assets as well as human lives, some contemporary techniques are essential.
Advanced structural engineering also means continuous research on designing new structures which will not only be safe but economical. Tensile structures are the best solution against earthquakes due to their light weight. Such structures are gaining popularity due to their high strength and lightweight properties.
Advantages of tensile structures
The main advantages of tensile structures are:-
1. These structures have good durability and strength.
2. They have high fire resistance properties and can be used for industrial too.
3. Due to their light weight they have high structural efficiency against earthquakes.
4. These structures have better aesthetic appearance and satisfy architectural designs.
5. Due to lesser structural weight, they are more economical.
6. These structures are not subjected to bending, shear or compression, which makes the design easy and economical.
7. They are pre-stressed to the extent that they are stable even after application of external load.
8. Due to their non-linear behavior they are seismically safe.
Utility of Tensile Structures
Due to these advantages tensile structures can be used in:
1. Hospitals, where large space is required with a lot of ventilation and good light.
2. Mosques, where column-free space is required to avoid obstructions.
3. Exhibition centers, where large space with good acoustic properties is required.
4. Shopping malls, where a lot of glass cladding is required.
Loading and structural behaviour
The change in load is likely to have a large effect on the size of structural membrane required and the deflection in these structures. The structures are usually ‘compensated’, such that the fabric material achieves predefined pre-stress levels at the correct geometry. The uplift due to wind pressure is a critical case for membrane and cable stresses in light weight membrane structures. It is considered a static load case defined by dynamic pressure multiplied by a pressure coefficient.
Here, it may be noted that earthquake acceleration has no impact on these structures due to the light weight.
Many engineers and the public are unaware about the structural behavior of tensile structures and it is essential to know their structural behavior and failure mechanism. Tensile structures have a membrane-type structure and the load applied should be resisted by tensile strength in the membrane.
The initial curvature is so kept in the membrane that stresses remain within acceptable limits. This requires that membrane be placed in a state of internal stresses or pre-stress. The members act like an umbrella and create tension in different ways, depending on the area to be covered.
Tensile structure VS traditional structure
Tensile structures are preferred as compared to traditional buildings. Buildings developed from traditional material, like masonry, are quite often built of other materials, which may be homogenous or heterogeneous, which creates a ‘wrong architectural vocabulary’. However, it is also a fact that the architectural reputation of membrane structures is yet to be established due to their recent development. But as familiarity increases, they will be employed more often for their architectural form.
Material used in tensile structure
The fabrics used in tensile structures are classified under three categories.
Coated Fabrics: These are symmetric structures of yarn arranged in two orthogonal directions. The structural function is provided by the yarns, and coating protects it from chemical and biological reactions.
Open Mesh: These are used where ‘weather lightness’ is not required.
Foils: They are used for pneumatic applications, but due to their reduced mechanical properties cannot be used for large cushions and single skin envelops.
These fabrics are given a coating of PVC, polymer, synthetic silicon and rubber polymer.
The fabric skin acts as a passive filter capable of modifying both thermal and light levels with the enclosed space to minimise the reliance on conventional energy resources. In addition to this, these tensile structures can be used for better climate regulation and utilising natural light, thereby reducing the usage of non-renewable resources.
—The writer is an assistant executive engineer with the PWD