To achieve desired strength of tower, use of HSS (High Strength Steel) instead of normal mild structural steel will result in lighter tower (Not necessarily cheaper tower because of cost differential of normal mild structural steel and HSS). Reduction in weight may be very important in certain applications e.g. airplanes, motor cars etc. In such applications, irrespective of the initial cost, low weight results in low operational cost. In case of towers, operational costs do not depend on weight of tower. Hence no special advantage is obtained by using HSS. HSS is normally used by manufacturer to achieve cost savings.
The strength of tower is determined by “Total Resistance” offered by the tower to the applied loads. Total resistance offered by the tower is ‘Strength of material per unit area’ X (multiplied by) ‘Area of material available for resistance’ If ‘Strength of material per unit area ‘ is less then designer provides more ’ area of material’ to get the same total resistance. If ‘Strength of material per unit area’ is more then designer will provide less ‘area of material’ to get the same total resistance and not increase the cost unnecessarily. Designer will normally build some reserve structural capacity in the tower. Other things being equal, a factor of safety of 1.5 me Answer a reserve capacity of 50% is provided in the design. Total structural capacity (required + reserve) is determined by the decisions of the designer and not by the strength of steel. As a matter of fact, tower can be constructed with any structural material. We do have towers made of reinforced cement concrete, Aluminum, wood, combination of RCC and steel etc. Whichever be the material of construction, adequate strength of tower can be achieved, provided the designer has done his work properly.
It depends on
a) weight of steel saved in tower by using High strength steel and
b) price differential between high strength steel and normal mild structural steel.
Some members are designed on the basis of permissible slenderness ratio. These members are not made from HSS. Only few members of tower are made from HSS generally.
If the steel used is normal mild structural steel, which has not been heat treated to achieve any increased strength then galvanizing does not affect the properties in any way. If steel used is work hardened, then hot dip galvanizing (dipping in temp up to 415deg C will certainly have some annealing affect on steel, which may reduce the strength of steel.
In a situation, where it is not possible to measure the various parameters which affect the quality of material, country of origin of material may be considered to be a reasonable assurance of quality. Today, when all parameters, which have effect on quality, can be quantitatively measured, there is no significance or relevance of the origin of materials. Quantitative testing to ascertain value of desired properties should be used to establish quality of steel. For quality of tower to be as desired, the strength of steel should be as assumed by designer in his design calculations.
End use of steel determines the properties required from raw material used. If the end use of steel is to cut another piece of steel on a lathe machine then it is important the steel must maintain its strength even when it is red hot. If steel is to be used for making an acid tank, it becomes necessary that the material should not be attacked by that acid. Resistance for other acids may not be of any consequence. If steel is to be used as stampings, then the magnetic characteristics of the steel are important. All properties are not important for any one end use. For example the magnetic properties of steel may not be of any importance, if it is being used as a structure. When end use of steel is tower manufacture, then the following properties should be evaluated.
a) Yield strength - elastic strength of steel
b) Percent Elongation - ability to bend and return to original shape
c) Chemical composition - for establishing grade of steel.