• Question21. Why each country makes its own standards for same product?

    There is always urgent need of standardization, whether it is a small manufacturing unit, a small community or a country.  For example Britain has made a standard for electrical Plugoe for making electrical connection to a wall socket. The diameter, distance and number of pins of the electrical plug is different from other countries. This difference does not make it better or worse than plugs made according to standards of other countries. But it is necessary to adopt a standard so that all electrical equipment manufacturers of that country make provision for that type of electrical connection in all their equipments. Each country has different types and grades of materials and technology available within the country. Each country makes its standards to make best use of
    a)the materials available within the country
    b) the technology and manufacturing facilities available within country
    c) the environment of country
    Objective of standard is always to make a predefined quality/size of product to ensure compatibility /inter-changeability with other products available in that country. As the scene changes or new technology becomes available, the standards also are modified or deleted and new standards are introduced to suit the new environment of the country.
    Today when the world is becoming a global village, there is more and more need of exchange and compatibility of equipment and products between the countries, therefore more and more work is being done on International Standards. Some International standards are already available today. As long time and effort is required to finalize a standard, it will be quite some time before International standards are available for all products. 

  • Question22. Is tower steel as per Standards of different countries equivalent?

     Yes. Each standard of any country normally describes several grades of steel. If end use of steel is tower manufacture, only structural properties need be compared. It is always possible to find an equivalent steel for the grades normally used for tower manufacture.

  • Question23. Which is the controlling load in tower design?

    For reasons of economy, all the loads are not considered to be acting simultaneously. For example: When there is earthquake, then wind load is not considered and vice versa. Normally wind load + ice load (whenever it is present) is the controlling load.

  • Question24. Are towers designed according to standards of different countries comparable?

    YES. To a large extent, objective of standard of any country is a get a safe design with minimum cost/materials. This objective is achieved by makers of standards in their own way, which is suitable for that country. Depending on the type of data available in that country, they work out the load on the structure.
    If there are any gray areas, where data is not being gathered in that country, then that shortcoming is covered by a factor of safety (typically a multiplier up to 1.5 , figure depending on percentage of gray areas ). In a situation, where all relevant data is being collected and is readily available, then standards are so made as to take care of each load contributing factor. Factor of safety in such a situation is kept low e.g. 1.05 or 1.1. In either of above 2 cases, the tower after design and fabrication is destructively type tested .This test will confirm that the design is correct and safe in all respects. Although approach to design will be different but equal level of confidence in safety of final product can be expected in either of the above two cases. In most cases, customer is interested in building some reserve capacity in his tower for future loading of antennas. In such a case, he specifies overriding factor of safety as 1.7 or 2.

  • Question25. How does quality of FABRICATION affect the strength/ Safety of tower?

    Fabrication is an important factor of design for the quality of tower. If fabrication of tower is not proper then structural capacity of tower may be reduced considerably.
    Following situations explain, how that happens-

    a) If distance between two holes in a member is less than required (by say 5 mm), then this member will have to be fitted forcefully. After forceful fitment, this member will exert a tremendous force on members to which it is attached. This force is over and above the forces which will be generated by external loads. Hence this member has been effectively reduced in strength to extent it is experiencing forces in no external load condition. If several members are fitted in this forceful manner then effective strength of tower will be reduced.

    b) In a leg joint, all the bolt in a joint are expected to act together, when external loads are applied. If during fabrication, distance between holes is not maintained uniform, then all the bolts do not act together. In such a situation, phenomenon of progressive failure takes place. This will be equivalent to reduction in strength of joint.

    c) If the holes for bolts are made larger than required, then the effective strength of bolts will be reduced by 15%.Hence , if designed strength of tower is to be achieved, it is necessary that fabrication of tower material be done by an experienced fabricator. All the advantages obtained by good design can be more than offset by poor fabrication. MEGATRO takes pride in quality of fabrication and has experience of more than a decade of fabricating towers.

  • Question26. How does workmanship in erection affect the strength of tower?

    Self supporting tower has a large number of members. Quite large numbers of members are only marginally different from each other. If during erection, erectors fix a member with slightly larger length in position of member with smaller length, then the member will be stressed even without any external load. The strength of the member will be compromised to that extent. If the tightening sequence is not properly followed, then that will also result in stresses in members of tower.

  • Question27. How does detailing of design affect the strength of tower?

    Designer provides size of each member. He also provides number of bolts and diameter of bolts at each joint. After that it is responsibility of detailer to make production drawings without sacrificing the strength of tower. For example, if the distance of each hole from rolled edge/ cut edge is not maintained, according to recommendations in standards, then the joint does not get the desired strength, notwithstanding that diameter of bolt and numbers of bolts are according to recommendation of designer. British standards insist that detailing of tower design be appraised by an independent appraiser, before production is taken up.

  • Question28. How does maintenance affect the available strength of tower?

    Tower is a steel structure. Available structural capacity of tower at any time will be determined by available strength of each member. If members have corroded then obviously the available strength will be reduced to that extent. Looseness of nut bolts also affects the strength of a joint. As a matter of fact, if high grade (8.8 or higher) bolts have been used, then the joint strength depends on amount of tightening also. Hence it is mandatory that maintenance of tower be done as recommended by the manufacturer, if the design strength of tower has to be maintained.

  • Question29. What are the pitfalls of specifying required performance of a tower?

    The requirement specifications given by the buyer can be read in different ways. The result could be that, although the supplier claims that product offered by him meets the requirements specified by buyer yet it could be 30% less than some other supplier, who has read the requirements in a different manner. Variation occurs due to different interpretations, which are possible in absence of clear definition of requirements. Described below are certain terms, which can be interpreted in different ways. Wind speed: Wind speed varies with time and height. Different countries have different averaging intervals. For example, Canada has one hour. Denmark has 10 minutes. Soviet Union has Approx 2 minutes. India has 3 seconds. UK has 3sec to 15 seconds depending on vertical dimension of the obstruction. USA has fastest mile speed, which translates into 30 seconds for 120 miles per hour (or 60 seconds for 60 miles per hour). Similarly the height for basic wind speed also varies with country. Uk has sea level. India has 10 meters above existing ground level. When the buyer specifies wind speed for design purpose, then he does not mention which type of speed is being specified. 3 sec wind speed is about 30% more than average hourly wind speed. It is suggested that design wind speed be specified. Or alternatively Design wind pressure in tons/sqmeter or pounds/sqft may be specified. i.e for telecom tower, Antenna loads Buyer specifies the size, weight and level of installation of antennas. Direction of antenna installation is either not known or not specified. Amount of shielding provided by various antennas has to be decided by designer. This decision could make a lot of difference in loading on the tower. And hence on cost of tower also, It is suggested that antenna loading may be specified in square meters of wind area. Factor of safety If buyer has not asked for any factor of safety, supplier may decide to offer a tower with factor of safety as 1.0 to make his offer most attractive. It is suggested that a factor of safety be specified.

  • Question30. Should the bolts used in my tower be high strength only?

    Strength of a joint is determined not only by the strength of bolts but by the number of bolts also. Whenever a joint is designed with high tensile bolts, designer will reduce quantity of bolts per joint because lesser quantity of high tensile bolts will provide the required strength in the joint. MEGATRO prefers the option of using lower strength and larger quantity of bolts on each joint. This is a better option because:
    a) Kl/r ratio is improved and hence better rigidity is achieved.
    b) Tightening of bolts up to a predetermined level does not remain important.
    c) Presence of paint or dust or grease under the bolt surface is not of any consequence.
    d) Tolerance for misalignment of members is increased.