2. Review 2.1 History ofRooftop

Even though, this rooftop concept became popular only in recent times, it has relatively long history back to early stages ofNineteenth century. Rooftop towers have been constructed on rooftops of numerous in developed countries for various purposes in mid nineteen sixties. Transmission tower on Marina City (USA) is one good example for these old J roof top towers.

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Fig 2.1 -The transmission tower on Marina City (photograph has been taken in 1%7)

In Sri Lanka, there are some roof top towers, which have been constructed over 25-30 years ago. The tower at rooftop of SLT headquarters building at Colombo Fort is a notable example. This tower has a height of 30m.

Construction of a large number of roof top towers in Sri Lanka was initiated only in recent times after the introduction of mobile technologies such as CDMA, GSM. Presently hundreds of rooftop towers, having heights from 2m to 30m, are observed especially in urban and sub urban areas.

PageS Fig 2.2 -30m Roof top tower on SLT headquarters building

2.2 Configuration of Towers , <~· Roof top towers have different structural forms. These forms can be categorized under two main categories.

Self Supporting towers:

Towers which are having self stability without the additional supporting systems are called as self supporting towers. Following structural forms come under this category;

Monopoles

Self supporting towers

Non self supporting towers: ~ Towers, which require additional supporting systems to maintain the stability of tow~~ - are called as Non self supporting towers. Structural forms that come under this category are;

Guyed Mast

Page6 Pole with supporting struts

Selection of a structural form for a particular case depends on various aspects. Some of those aspects are;

• Space availability on roof top

• Required height for the tower

• Aspects related to fixing of tower to the existing structure

• Aesthetic aspects

One has to carefully consider all of the above aspects as well as any other specific aspects for a particular case, before selecting a structural form as an effective and efficient solution.

2.2.1 Monopole

A single self supporting vertical pole without guy wires or supporting struts is referred to as a monopole. Normally, a monopole is formed by tapered steel tubes that fit over each other to form a stable pole. Usually, these tubes have cylindrical or multi sided shapes like octagonal, hexagonal cross sections and these are produced out of galvanized steel or high tensile steel. But in some instances, the entire pole is constructed by tubes of the same size anti tubes are connected to each other by using connecting plies. This method is adopted when the height required is comparatively less.

Fig 2.3-Two Monopoles on a rooftop ofbuilding in Colombo

Page7 Monopole is becoming a popular structural form especially for shorter towers. Possible reasons for this popularity are that the,

• Space required for a monopole is quite small when compared with other structural forms.

• Monopole seems to be the most economical solution; if the height required for the tower is less and wind area of antennas is less.

• Effects on aesthetic aspects of the existing building is minimal, if it can use a monopole as the roof top tower.

However, use of monopoles is not possible when greater heights are required for towers. Fixing of monopoles to existing structures by chemical anchoring becomes impossible under large induced bending moments that develop at the base level of tower, in monopoles that have greater heights.

In Sri Lanka Monopoles are normally used only if the height required for the tower·is less than lOrn.

2.2.2 Self Supporting Lattice towers

Self-supporting is the most commonly adopted structural form for both rooftop and green field towers. A self-supporting lattice tower behaves as a vertical cantilever structure, when the whole structure is considered as one unit. However, it can be considered as a space truss in the structural analysis and design for convenience.

This structural form can be considered as the strongest form of tower structure and it can be designed to withstand very large wind loads, where it is not possible to use any other structural form. However, self supporting lattice towers are comparatively expensive compared with other structural forms. But the space required at the base level is comparatively less, when compared with structural forms such as and pole with supporting struts.

PageS Fig 2.4 -Self supporting roof top tower in Kandana area

Four legged and three legged towers are equally used as self supporting lattice towers. Neither of these can be identified as the optimum shape. ,•·

Self-supporting lattice towers are very commonly used as rooftop towers in Sri Lanka. This form of towers adopted for towers with heights of 30m. However, most of roof top lattice towers have a height of7m to 20m.

There is speculation that this structural form is uneconomical when towers are relatively short and wind area of proposed antennas are relatively small. Therefore, self-supporting lattice towers, which have heights less than 7m, are not a common sight.

2.2.3 Guyed Mast

Guyed mast is a structure, which is pinned connected to base and braced with guys. Center mast can be formed by a triangular or a square lattice structure or as a monopole and it may have tapered or straight shape in vertical profile.

Page9 Fig 2.5 -Guyed mast with square lattice structure as center mast in Wattala area

Generally, guyed masts are lighter in weight and relatively inexpensive when compared with other structural forms, but it requires the greatest amount of area in plan. This is due to the space required for cable guy wire stays, which are to be anchored at least in three directions since these wires can only take tensile forces.

This structural form is selected under local conditions when height of the tower is relatively ,"· short. Normally, this structural form is used for towers, which have height from Sm to 15m in Sri Lanka.

However, Guyed mast towers are relatively less in number compared with other structural forms like self supporting structures and monopoles in Sri Lanka.

2.2.4 Pole with supporting struts

A vertical pole with inclined supporting struts (at least two or more) is another structural form used in roof top tower . This structural form has been developed to overcome the shortcomings of structural forms such as monopoles and guyed masts.

Bending Moments develop at the base level of the pole can be significantly reduced by providing supporting struts to the pole. Therefore, this structural form can be used when it is required to erect a pole of greater height.

Also, installation of struts only in two directions is enough to stabilize the pole effectively,

Page 10 since struts can take both tensile and compressive forces. But, this is not possible if cables are used instead of struts (as in guyed mast) since cables can only take tensile forces. Therefore, this structural form can also be considered as an advanced form of the guyed mast system.

Fig 2.6 -Pole with Supporting struts on a roof top

Space required to install a pole with supporting struts is comparatively lesser than in guyed mast due to above reason. Also, this form seems to be quite an economical alternative for self­ supporting lattice towers in cases, where design wind area of the antennas is relatively small.

Presently, Poles with supporting struts is a common sight in most of rooftops in urban and suburban areas of Sri Lanka. Usually, height ofthis type oftowers varies from 5m to 15m.

2.3 Summary

Review that is carried out on roof top towers can be summarized as follows;

Erection of roof top towers is relatively new concept for Sri Lanka, even though there are few roof top towers, which are 25-30years old. However, it can observe hundreds of rooftop towers, which are having different structural forms all over the country at the moment. These structural forms and features of those forms can be tabulated as follows;

Page 11 Type Range of Heights Features Self Supporting Monopoles upto lOrn Space requirment is minimum Usually used when wind area of antennas is less Self Supporting lattice towers 7m- 30m Usually used when wind area of antennas are large Non self supporting Guyed Mast 5m- 15m Space requinnent is comparatively large ,Po!ewith supporting struts 5m -15m Space requirment is comparatively large

Table 2.1 ~ Structural forms of rooftop towers

2.4 Selection of four legged self supporting towers having 15m and 20m heights

As explained in previous chapter, self supporting lattice towers are the most common structural fonn for rooftop towers especially if the heights of the towers are more than 1Om in Sri Lanka. According to records of telecommunications operators, at least half of the rooftop towers in the country are taller than 1Om and most of those towers are self supporting towers. Also, four legged type seems to be the dominant structural form for these self supporting lattice towers.

It is decided to focus mainly on four legged self supporting rooftop towers considering this situation. In this research, 15m and 20m height are selected since those are the most common ,"· heights of existing four legged self supporting towers in Sri Lanka. Rooftop towers, which are taller than 20m are very rare, since most of these towers are installed in rooftops of buildings, which are not initially designed for tower loads.

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