Australian standards on Tower classification

 

1.5 TYPE OF STRUCTURE:

1.5.1 Type I A structure shall be classified as Type 1 where- (a) the structure is designed to provide post-disaster communications services; or (b) the collapse of the structure and loss of services provided causes unacceptable danger to life or extensive economic loss. 1.5.2 Type II A structure may be classified as Type II where- (a) the danger to life in case of collapse may be negligible and adequate warning arrangements are incorporated to ensure the general public is not unduly endangered; and (b) the loss of services provided is not critical, e.g. where alternative means f communication can be provided. 1.5.3 Type III A structure may be classified Type III where all consequences of failure and more tolerable than those specified in Clause 1.5.2

 

 

LOADING:

1.6 LOADING the following loads shall be considered in the design of the lattice towers and masts, and their structural members and connections: (a) Dead loads Soil Types- Soil gauge Wind Regions throughout Australia and Asia Pacific. *mapping along with details descriptions

 

Structural Analysis and Design

3.1 STRUCTURAL ANALYSIS

3.1.1 General Freestanding lattice towers shall be analysed using a first order linear elastic method/ For the analysis of lattice structures which are supported by guys. the non-linear properties of the guys and other secod order effects shall be taken into account. For the application of the design provisions of the section, the axial forces in triangulated lattice towers shall be determined by assuming all members are pin connected. A rational design may be used in lieu of the design procedures provided in this Standard. In such case, it shall be ensured that the adopted procedure will lead to a level of safety and performance equivalent to tat envisaged by, or implicit in, this Standard. Where guys provide supplementary stability the inherent stiffness of a tower, the analysis of guyed lattice towers shall be in accordance with this Standard, taking into account the non-linearity of the guy and tower interaction. Adequate allowance for torsional effects due to asymmetric positioning of antennas shall be considered in the structural design. For guyed masts with torsional outriggers, secondary torsional effect due to the outriggers' reaction shall be considered. Members of the torsional outriggers shall be designed to allow for the worst combination of guy tensions. 3.1.2 Guyed Masts The stability of guyed masts shall be ensured by considering guy displacements under critical loading conditions. In addition, serviceability performance under lesser loading conditions shall be considered. As a minimum, a quasi-static analysis shall be considered, taking into account- (a) the full non-linear, second order effects caused by the changes in the structure geometry; (b) all loads on guys; and (c) maximum shear forces and moments resulting from a patch loading analysis for guyed masts taller than 150m

Footing and Design

4.1 GENERAL

Footings for freestanding lattice towers and guyed masts shall be designed to resist all load combinations applied to the structures in both vertical and horizontal directions combined with bending moments where appropriate, particularly at the ground line of freestanding lattice towers. A rational analysis shall be used to determine the strength and deflection of the footing.

4.2 PERFORMANCE OF FOOTINGS

The performance of the footing under short-term and long-term loading's shall be considered. The resulting strength and deflection shall be considered in the design of the lattice tower. The effects of variation of water content of the soil shall be taken into account and shall include the allowance for reduction in the weight of materials due to buoyancy.

Criteria for Assessment of Existing Structures:

5.1 STRUCTURAL ASSESSMENT

Existing steel lattice towers and masts, and other supporting structures, shall be assessed when changes occur to- (a) the antenna or ancillary conditions leading to a loading variation; and (b) the operational requirements. e.g. twist, sway.

5.2 GENERAL DESIGN REQUIREMENTS

Before an existing structure is modified or additional loads applied to it, the following shall be considered (a) The physical condition and loading o the structure shall be verified by inspection. (b) The structural adequacy of the structure shall be evaluated in accordance with the Standard. The application of a reduced importance factor (see clauses 1.5 and 1.6.5) may be considered excessive in some instances, especially when the structure has survived quite well for some years. In these cases and where the reliability of the structure has been proven, it is considered appropriate to introduce a reduced importance factor concept into the evaluation of existing structures.

Maintenance and Inspections:

A1 GENERAL

The reliability of a structure depends on the quality of materials and construction, and the adequacy of maintenance provided after installation. This Appendix provides a general framework for maintenance and inspection. A maintenance program will involve planned inspections and repairs during the life of the structure. Records should be kept of all inspections, modifications, facilities and equipment placement, and repairs carried out.

A2 SCOPE OF MAINTENANCE INSPECTION

the scope of maintenance and integrity, and to the servieability of the structure and its communications functions. The inspections should cover, as far as possible, the following: (a) Loose or missing bolts. (b) Fatigue cracking. (c) Damage from structural overload. (d) Vandalism (including firearm damage) (e) Corrosion of galvanised steel work. (f) Degradation of paint systems. (g) Lightning damage (h) Foundation deterioration an cracking. (i) Loose or damaged guy wires and fittings. (j) Vibration. (k) Ground surface erosion. (l) Evidence of soil creep or landslides (m) Settlement. (n) Earthing integrity. (0) Auxiliary antennas, mountings and feed systems. (p) Maintenance of safety facilities. (q) Site security. (r) Guyed mast vertically and twist. (s) Navigation lights (t) Condition of insulators.

A3 MAINTENANCE INSPECTION FREQUENCY

The inspection intervals need to be tuned to the operational environment and structure/service functional needs. Structures that have known vibrational problems, or are in a very corrosive environment, or are in a very windy or ice environments may require more frequent inspections. The interval between maintenance inspections in particular will depend on factors suck as- (a) corrosion potential of the environment and the degree of protection required for maintenance and design reliability; (b) importance of the structure to its service; (c) severity of local conditions (i.e. wind, ice); (d) sensitivity to structure response; and (e) influence of ground conditions. It is recommended that the interval between inspections should be between two and five years according to the relative importance and the above factors.

A4 SCHEDULED MAINTENANCE AND REPAIRS

Maintenance and repair tasks should be undertaken by experienced crews with appropriate equipment. The replacement of any structural members should be approached with caution and an engineering valuation may be necessary before work commences. On guyed masts, variation in guy tensions may be critical to the performance of the facility. Where inelastic construction stretch is not removed from guys prior to installation, it may be necessary that retensioning be undertake at the end of 12 to 18 months after construction, Guy tensions should be maintained within -+5% of the design values.

A5 ELECTRICAL HAZARDS

Maintenance crews should be aware of any electrical hazards, particularly radiation, while undertaking work on communication structures. Advice on these aspects should be sought from the site owner and referenced to AS 2772.

Guidance for Footing Design:

I1 GENERAL

Theories used to determine the footing strength and deflection which are based on the characteristic soil properties are preferred. The truncated cone earth theoru is widely used but is not always reliable, particually in cohesionless soil or for deep foundations. The following should be taken into account when designing footings: (a) Expansive clays. (b) Changes of the water table. (c) Loss of soil around footing. (d) Build-up of soil around footing. (e) Long-term settlement of fill materials (f) Allowable long-term movement. (g) Maximum allowable movement at maximum load. (h) Suitable placement of reinforcement in any concrete to ensure load-carrying capacity and durability. (i) Selection of footing type and details to minimise construction difficulties. (j) Changes to loading on the tower during the life of the structure. (k) Routing of earth cabling thought the foundation and the effect of settlement. (l) Protection of ground-line components from accidental or malicious damage. (m) Provision of secondary guy anchor points for use during maintenance. (n) Sloping of all surfaces to avoid any collection of water and water born solids.

I2 PERFORMANCE OF FOOTINGS

The sides of all footings and guy anchorages should be placed against undisturbed soil wherever possible. Backfill should be compacted in accordance with assumptions used in the design. The designer needs to ensure that likely deflections of the foundation (soil and footing) will not be detrimental to the tower. Considerations need to be given to- (a) the variation in the load magnitude and duration; (b) changes in soil properties, such as drained or undrained; and (c) changes in the water table level. There are many suitable references available for guidance. In most cases, the worst load condition is for short-term maximum load. For most soils, deflection is not critical. The designer will need to satisfy the requirement to provide a foundation with adequate security. To achieve this, it will often be necessary to take into account the following; (i) The method used to obtain soil parameters. (ii) The reliability of the theory used to predict the foundation capacity. (iii) The degree of supervision during the construction. (iv) The need to minimise damage to the foundation should the tower ever be overloaded. (v) Use of another structure, such as a building, to provide the foundation for the tower. The loads applied to the footing by the tower are those determined from the analysis of the tower. The strength of the footing is assessed on the basis of the degree of certainty of each of the parameters used in the design and construction. Experience has shown that a mass concrete gravity footing is most suitable for guy footing.

I3 SOIL PROPERTIES

The extent of the site investigation will depend on the design methods adopted and the accuracy of information required. Experience has shown that normal design and construction practices in the transmission industry produce adequate strength factors. The following information is provided as a general guided based on Australian experience: (a) Loading type: short-term maximum load due to wind. (b) Dry density: 16kN/m3. (c) Cohesion: (i) For soft clay: 40 kPa (ii) For medium clay: 60kPa. (iii) For stiff clay: 80kPa. (iv) For very hard clay: 150 kPa. (d) Internal angle of friction equals 35* in cohesion-less soil. (e) Friction between in situ soil and compacted backfill (f) Friction on driven or bored piles in soft and saturated soil: 20 kPa Local conditions may however need to be considered in any of the above application. The values given in Items (a) to (f) are ultimately strength values. The soil parameters can be used in the determination of the strength of the footing by using theories provided in many references on soil and foundations.