UNDERSTANDINGFAILURE OF LATTICE TOWERS

 

A SEMINAR PRESENTATION IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE AWARD OF MASTERS DEGREE IN CIVIL ENGINEERING, DEPARTMENT

OF CIVIL ENGINEERING,

UNIVERSITY OF ILORIN, NIGERIA.

SHITTA, RAZAK OLASUNKANMI 

00/30GB064APRIL 2011

Steel Towers have been employed to aid the infrastructural development in the Telecommunication Industry, Electrical Power Transmission Grid, Radio Communication, Military Communication etc.

Within the last few decades the need for tall structures has accelerated with the requirement for effective communication especially with the advent of radio, radar and television. The recent exponential growth in the use of cellular phones has led to a new era of self supporting and guyed towers particularly in a developing country like Nigeria.

LATTICE TOWER USES

Electrical Transmission towerFree standing communication tower

Incidences of Lattice tower failures abound around the world with its inherent economic consequences with having been recorded for both free standing tower and the guyed mast.

It is therefore important for wireless provider, tower owners, Regulatory Organ of Government and engineering practitioners to understand the behavior and failure of lattice towers.

This presentation aims to enlighten Engineering personnel and Tower owners on the need to understand the possible causes of tower failure for them to take appropriate actions to safeguard such occurrence

Win

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Microwave antenna

GSM antenna

Load Schematic on a communication tower

Dead Load on Tower

The predominating design parameter for lattice tower structures is the wind load on the structure itself and on the ancillaries both discrete and linear.

The wind load on the structure depends on the climate Conditions and the wind resistance of the structure, antennas etc. Apart from the wind load on the antennas, cables,

and other ancillaries, the lattice tower itself contributes significantly to the wind load on the structure.

The wind resistance of the lattice tower is dependent on various parameters: e.g. type of cross section, solidity ratio and type of members.

In some regions heavy ice loads can occur on the structure and the dimensioning load can be the weight of the ice or the combination of ice load and wind load. Atmospheric icing on the structure may have significant influence on the design of the structure.

Guidance on the analysis and design of Lattice towers are available in some International codes of practice such as BS8100 [Lattice towers and mast Part 1-4], ANSI/TIA 222G etc which provide a basis for the design of lattice towers in different environmental conditions.

The wind load on the tower sets up a dynamic effect on the structure, it is therefore essential that self supporting towers and guyed masts are analyzed for the dynamic response of the structure to the wind gust load.

Guyed mast are essentially of a more complex nature due to the static system of the mast shaft as a column subjected to bending moments and elastically supported by guys with non-linear stiffness.

The tower is sub-divided into a series of panels with the estimated wind force applied on each panel point and analysis carried out to determine the gross response of the tower as a whole.

lattice tower failure

Design/ constructio

n flaw

Construction Induced

Failure

Design Flaw

Fabrication flaw

Installation flaw

In-service FailureO

verloading

Failure to maintain

Extreme Icing

Special wind

Aircraft Collision

Anchor Failure

Associated Causes of Lattice tower Failures

Oversight in accurately estimating the loads.

Critical design consideration for guyed tower to able to withstand wind pressure in still air conditions and at a reduced wind pressure should guy rupture occur.

Analysis and design of lattice towers should consider both the axial force and bending moments in the main legs

Design Flaws

Antenna Area Guide for Ancillaries

Reproduced from Mast Project Documentation

The Maximum Number of ancillaries is estimated and the total antenna area used as a guide in the design and procurement of a new Tower

Installing more than the designed Antenna area loading induces excessive wind load on the tower which can impair the safety of the tower and possibly the cascading failure of such tower.

Typical Backbone towerTypical Cellular tower

Collapse of Tallest Mast in the world (1991) due to guy rupture

A critical design consideration for guyed tower is to be able to withstand wind pressure in still air conditions and at a reduced wind pressure should guy rupture occur.

Adequate design of guys for the tensile actions is also of grave importance to prevent guy failure as result of poor design..

Guyed mast are essentially of a more complex nature due to the static system of the mast shaft as a column subjected to bending moments and elastically supported by guys with non-linear stiffness.

Failure of this tower was attributed to insufficient weld penetration resulting in the tower leg breaking free from the flange.

TOWER FAILURE ASSOCIATED TO MATERIAL FAILURE

Lattice towers are made up of built up steel members with Material specification and Connection of paramount importance.

Quality Control and assurance manufacturing process of the tower members to ensure that member strength is as designed

Failure of any component of the tower or welded connection details can cause the failure of tower.

TOWER FAILURE ASSOCIATED TO INSTALLATION FLAW

GUY SLIPPAGE FAILURES

Installation of guys for guyed towers has to be to precision in terms of location and required tension.

Poor installation of the guys can cause guy slippage failure which imposes excessive loads and possibly reversal loads on the members and can eventually leads to the total collapse of the tower.

A detailed understanding is of great importance because most radio towers in Nigeria are procured from fabricators directly who have no knowledge of such design but base their fabrication on experience.

TOWER FAILURE ATTRIBUTED TO EXTREME ICING:

Ice accumulation on a structure increases both the area and weight, causing additional force on the tower.

The increased surface area due ice captures more wind resulting in significant increase on the wind force on the tower and the appurtenances.

Earlier design standards under estimated the magnitude the increased weight placed on the structure caused by ice accumulation

The above real-life iced towers give us a better understanding and appreciation of the devastation that might arise because of the increased surface area and weight due to ice accumulation of this magnitude.

Towers have been popularly used to support various antenna systems since the 1940's with very little attention given to corrosion of buried tower components.

Many tower Facilities are coming of age and the problem of anchor shaft corrosion is now becoming an industry issue.

Anchors are employed in guyed towers to support tension in forces in the tower.

Corrosion causes the deterioration of the anchor reducing the strength required to support the tensile forces thereby causing the failure of the anchor. This corrosion is caused as a result of electromechanical or galvanic action causing the metal to deteriorate in thickness and invariably the strength

ANCHOR FAILURE

Guyed Tower Anchor schematics

The effect of the anchor failure or failure of any of the system is a sudden redistribution of the load when the guy is loose resulting in load reversal and excessive load in some members.

This can result in the failure of the members due to load reversal and excessive loads and ultimately the failure of the guyed tower.

COLLAPSED TOWER IN SAN BERNARDINO COUNTY, CA AUGUST 2005

Example of such tower failure is the December 14 2009 Tusla tower in Oklahoma and the collapsed tower in San Bernardino County, CA. August 2005. The causes of the failures was attributed to electrolytic corrosion on one of the guy wire anchors.

Collapse Tusle tower Oklahoma due to anchor Failure

Incidence of tower failures imposes economics consequences as well as risk to Human lives and as such there is a need for a concerted to better understand such failure.

Some of the associated tower failures has been discussed to enlighten the engineering personnel and tower owners on the risk associated with the installation of lattice towers.

Tower Owners need to appraise the integrity of towers at regular interval to ensure it meet minimum safety standard. Owners of abandoned towers should remove such to eliminate risk to human lives.

There is also the need for regulatory agency of Government to revisit the issue of citing of towers to minimize the causalities and the economic consequences in the event of tower collapse

CONCLUSION

REFERENCES. 

 [1] F. Al-Mashary, A. Arafah, G. H. Siddiqi and Y. Al-Salloum ( ) Investigation Of Failure Of Six Transmission Towers, Civil Engineering Department, King Saud University, Kingdom of Saudi Arabia http://faculty.ksu.edu.sa/2639/Publications

 

[2] Mogens G. Nielsen (2009) The analysis of Mast and Towers, International Journal of Space Structures Vol 24 N0.2 2009.

[3] BS8100 Part 1-4, (1986) Lattice towers and Masts Part 1-4 British Standard Institution

[4] STØTTRUP-ANDERSEN, Ulrik (2009) Mast and towers, Proceedings of the International Association for Shell and Spatial Structures (IASS) Symposium, Valencia Evolution and Trends in Design, Analysis and Construction of Shell and Spatial Structures.

 

[5] BS5930, (2005) Code of practice for site investigations, British Standard Institution

 

 [7] ANSI/EIA-222-G 2006. Structural Standards for Steel Antenna Towers and Antenna Supporting Strucutres, Telecommunication Industry Association (TIA), U.S.A

 

[6] F. Al-Mashary, A. Arafah and G.H. Siddiqi (2008) Effective bracing of Trussed towers against Secondary moments, Civil Engineering Department, King Saud University, Kingdom of Saudi Arabia. http://faculty.ksu.edu.sa/2639/Publications.

 

[7] ANSI/EIA-222-G 2006. Structural Standards for Steel Antenna Towers and Antenna Supporting Strucutres, Telecommunication Industry Association (TIA), U.S.A

 

[8] D. Jatulis, Z. Kamaitis, A. Juozapaitis (2007) Static Behaviour Analysis Of Masts With Combined Guys, Journal Of Civil Engineering And Management, 2007, Vol XIII, No 3, 177–182.

 

[9] C. T. Georgakis, U. Støttrup-Andersen, M. Johnsen, M. G. Nielsen, H. H. Koss (2009) Drag coefficients of lattice masts from full-scale wind-tunnel tests, EACWE 5 Florence, Italy.

 

[10] Craig M. Snyder () Understanding and Preventing Guyed Tower Failure Due to Anchor Shaft Corrosion, Sioux Falls Tower Specialists, Inc, South Dakota USA

 

[11] www.eriinc.com.

THANK YOU