ON THE SEISMIC RESPONSE OF TALL TELECOMMUNICATION TOWERS …irf/Proceedings_IRF2016/... ·...

Proceedings of the 5th International Conference on Integrity-Reliability-Failure, Porto/Portugal 24-28 July 2016

Editors J.F. Silva Gomes and S.A. Meguid

Publ. INEGI/FEUP (2016)

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PAPER REF: 6408

ON THE SEISMIC RESPONSE OF TALL TELECOMMUNICATION

TOWERS OF DIFFERENT TYPOLOGIES

Fábio Monteiro1, Rui Carneiro Barros

2(*)

1MSc, Colaborator of PRODEC at FEUP, Mota-Engil (Africa), Zambia 2Habilitation - PhD - MSc - Civil Engineer, Prof at Dept Civil Engng, Structural Division, FEUP, Porto, Portugal (*)Email: [email protected]

ABSTRACT

The design of telecommunication tower structures is usually done considering only wind

action. This study aims to carry out a seismic analysis of three different types of support

structures, which consist of two lattice towers typologies and tubular masts, which were

designed taking into consideration only the action of the wind. It is intended to understand if

the seismic action can somehow condition and control the design of these structures. To this

end, seismic studies were performed based upon Eurocode 8 provisions and on existing

seismic records of previous earthquakes. The results of parametric studies for the three

typologies were obtained using the calculation Robot Structural Analysis software for lattice

towers, and with the support of Excel worksheets for tubular masts.

Keywords: Telecommunication towers, seismic response, spectral analysis, time-domain

analysis, lattice towers, tubular steel masts, parametric studies.

INTRODUCTION

The globalization that has been occurring over the years has become an essential factor in the

life of world citizens. Thus, use of the telecommunication media has been growing constantly.

Without the support telecommunication structures of the technological devices and

equipment, nothing would be feasible. These support structures require special consideration

with respect to their design, because the smooth operation of communication devices is

intrinsically linked to the structural behavior of the medium in which they are supported.

These structures must meet certain requirements in order to provide support service

sufficiently stable, so that the transmitters installed in them maintain continuous emission

signals within some tolerances. The support structures are very important especially after

natural disasters, and their integrity is essential to ensure safety of human lives. The design of

these tower structures is usually done considering only wind action. Indeed, it is established

from the outset that wind will be determinant, thus discarding the seismic action.

This study aims to carry out a seismic analysis of three different types of support structures,

which consist of two lattice towers typologies and also tubular masts, which were designed

taking into consideration only the action of the wind. It is intended to understand if the

seismic action can somehow condition and control the design of these structures. To this end,

seismic studies were performed based upon Eurocode 8 provisions and on existing seismic

records of previous earthquakes, the latter being considered to carry out time-domain analysis.

The results for lattice towers were obtained using Robot Structural Analysis software, while

the development of Excel worksheets was crucial for the tubular masts cases. Parametric

studies for the three typologies allowed to have better perception of the towers performances.

Symposium_24: Structural Dynamics and Control Systems

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TELECOMMUNICATION TOWER’s TYPOLOGIES

AND MAJOR FAILURES

Over the years there has been a steady evolution in the types chosen for communication

equipment support structures (telecommunication towers), majorly sub-divided as: lattice self-

supported towers, lattice guyed towers, self-supported tubular towers and guyed tubular

towers.

The metallic lattice self-supported towers have been an economic solution in the

communications industry over the years. These towers are designed to obtain maximum

efficiency, which correspond to minimize the weight of steel. Typically, this type of towers

have variable tower section from the bottom to the top, and depending on the structure

different types of bracing can be adopted appropriate to the type of load that the structure will

support. Such structures may have heights ranging from 10 to 200 meters. The main

advantages of such structures lies in their good torsional rigidity and in the fact that they

require les implantation space around (as self-supporting they do not require cables to be

anchored to the ground). The torsional stiffness is important for good performance of the

antennas, which can transmit eccentric loads to the supports due to wind action. These

structures can be usually subdivided into two categories: square section towers (Figure 1, left)

and triangular section towers (Fig. 1, right).

The self-supporting lattice towers of triangular base are usually made of tubular metal

sections where the links between the various sections are held by bolted flanges, and the

diagonal bars and sleepers can be screwed or welded. The self-supported lattice towers of

square base are made of metal type bracket profiles for low to medium heights; in such cases

the links between the various sections are carried out by coupling together plates, diagonal

bars and crossbars using generally bolted features. For very tall cases, these square base

towers are made of steel tubes of various diameters and thicknesses along the height, and the

links are done with bolted connections at special welded flanges at the major spatial nodes

between the tower corner legs and the space beams.

Fig. 1 - Lattice square tower (left) and lattice triangular tower (right)

Proceedings of the 5th International Conference on Integrity-Reliability-Failure

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The lattice guyed masts are typically constituted by a square or triangular section (Fig. 2). The

leg members generally comprise tubular and/or solid circular sections, and with bracing

welded connections. The mast is generally very thin and consists of modules containing

horizontal, vertical and diagonal profiles, whose connections between them can be screwed or

welded. The wind resistance in this structure significantly depends on: the choice of the

section; the type of brace; and the types of profiles used for braces and legs.

In general, the section of a guyed mast can be smaller than any other type of communications

structures. This type of structure is fixed laterally along its height by cables anchored to the

ground, which absorb the horizontal actions. The number of plane guy directions are

generally two per tower corner (the number of guy planes is usually twice the number of

corner nodes of the cross section, for usual cases of lattice triangular guyed tower), but the

number and levels of cables along the tower height greatly influences the design of the

structure, as well as the number and position of the cable foundations.

Another choice that must be made is whether the mast is embedded in the foundation, or

simply hinged at the base. Generally, the masts are hinged at the base when their section is

triangular. This has the advantage of eliminating the bending moments at the base, which

leads to a better distribution of the stress state in the lower sections. The disadvantage lies in

the fact that practically there is no restriction of rotation at the base, which may be

unacceptable for certain types of equipment installed at the top of the communication tower.

But in general such towers have the advantage of being easier to mount and are more

economical than self-supporting; but have the disadvantage of requiring a larger area of land

for the implantation of the tower itself and appendices, in the order of 10 times the area of a

self-supporting structure that has the same height (Mendonça, 2012).

Fig. 2 - Lattice triangular tower guyed with tendons


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According to Antunes (2008), the self-supporting tubular towers present a polygonal tubular

section which decreases diameter in height according to a particular gushing or spouting rate.

They enable the same applications as those of lattice self-supporting towers but occupying a

smaller area of land, thereby causing lower costs; also its assemblage involves little hand

labor. They have the disadvantage of their lower stiffness when compared with the lattice

structures and are therefore very flexible structures, which can cause unacceptable

consequences for communications equipment. They provide however a good landscaping, and

there are disguising techniques for these structures when they have to be deployed in

ecological protection sites.

The guyed tubular masts have a cylindrical tube section from the base to the level of the

supports for the equipment (Fig. 3). The hot-dip galvanizing method is used as the main

protection against corrosion. The advantages of this structural type are: (a) the drag

coefficient for the wind pressure is approximately half that of a flat surface; (b) the effect of

icing on wind action is less than for the lattice structures; (c) assemblage is simpler than that

associated with the lattice construction. It also has the following disadvantages: (d) tendency

to vibrations resonance under certain wind velocity conditions; (e) the installation of

additional antennas is difficult on the smooth outer surface; (f) moisture may condense on the

inside; (g) the total cost is greater than that of the equivalent lattice mast.

Fig. 3 - Guyed tubular mast

(http://www.skyscrapercity.com/showthread.php?t=568982 )

There are several factors that may lead to structural failure of such telecommunication towers

structures. Among them are included: (a) ice (its own weight is an additional action that can

reach very high values for tall towers in cold regions); (b) wind action combined with ice (ice

increases wind exposure area, generating increases of horizontal forces acting in the


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structure); (c) wind gusts and sharp fluctuations of wind velocities due to distributions with

high gradients; (d) errors in the tower design and/or poor characteristics of the materials used;

(e) acts of vandalism; (f) faults occurring during maintenance operations (e.g., during such

operations, the tower structures may be subjected to actions exceeding its load carrying

capacity); (g) very high towers can suffer collisions caused by air traffic (in the last 15 years

there have been at least 11 accidents of this nature).

CONSIDERED TOWERS FOR THE PARAMETRIC STUDY AND OTHER

CHARACTERISTICS OF THE ANALYSIS

The tower typologies analysed herein under seismic actions, for towers initially designed and

primarily controlled by wind actions (location of towers in zone A), were: lattice self-

supported tower triangular base and lattice self-supported tower square base (Fig. 4) and also

tubular self-supported cylindrical tower. Each typology has a specific design height (65 m for

the triangular based; 50 m for the square based; and 15-18-21-24-27-30 m for the tubular

mast) and were designed for a certain seismic site but with possibly varying soils.

Also a variable top mass of the telecommunication equipment and auxiliary items was

considered in this parametric study for the considered typologies (starting with 200 kg until a

maximum value of 2400 kg, in steps of 200 kg of additional masses). More details of the

comprehensive seismic analysis done for the three tower typologies mentioned above, can be

seen in the master of science thesis of the first co-author (Monteiro, 2014) supervised by the

second co-author.

Fig. 4 - Lattice self-supported tower typologies: triangular base (left) and square base (right)

The above considered towers are supposed located in Sagres that constitutes the worst seismic

site in the national territory, to which correspond a design value of the ground acceleration ag

of 4,88 m/s2 (for soil type A, for which soil coefficient S=1).

The Eurocode 8 allows the seismic analysis of structures using recorded accelerograms of

past earthquakes. Therefore, it is possible to use any accelerograms caused by any earthquake

that has occurred in any part of the world. The use of these accelerograms are allowed

provided that their values are graduated or scaled in function of the local peak ground

acceleration (PGA), which corresponds to the value of ag.S (in Eurocode 8).


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For the present case used in this study (Sagres), ag.S takes the value of 4,88 m/s2 so that the

normalization constant for each considered i earthquake ( KPGA , i ) is given by the ratio:

KPGA , i = PGA local / amax , i = ag S / amax , i = 4,88 / amax , i

where amax , i is the maximum peak acceleration obtained for the i earthquake.

Multiplying all seismic records used by the corresponding normalization constant, the scaled

accelerograms are obtained. After obtaining these scaled accelerograms time-histories, one is

able to make the temporal time-domain analysis applying the earthquakes independently in

each direction of the structure.

As seismic excitations were considered accelerograms and acceleration spectra generated by

earthquakes of Kobe and Northridge, taken from the site of the Pacific Earthquake

Engineering Research Center (PEERC). After scaling of these seismic records, the time-

histoy integration of the two earthquake accelerograms was done using Newmark Method

(Clough and Penzien, 1975).

The Eurocode 8 (EN 1998-6 , Part 6 , section 3.3 ; 2005), quite specific for structural

typologies that include the towers analyzed in this paper, proposes two alternatives for the

behavior coefficient as transcribed below:

- Paragraph (2): For towers, masts and chimneys, depending on the structural sections of

the elements, the design calculations for elastic behavior until the ultimate limit state may

be appropriate. In this case, the behavior factor q should not exceed q = 1,5 .

- Paragraph (3): Alternatively to (2), the design calculations may be based on the elastic

response spectrum with q = 1.0, and with damping values selected so as to be suitable for

the particular situation (according with paragraph 4.2.4).

In the present study the second option was chosen, because the goal is not to work with design

values but to analyze the behavior of the structure and verify that it remains always in the

elastic range under the seismic action. The damping ratio used was 2% for both towers and

masts, since they are steel structures for which the adopted ratio is a good estimate of the

damping characteristics of the material.

All the analysis presented herein rely on the three types of self-supported tower structures

considered: triangular lattice towers, square lattice towers, and tubular masts. Structural

models of both lattice towers have been provided by Metalogalva; these models correspond to

existing towers in reality, where the design has been made taking account the wind action and

discarding the seismic action. It was therefore the first co-author responsibility to check the

design for seismic actions, under the supervision of the second co-author.

The entire studies are based on the evolution of the behavior of the towers typologies for

various increments of mass at the top, which correspond to mass increments of the antennas

and other devices and equipment that may be wished to install. In the case of tubular masts,

they were pre-designed by the first co-author, having the wind as variable action, and

subsequently was performed a verification for the seismic analysis. Note that these analysis

were performed for different tower heights and different top masses. All the spectral and

temporal analysis were performed using the Robot Structural Analysis because this is the

software used by the company Metalogalva; it was therefore imposed as imperative the use of

the same design software for lattice towers. In relation to the tubular mast, seismic analysis

were performed with support of a specifically developed personal Excel worksheet.


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ANALYSIS AND RESULTS FOR THE SELF-SUPPORTED LATTICE

TRIANGULAR TOWERS

The triangular tower analyzed in this study has a height of 65 meters from the ground to the

top, and the legs are buried (Figure 5). In the design model, the base of the tower was

considered perfectly built-in and the characteristics of the steel are those corresponding to a S-

275, with an elastic modulus of 210 GPa and a specific density of 77 kN/m3. All structural

member components of the tower are made of tubular profiles. As can be seen, the structure

will have different behavior depending on the X or Y directions of seismic actions, since it

does not present complete symmetry along those axis.

The analysis was performed for a basic model structure that has a technical platform at the

height of 61 meters and stairs along its entire length. The platform transmits to the structure a

service minimum load (antenna and equipment) of 150 kgf (1500 N) that can be increased,

and was modeled as a surface load evenly distributed. The stairs transmit a 16 kgf weight

(160 N) per linear meter of height (which was modeled as equal concentrated loads, applied at

the level of each cross beam member).

Fig. 5 - Frontal lateral and perspective views and details of the platform for antenas,

for the self-supported triangular tower

Masses were added at the top of the basic model tower structure (corresponding to different

weights of the antennas and other equipment), with increments of 200 kg and belonging to the

range from 0 to 2400 kilograms (in 12 steps); the latter higher mass is found to be a perfectly

valid upper limit, taking into account that the weights applied on top of these structures will

never exceed this value.

From the modal analysis using Robot Structural Analysis software it was possible to determine

the number of modes to be used in the seismic study of this triangular tower; a number of

required modes was used such that the sum of the effective modal masses (EMM) were at

least 90% of the total tower mass, for vibrations in the X and Y directions (Fig. 6 and Fig. 7).

The evolution of natural frequencies (in Hz) for the first 60 modes is given in Fig. 8; and the

characterization of the first 5 modes and 8th mode is given collectively in Fig. 9 to Fig. 11.


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Fig. 6 - Efective modal mass for every mode until order 60

th, for vibrations along X-Y-Z directions

Fig. 7 - Cumulative efective modal masses mobilized, for vibrations along X-Y-Z directions

Fig. 8 - Evolution of the natural frequencies (Hz) until 60

th mode

The lack of complete tower symmetry is the reason for which 1st and 2

nd modes (flexural

modes in X and Y directions) are very similar however not completely equal (Fig. 9).

Fig. 9 - First mode (left): f 1 = 1.38 Hz, 1st pure flexural along X-direction, mobilizes 38.6% of EMM ;

Second mode (right): f 2 = 1.38 Hz, 1st pure flexural along Y-direction, mobilizes 38.5% of EMM


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Fig. 10 - Third mode (left): f 3 = 3.98 Hz, 2nd pure flexural along X-direction, mobilizes 24.4% of EMM ;

Fourth mode (right): f 4 = 4.02 Hz, 2nd pure flexural along Y-direction, mobilizes 23.7% of EMM

Fig. 11 - Fifth mode (left): f 5 = 3.98 Hz, 1st pure torsional along Z-direction, no mobilization of translational

EMM ; Eighth mode (right): f 8 = 6.66 Hz, global flexural along X and Y directions, mobilizes 6.35% of EMM

Part of the parametric study performed with variation of the top additional masses (of

additional antennas and auxiliary equipment) can be synthesized in Figures 12-14-15. For

additional details the reader should refer to (Monteiro, 2014).

Fig. 12 - Decreasing variation of the 10 first natural frequencies with the increase of top additional mass

The decreasing variation of natural frequencies shown in Fig. 12 is more pronounced for top

additional masses up to 1000 kg, and from that mass onward begin to be less and less

pronounced. The highest frequency variation (in percentage terms) occurs for the modes 1 and

2, with frequencies decrease of about 46% (the frequency thereof changes from 1.38 Hz to


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0.74 Hz, for additional top masses changing from zero to the maximum mass considered).

Mode 5 shows the second highest frequency variation (with increase of top additional masses)

with a decrease of about 32%; followed by the case of modes 3 and 4, with variations in the

order of 24%. Finally modes 8 and 9 show a decrease of about 10%; while modes 6 and 7

show very slight variations, being about 4% for mode 7 and almost null for mode 6.

Part of the parametric results (with varying top additional mass) of regulatory (EC8) spectral

seismic analysis for earthquakes solely in X-direction are shown in Fig. 14, about the modal

contributions for the axial reactions in base supports 1 - 2 and 3 (refer to Fig. 13).

Fig. 13 - Location of base supports for the triangular tower

Fig. 14 - Spectral modal contributions for axial reactions in the base supports 1 - 2 and 3

(top for mode 1, bottom left for mode 2, bottom right for mode 3), for earthquakes in X-direction


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When the earthquake is considered solely in Y-direction, the spectral values show a similar

variation as the one shown in Fig. 14 but now with exchanging roles of mode 1 and mode 2,

and a similar variation with mode 4 (pure flexural in Y-direction, which now replaces mode 3

of pure flexural vibration in X-direction).

But the regulatory seismic action is the combination of seismic action according to a main

direction, with thirty percent of the seismic action in the orthogonal direction. Thus this fact is

also shown in the spectral reaction results of Fig. 15 (for the same parametric variation of top

additional mass), which were obtained by regulatory quadratic combination of the axial forces

for the four vibration modes (1-3 for vibration in X-direction, 2-4 for vibration in Y-

direction). In Fig. 15 (left) for spectral seismic action along X-direction and with the increase

of 30% of spectral seismic action along Y-direction; and the reverse, for Fig. 15 (right).

Graphs in Fig. 15 clearly indicate such almost symmetry of roles.

Fig. 15 - Spectral regulatory quadratic combination (only with 4 modes) for axial reactions in base supports 1 - 2

and 3 for: spectral seismic action along X-direction and with the increase of 30% of spectral seismic action along

Y-direction (left); spectral seismic action along Y-direction and with the increase of 30% of spectral seismic

action along X-direction (right)

Fig. 16 - Total axial reaction at the 1-2-3 base supports (static and regulatory seismic spectral),

for soil type D (left) and soil types A-B-C (right)

Similar variations could be obtained for the regulatory seismic values of basal shears and

basal moments at the 1-2-3 base supports. Due to its importance in controlling the tower

response and service or usefulness performance of the antennas, in Fig. 17 is shown the

parametric variation of the tower top displacements and top rotation at platform level (which

influences the signal emission and reception) both with the value of top addition mass as well

as with soil type A-B-C-D (for displacements) or solely for soil type D (for rotations). Notice

that Fig. 17 shows the total spectral values associated with a complete quadratic combination

(CQC) of 64 modes.


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Fig. 17 - Top displacements at platform level for soil type D (graph top left) and soil types A-B-C (graph top

right); top rotation at platform level for soil type D (graph bottom)

According to the limit values expressed in the American regulatory normative (TIA 222-G,

2006; section 1.8 of chapter 1) the maximum top displacements allowed, for this particular

type of telecommunication tower structure, is 98 cm; a much higher value (insuring good

signal performance, with respect to expected displacements) than those conservative safe

values obtained for the different types of foundation soils. In terms of maximum allowable

rotations, these are 0.07 radians for VHF antennas (according to the TIA 222-G) and 0.03

radians for UHF antennas according to Telebras requirements (Mendonça, 2012).

Only the latter slightly exceeded the threshold for ground soil type D. Generally it can be said

that the seismic action poses no constraints on the operating requirements of the antennas

considered (Smith, 2007).

Finally similar graphs can be obtained for the maximum normal stresses at the most stressed

bracing member of the tower. While for top additional masses up to 1200 kg, the most

stressed bracing member occur in the first third part of the tower, for additional masses above

that value the most stressed bracing member occurs at the top of the tower (Fig. 18) requiring

local re-design of a few bracing bars modifying the tower structure at the top.

This fact is responsible for the jump or discontinuity in the graphs of Fig. 19, associated with

the parametric variation (with the top additional masses and with the soil type) of the

maximum normal stresses at the most stressed bracing bar member of the tower.


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Fig. 18 - Location of bracing members with highest axial stress in the triangular base tower,

with increase of top additional masses

Fig. 19 - Evolution of the normal stress of the most axially stressed bracing member, with increase of the top

additional mass (left: for soil type D; right: for soil type A-B-C)

Seismic time-domain analysis were also done for scaled Kobe and Northridge earthquakes,

acting independently along X and Y directions. Some results are given in Fig. 20 for the

Northridge earthquake acting along X-direction, respectively for maximum traction (Fig. 20,

left) and maximum compression (Fig. 20, right) at the 1-2-3 base supports.


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Fig. 20 - Maximum traction (left) and maximum compression (right) at the 1-2-3 base supports, for scaled

Northridge earthquake along X-direction

Comparing the results of Fig. 14 and Fig. 20, for example for base support 2 and with the

value of top additional mass of 1600 kg, from Fig. 14 the spectral value of axial force (at 2)

with 3 modes contribution (1, 2 and 3) would be (0+4002+160

2)1/2 = 430 kN, while for the

scaled Northridge earthquake the traction force would be 580 kN and the compression force

would be 610 kN. Leading to a relative difference in magnitude of the order of 30%, from

results relative to X-direction. However, when comparing the regulatory quadratic

combination of Fig. 15-left (predominantly X-direction value of 440 kN) and Fig. 20 (value of

610 kN used before) the error difference decreases to 28%.

Fig. 21 shows the evolution, with increasing value of the top additional mass, of the top

displacements along X-direction as well as the platform top rotation, for triangular base tower

under scaled Northridge earthquake in X-direction. Comparing the maximum value of 71 cm

(for top additional mas of 1600 kg) for such time-domain information, with the value of 53

cm determined by CQC of spectral values (Fig. 17), it is proven that the spectral values

underestimates the scaled earthquake values in about 25%. With respect to top platform

rotation, the 0.026 rad (in Fig. 17) underestimated the scaled earthquake value of 0.028 rad (in

Fig. 21) in about 7%. For additional details on the seismic response of this triangular base

towers, the reader should refer to (Monteiro, 2014) where specific features of non-symmetric

behavior are more fully discussed as well as their light consequences.

Fig. 21 - Top displacements along X-direction and platform top rotation,

for scaled Northridge earthquake along X-direction


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For the same triangular base towers subjected to the scaled time-history of Kobe earthquake,

in general the curves of variation of structural quantities (for the parametric study taking into

account top additional masses of antennas and equipment, bigger than the basic design

structure considered, and also ground soil type) show less magnitude of the quantities

involved and a decreasing trend as the top additional mass increases. This is specifically due

to the frequency content of the Kobe’s spectrum, exciting the towers in such a way that they

contribute less to the response than for the Northridge earthquake.

For the seismic response of the square base towers, assumed in the original study with a

height of 50 meters above the ground, also the tower legs are buried and the base is

considered as built-in. The characteristics of the steel used are those already used for the

previous model of the triangular base tower. All the structural members are materialized as

angle bars. Also in this structure was considered the existence of a platform at the level of

47.5 meters height and stairs along its length. The loads transmitted to the tower platform by

antennas and additional equipment are equal to those used for the triangular base tower.

In this case it is considered that the tower has double symmetry (equal behavior for both the X

and Y directions), however this is not strictly correct since the stairs are not located in the

vertical plane of symmetry of the structure; whereby there will be a very slight asymmetry

caused by the ladder loads, nevertheless with almost non-disturbing symmetric behavior and

results (and never in the abnormal manner associated with the triangular base tower, described

earlier). Therefore, for question of space, the results of the square base tower are no longer

mentioned (they are simpler and more regular than the earlier one’s for triangular base tower)

and the interested reader may refer to (Monteiro, 2014).

ANALYSIS AND RESULTS FOR THE SELF-SUPPORTEDTUBULAR TOWERS

The parametric study associated with tubular self-supported cylindrical towers considered

tubular tower masts 15-18-21-24-27-30 m in height, materialized in the same steel properties

considered earlier and designed for a certain seismic site (Sagres, with maximum national

value of ag.S = 4,88 m/s2) but with possibly varying soil types. The most important results are

in figures shown below and their variations are somehow self-explanatory.

Fig. 22 - Variation of fundamental frequency with tubular mast heights and top additional mass


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Fig. 23 - Variation of maximum top lateral displacements (top), maximum basal shear (centrum) and

maximum basal moment (bottom), for soil classes A-B-C-D and with the top additional equipment

mass, for tubular towers 15 meters high


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The top lateral displacements for tubular mast towers 15 meters height (Fig. 23) range from

12 cm to 60 cm for ground soil type D and between 12 cm and 45 cm for any other soil type

A-B-C. There are three different zones in the curves, corresponding to different levels in the

regulatory (EC8) spectrum of accelerations. The displacements remain approximately

constant for additional masses in excess of 1600 kg (since such masses lead to natural

frequencies that are in square hyperbolic branch of the spectrum).

The same applies for the curves of basal shear and basal moments. The basal shear, for these

15 meters height towers, range from 4 kN to 10 kN for soil type D, and range from 4 kN to 7

kN for soil type A-B-C. The basal moment, for the same towers, reach maximum values of

140 kNm (soil type D) and 100 kNm (soil type A-B-C) respectively.

In this particular case of tubular tower 15 meters high, the basal shear and basal moment

curves are similar in configuration to the curve of top displacements. This happens precisely

because the tower mast of 15 meters, in addition to having a smaller height also has the

smallest section, and consequently has a distributed mass not very significant. The other

variations for all tubular mast towers of the heights considered, are shown in Figure 24 to

Figure 27; such figures show in general a clear discrepancy in relation to the configuration of

the displacement curve (contrary to what happened in Fig. 23).

Fig. 24 - Variation of tower top displacements, with soil types and tower height

Fig. 25 - Variation of tower top platform rotations, with soil types and tower height


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Fig. 26 - Variation of tower basal shears, with soil types and tower height

Fig. 27 - Variation of tower basal moments, with soil types and tower height

It was verified (Monteiro, 2014) that all the tubular tower masts satisfied structural safety.

Since the tubular masts have been pre-designed with regards to wind action, one can conclude

that the seismic action is not conditioning. The tubular mast of 24 meters is the ideal solution,

because it is the one that in general is subjected to smaller seismic basal generalized forces

and having intermediate displacements in the interval of values obtained.

The final three graphs in Figure 28, compare maximum displacements and maximum

rotations of the tubular mast towers (for different soil types A-B-C and D) of different

heights, with the regulatory limits imposed by TIA-222-G (2006) and Telebras (Mendonça,

2012) for VHF and UHF antennas. In terms of maximum displacement it is verified that the

masts heights of 15 meters or of 18 meters do not meet the limits if located in soil type D.

With regard to maximum rotations verifications, only the masts of 27 meters and of 30 meters

comply with the limits set by the TIA-222-G, for all types of soil. When dealing with UHF

antenna, none of the poles meets the limit imposed by Telebras. Being rotations the most

conditioning parameter of the good performance of the antennas, it is here clearly verified that

the earthquakes will have an important interference in the functioning of the tubular self-

supported telecommunication towers.


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Fig. 28 - Comparisons of maximum tower displacements and maximum tower rotations with the

regulatory limits of TIA-222-G and Telebras, for various tower heights and soil types: Displacements

regulatory limits (top), Rotations regulatory limits for VHF antennas (middle), Rotations regulatory

limits for UHF antennas (bottom).

CONCLUSIONS

Parametric studies about the seismic response of a lattice self-supported triangular base

telecommunication tower and of tubular self-supported circular base telecommunication

towers, pre-designed for wind action, were conducted and explained in detail; both seismic

spectral analysis and time-history analysis of scaled earthquakes were addressed. The

variation parameters for the lattice triangular base tower were the top additional masses (from


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a basic design configuration) and the soil types A-B-C-D; for the tubular circular base tower,

the variation parameters were the tower height from 15 m to 30 m, the top additional masses

and the soil types. A small reference is made on the simpler seismic behavior of lattice square

base tower, very slightly asymmetric (because of the ladder) but with a very negligible effect.

It was verified that although the telecommunication towers have been pre-designed with

regard to wind action, in general they do satisfy structural seismic safety; i.e., in general the

seismic action is not conditioning of the structural safety. However, the regulatory limits (on

displacements and rotations) imposed by TIA-222-G and Telebras for VHF and UHF

antennas, have to adequately introduced in the design and selection process. Being rotations

the most conditioning parameter of the good performance of the antennas, it was here clearly

verified that the earthquakes will have an important interference in the functioning of the

telecommunication towers (especially the tubular self-supported).

ACKOWLEDGEMENTS

The authors acknowledge the financial support received from the research unit CONSTRUCT

(sub-unit LESE) of the Department of Civil Engineering at FEUP, for the publication of this

work. It is worth noting that the second co-author has been carrying out R&D on this type of

special structures since early 2000’s, with noticeable concentration of activity since 2008. The

authors also acknowledge the thematic technical support given by Metalogalva Group (Trofa,

north of Portugal) for this research, done through a technical training of the first co-author in

the design environment of Metalogalva, during the second semester of 2013/2014.

REFERENCES

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[3]-EN 1998-1 -- Projetos de estruturas para resistência aos sismos: Regras gerais, ações

sísmicas e regras para edifícios. LNEC, Lisboa, 2010.

[4]-EN 1998-6 -- Projetos de estruturas para resistência aos sismos: Regras para torres,

mastros e chaminés. LNEC, Lisboa, 2005.

[5]-Mendonça, C. -- Dimensionamento de torres espiadas de grande altura. Integrated Master

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[6]-Monteiro, F.F.J. -- Análise Comparativa do Efeito da Ação Sísmica em Torres Metálicas

de Telecomunicações de Grande Altura. Integrated Master of Science thesis (supervised by

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[7]-Smith, B.W. -- Communication Structures. Thomas Telford Ltd, London, 2007.

[8]-TIA-222-G -- Structural Standard for Antennas: Supporting Structures and Antennas.

Telecommunications Industry Association, Arlington VA, 2006.

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