Failure Investigation Of Microwave Towers During Cyclones – A Case Study

No. 32-14

Journal of Structural Engineering Vol. 32, No.3, August–September 2005 pp. 147–157

Failure investigation of microwave towers during cyclones – A case study A. Abraham∗ , P. Harikrishna∗ , S. Gomathinayagam∗ and N. Lakshmanan∗∗

Open latticed steel towers are used widely in a variety of civil engineering applications. Along the east coast of India, many latticed towers, located in cyclone prone zones, have collapsed during cyclones with lesser wind speeds than the design basic wind speed. Dynamic effects of wind for design of lattice towers are simplified by most of international codes of practice with the use of Gust Effectiveness Factor (GEF). The application of gust effectiveness factor have been evaluated as per various international codes of practice and are compared in this paper. Case studies of failure analysis to identify the causes of possible failure of two microwave latticed towers, which collapsed in cyclonic wind conditions, are presented. Characteristics of a measured cyclone wind turbulence spectrum are used along with a practiced gust effectiveness factor approach or equivalent for the dynamic analysis.

Open latticed steel towers are used for a number of diverse purposes, such as radio and television broadcasting, observation (e.g., security rural, fire fighting), lighting supports, offshore deck areas and lifeline structures. Power transmission towers, electric poles, poles for telephone connections and communication towers form a major set of lifeline structures. Among these, microwave communication towers invariably adopt a open latticed type of design. The microwave towers carry one or more antennae at the required levels oriented in specified directions. There are many types of antennae varying in size and construction. The structures like masts and towers are sensitive to dynamic wind load. The need to design a lattice tower considering resonant dynamic response to wind loads arises when their natural frequencies are low enough to be excited by the turbulence in the natural wind1,2 . These types of structures, which are vulnerable to wind induced oscillations are required to be examined for dynamic effects of wind. Further, the structural loads produced by wind gusts depend on the size, natural frequency and damping of the structure in addition to the inherent wind turbulence. One of the approaches used for evaluating the dynamic response of lattice towers is the gust factor method2 . The design procedures for lattice towers are more complicated than for the case of buildings and other cladded structures because of the fact that, the wind, as it flows through the structure applies varying magnitudes of loads on the tower members along its path by their spatial dispersion and the direction of wind3 . For such open latticed towers, it is imperative that the dynamic wind loads are computed accurately, and, the resulting tower responses are ensured to be within acceptable limits. Therefore, it is necessary to estimate the wind ∗

loads properly for the design of these towers from the consideration of economy and safety. Collapse of microwave lattice towers during cyclones emphasises the importance of understanding the cyclone wind characteristics and the wind induced dynamic response of these structures. Fullscale experiments carried out by SERC, on wind characteristics during extreme wind conditions like cyclones indicated increased levels of turbulence as compared to normal pressure system wind conditions wherein the turbulence is dominated by terrain and structure sizes4 . The necessity of inclusion of background enhancement factor in evaluation of gust effectiveness factor has already been discussed5 . In addition, the important effect of aerodynamic damping which increases with increase in mean wind speed has been reported based on many full-scale experiments4 . In the present investigations, two microwave lattice towers which collapsed during two cyclone events6,7 were considered for failure analysis. Unlike the usual failure analysis involving nonlinear, transient, elasto-plastic analysis or shakedown analysis, a designers approach coupled with a “cause and effect” concept is adopted, failure causing extreme wind being the cause and the collapse of the tower being the effect. Initially, gust effectiveness factors were evaluated for these towers as per different international codal provisions, viz., AS code8 , BS code9 , ASCE code10 , China code11 , Canada code12 , Japan code13 , and were compared with the values obtained using IS code14 . The failure analysis involved identifying the basic wind speeds at which the collapse would have initiated as per IS code14 with the modified parameters suggested for towers under cyclone wind conditions15 .

Scientist, ∗∗ Director, Structural Engineering Research Centre, CSIR Campus, Taramani, Chennai 600 113, India.

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COLLAPSED MICROWAVE TOWERS 101 m Tall Steel Lattice Microwave Tower

The first failure investigation pertains to the total collapse of a 101 m microwave tower during a severe cyclonic storm, which crossed (Fig. 1) the Andhra Pradesh coast on 6th November, 1996, about 50 km southwest of Kakinada6 . The tower site (with latitude of 16.42◦ N and a longitudinal of 81.31◦ E) was observed to be over 60 km away from the coastline. Kakinada is situated about 8–10 km from the east coast of India with latitude of 16.57◦ N and a longitudinal of 82.15◦ E.

FIG. 2. 3D FINITE ELEMENT MODEL OF 101 m MICROWAVE TOWER WITH (i) MAIN AND SECONDARY MEMBERS. (ii) MAIN MEMBERS. ∗ - by failure analysis

FIG. 1. TRACK OF KAKINADA SEVERE CYCLONE

The 101 m tall steel lattice tower was square in plan, with a width of 14.274 m at bottom and a width of 1.8 m at top. The tower was fabricated using steel angle sections with bolted connections. One dish type antenna having a diameter of 3.0 m and weight 245 kg, and two grid type antennas each of which had a diameter of 3.0 m and weighing 120 kg were supported at 40 m, 50 m and 60 m levels respectively. The tower legs were covered with 1.5 m × 1.5 m square concrete blocks for a depth of 2 m, above ground level. The FEM-model of the 101 m tall microwave tower (704 nodes; 2012-3D truss elements) is shown in Fig. 2. A low-pressure area developed very close to the east coast of India on 2nd November 1996. It turned into a depression on 4th November, intensified into a cyclone storm on 5th and into a severe cyclonic storm on 6th . Finally the cyclone crossed the coast on 6th November 1996. The maximum wind speed, sustained over 3-minutes as per regional practice, of the cyclone was reported by the India Meteorological Department (IMD) as about 140–150 km/h (28.9–41.7 m/s)16 . Observed damage at the failure site showed that two of the concrete blocks covering the tower legs on one face had no/partial damage, while the other two blocks had 148

completely been damaged. One of the main leg members failed in shear (Fig. 3) at the bottom bolt hole location, which had possibly led to the total collapse of the tower as shown in Fig. 4. The site specific wind speed would be less than the maximum wind speed as mentioned above, since the wind speed reduces from the maximum wind speed, VR , as the distance from centre of cyclone increases beyond the radius of maximum wind speed, R as typically shown in Fig. 517 . Since exact cyclone wind speeds were not available at the failure site, a site specific maximum wind speed is estimated as 35.46 m/s following Fig. 5 with R = 40 km, r = 60 km and with a nominal decay coefficient, δ = 0.4 using the upper bound of maximum wind speed reported (41.7 m/s).

FIG. 3. SHEARED LEG MEMBER INSIDE THE CONCRETE BLOCK AT BOTTOM

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on 9th November, 1989 near Kavali7 . Kavali is situated about 10 km from the coast with a latitude of 14.55◦ N and longitude of 80.03◦ E. The 91 m tall steel lattice tower was square in plan. The width at bottom was 12.874 m and at top it was 1.8 m. Two dish antennae of 3.66 m diameter and each of which weighed 386 kg, were supported at 80 m and 90 m above the ground level. The tower was fabricated using steel angle sections with bolted connections. The FEM-model of the 91 m tall microwave tower (645 nodes; 1824-3D truss elements) is shown in Fig. 7. FIG. 4. COMPLETE COLLAPSE OF THE 101 m TALL STEEL LATTICE TOWER

FIG. 5. VARIATION OF WIND SPEED WITH RADIAL DISTANCE

91 m Tall Steel Lattice Microwave Tower

The second failure investigation pertains to the total collapse of a 91 m microwave tower during a severe cyclonic storm, which crossed (Fig. 6) the Andhra Pradesh coast

FIG. 6. TRACK OF KAVALI CYCLONE

FIG. 7. 3D FINITE ELEMENT MODEL OF 91 m MICROWAVE TOWER WITH (i) MAIN AND SECONDARY MEMBERS. (ii) MAIN MEMBERS. ∗ - by failure analysis

A cyclone originated in the gulf of Thailand on 1st November 1989, and this intensified into a severe cyclonic storm with a core of hurricane winds on the evening of 5th November. On the morning of 6th November, the storm crossed North Andaman and after intensifying further, it lead towards Indian main land, and hovered in the Bay of Bengal for two subsequent days, and finally crossed the east coast of India on Thursday, the 9th November 1989. The maximum wind speed, sustained over 3 minutes as per regional practice, of the cyclone was reported by IMD was about 100–120 knots (51.4–61.7 m/s) with in a radius of about 10 km around Kavali and as 80 knots (41.1 m/s) between 10 and 17 km from Kavali18 . Observed damage at the failure site includes, the main leg members were ruptured at the base level, leading to complete collapse of the tower as shown in Figs. 8 and 9. The tower site was observed to be 20 km away from Kavali. Using the upper bound of reported maximum wind speed

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(61.7 m/s) following Fig. 5, with R = 10 km, r = 30 km and a decay coefficient, δ = 0.4, the site specific maximum wind speed for the cyclone is estimated as 39.74 m/s.

FIG. 8. RUPTURE AT THE BOTTOM OF THE 91 m TALL STEEL LATTICE TOWER

effectiveness factor (function of wind, terrain and structure characteristics). Free vibration analyses of the two towers were carried out and the fundamental natural frequencies of the 101 m and 91 m tall towers were obtained as 1.38 Hz and 1.47 Hz, respectively. Figure 10 shows the first three bending mode shapes of the towers under investigation. Even though, the fundamental natural frequencies were more than 1 Hz, the towers need to be considered as dynamically sensitive as per IS code14 since their aspect ratios (height/width) were observed to be more than 5. Hence in the present study, gust effectiveness factor based wind loads were considered for the failure analysis. Assuming that the towers collapsed were designed as per Indian practice, the failure analysis is being done with respect to IS codal14 provisions. It is evident that most of the international codes are also based on the assumption of stationary wind process resulting in background (due to low frequency wind excitation) as well as resonant (due to energy in wind around natural frequency of the structure) components of responses of structures2 . The gust effectiveness factor obtained using IS code14 were compared with those obtained using various international codes, viz., AS code8 , BS code9 , ASCE code10 , China code11 , Canada code12 , Japan code13 , in order to review the usage of various wind and response parameters to quantify the peak dynamic response.

FIG. 9. COMPLETE COLLAPSE OF THE 91 m TALL STEEL LATTICE TOWER

DYNAMIC WIND LOADS

The present failure investigation involves the estimation of the dynamic wind loads causing the failure of the two towers under the respective reported cyclone wind conditions. Wind force is basically random and dynamic in nature, and it is treated as stationary for simplicity in the analysis and design of wind sensitive structures. When the fundamental frequency of the structure is less than 1 Hz or the aspect ratio (height/width) of the structure is very high (> 5), most of the international wind loading standards recommend equivalent steady state wind loads based on gust effectiveness factor approach as given below, 1 2 F = GCf ρV Ae 2

(1)

where, Cf = force coefficient; V = hourly mean wind speed; ρ = air density; Ae = exposed area; and G = gust 150

FIG. 10. FIRST THREE MODE (BENDING) SHAPES OF THE TOWERS. (a) 101 m, (b) 91 m

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EVALUATION OF GUST EFFECTIVENESS FACTOR (GEF) BASED ON THE PROVISIONS OF THE INTERNATIONAL CODES

Gust effectiveness factors for the two towers have been evaluated based on expressions provided in various international codes for a standard terrain category of open fields with a terrain roughness height, z0 , close to 0.02–0.03 m (or) with a power law coefficient, α, close to 0.15–0.165.

ASCE Standard 7–9810

This code suggests that for the slender buildings and other structures that have a fundamental natural frequency less than 1 Hz, the gust effect factor shall be calculated by (clause 6.5.8.210 ),

  2 Q2 + gR2 R 2 1 + 1.7Iz + gQ  Gf = 0.925  (4) 1 + 1.7gv Iz

Australian Standard AS 3995–19948

The Australian code suggests two methods, viz., (i) simplified method, (ii) detailed approach, to evaluate the gust response factor for free-standing lattice towers, based on stochastic response of a linear single degree of freedom system (i.e., corresponding to the first mode of the structure). In this paper, the gust response factors were evaluated for terrain category 2 with the roughness length, z0 = 0.02 m, for both the towers6,7 based on the simplified method given in clause 2.3.88 . In this method, the gust response factor for both bending moment and shear force is to be the same but it varies with height and is given as,
  SE 2 2 (2) Gs = 1 + rH gB Bs + gR ζ where, Gs = gust response factor; r = roughness factor; H = height factor; gB = peak factor for the background response; Bs = background factor; gR = peak factor for the resonant response; S = size factor for resonant response; E = gust energy factor; and ζ = critical damping ratio. gB given in above expression includes the effect of factor, φ, as given in IS code14 expression. Since the values obtained using the above expression are being compared with the values obtained using the IS code14 expression, instead of gB only gu as given in AS code8 was used for consistency. British Standard BS 8100: Part 1–19869

where, Gf = gust effect factor; Iz = intensity of turbulence at height z; gQ = peak factor for the background response; Q = background response factor; gR = peak factor for the resonant response; R = resonant response factor; gv = peak factor for wind response. However, this gust effect factor is to be applied over wind loads obtained using 3-sec gust wind speed. Hence, the expression given in the denominator in Eq. (4) was excluded in order to be consistent with other international codal values that are applied over wind loads obtained using mean hourly wind speed. The gust effect factors were evaluated for terrain category C with α = 0.1538, for both the towers6,7 . National Standard of the People’s Republic of China GBJ 9–87–199411

This code suggests that for buildings/structures with a height greater than 30 m and a ratio of height to width greater than 1.5, and for high-rise structures such as tower frames, masts, chimneys, etc., with a fundamental natural period of vibration T1 which is greater than 0.25 s, the dynamic wind effect factor shall be adopted in consideration of the fluctuation effects of wind pressure. This means fundamental modes with frequencies upto 4 Hz must be checked for dynamic wind effects. The dynamic wind effect factor of high-rise structures or tall buildings at the height z, may be calculated as (clause 6.4.211 ): βz =

1 + ξ νϕz µz

The British standard suggests gust response factor for bending moment and shear force separately, which vary with height under the equivalent static method. In this paper, the gust response factor was evaluated for bending moment alone for terrain category III with the terrain roughness parameter, z0 = 0.03 m, for both the towers6,7 . The gust response factor for fluctuating bending moment at any height zm is given as (clause 5.2.39 and G59 ):   z 2  m G = GB 1 + 0.2 (3) H

where, βz = dynamic wind effect factor; ξ = magnification factor of wind fluctuation; ν = wind fluctuation factor; ϕz = vibration mode factor; µz = exposure factor for wind load. The dynamic wind effect factors were evaluated for terrain category B (open), for both the towers6,7 .

where, G = gust response factor; GB = basic gust response factor; zm = height above the ground at which bending moment is required; H = overall tower height. The gust response factor for total bending moment (= 1 + G) at the base of the tower is evaluated with α = 0.165, for both the towers and taking zm as zero. The British standard recommends the gust response factor for top deflection is the same as that for the base bending moment.

The code suggests a dynamic approach to the action of wind gusts to be called “detailed procedure” for tall buildings and slender structures. The gust effect factor, equal to the ratio of peak loading to the mean loading, is given as (clause 4.1.8.1. (6)12 ):   σ C g = 1 + gp (6) µ

(5)

National Building Code of Canada-199512

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where, Cg = dynamic gust factor; gp = statistical peak factor for the loading effect; σ = root-mean square loading effect; µ = mean loading effect. The value of σ/µ is given as,
  K SF σ = + B+ (7) µ Ce H β

Indian Standard IS-875 Part-3 (1987)14

The gust effect factors were evaluated based on this procedure for terrain category A, for both the towers6,7 .

where, G = gust effectiveness factor; gf = peak factor; r = roughness factor (twice the value of intensity of turbulence, σV /V ); B = background factor; S = size reduction factor; E = measure of available energy in the wind stream at the natural frequency of the structure; β = damping coefficient of the structure, σV = rms value of along wind fluctuations and V = mean wind velocity. The parameters 14 gf , r, B, S, E and β can be obtained from √ IS code and the parameter φ can be calculated as (gf r B)/4, and is to be accounted only for buildings less than 75 m high in terrain category 4 and for buildings less than 25 m high in terrain category 3 and is to be taken as zero in all other cases. The gust effectiveness factors were evaluated for terrain category 2 with φ = 0 for both the towers6,7 .

AIJ Recommendations for Loads on Buildings, Japan13

The code suggests two methods, viz., (i) detailed procedure I, (ii) detailed procedure II to evaluate the gust effect factor, for tall and relatively flexible buildings based on the basic wind speed corresponding to the 10-min mean wind speed. In this paper, the gust effect factor is evaluated based on detailed procedure II, for the design of structural frames and components/cladding of buildings when resonant effects are not negligible. This code suggests that the same procedure can also be used for the design of buildings when resonant effects are small. The gust effect factor can be calculated from the equation (clause 6.3.2. (2)13 ):

(8) Gf = 1 + gf rf Bf + Rf where, Gf = gust effect factor; gf = peak factor; rf = turbulence factor; Bf = background excitation factor; Rf = resonance factor. The gust effect factors are evaluated for category II with α = 0.15, for both the towers6,7 .

This code suggests the following expression for estimation of gust effectiveness factor,
  SE 2 (9) G = 1 + gf r B(1 + φ) + β

COMPARISON OF CODAL PROVISIONS

Table 1 gives the GEF values as per various international codal expressions. The gust effectiveness factor as per IS code14 for the 101 m tower was obtained as 1.828. The gust effectiveness factors for the 101 m tower as per AS code (with gu )8 , BS code9 , ASCE code (over hourly mean wind speed)10 , China code11 , Canada code12 and

TABLE 1 GUST EFFECTIVENESS FACTOR FROM VARIOUS INTERNATIONAL CODAL PROVISIONS Code(s)

Ht. (m) GEF Basic input values to evaluate the gust effectiveness factor 101 2.022 T.C.:2; h = 101 m; s = 0; n = 1.38 Hz; ws = w0 = 8.038 m; T = 3600 s; ζ = 0.05; V = 44 m/s; M (z,cat) = 0.841; M t = 1.0; Md = 1.0; z0 = 0.02 m AS:3995-19948 91 2.064 T.C.:2 h = 91 m; s = 0; n = 1.47 Hz; ws = w0 = 7.337 m; T = 3600 s; ζ = 0.05; V = 44 m/s; M (z,cat) = 0.8292; M t = 1.0; Md = 1.0; z0 = 0.02 m BS:8100-Part 1101 1.989 T.C.:III; H = 101 m; z = 0 m; zm = 0 m; z0 = 0.03 m; KR = 1; α = 0.165; z0 = 0.03 m 91 2.015 T.C.:III; H = 91 m; z = 0 m; zm = 0 m; z0 = 0.03 m; KR = 1; α = 0.165; z0 = 0.03 m 19869 101 0.871 T.C.:C; b = 0.65; h = 331 ft; z = 199 ft; α = 0.1538; n1 = 1.38 Hz; B = 26 ft; L = 26 ft; β = 0.02; ε = 1/5; gq = gv = 3.4; c = 0.2; V = 44 m/s; l = 500 ft 10 ASCE7-98 91 0.873 T.C.:C; b = 0.65; h = 299 ft; z = 179 ft; α = 0.1538; n1 = 1.47 Hz; B = 24 ft; L = 24 ft; β = 0.02; ε = 1/5; gq = gv = 3.4; c = 0.2; V = 44 m/s; l = 500 ft GBJ 101 1.927 T.C.:B; T1 = 0.73 s; v0 = 31 m/s; z = 101 m; H = 101 m; Bh = 1.8 m; Bo = 14.274 m 91 1.937 T.C.:B; T1 = 0.68 s; v0 = 31 m/s; z = 91 m; H = 91 m; Bh = 1.8 m; Bo = 12.874 m 9-87-199411 101 1.902 T.C.:A; H = 101 m; W = 8.037 m; n0 = 1.38 Hz; β = 0.01; V = 30 m/s; K = 1; Ce H = 1.4; NBC of B = 0.92; s = 0.07; F = 0.122 Canada-199512 91 1.929 T.C.:A; H = 91 m; W = 7.337 m; n0 = 1.47 Hz; β = 0.01; V = 30 m/s; K = 1; Ce H = 1.3; B = 0.96; s = 0.06; F = 0.107 101 1.646 T.C.:II; H = 101 m; B = 8.037 m; n0 = 1.38 Hz; ZG = 350 m; U0 = 47 m/s; nf = 0.02; α = 0.15; AIJ Recommenr = 100 yr; Eg = 1.0 13 Dations , 1.666 T.C.:II; H = 91 m; B = 7.337 m; n0 = 1.47 Hz; ZG = 350 m; U0 = 46 m/s; nf = 0.02; α = 0.15; 91 Japan r = 100 yr; Eg = 1.0 101 1.828 T.C.:2; Vb = 44 m/s; k1 = 1.0; k 2 = 0.9208; k3 = 1.0; h = 101 m; b = 8.037 m; Cy = 10; Cz = 12; IS:875-Part-2f0 = 1.38 Hz; β = 0.02 198714 91 1.858 T.C.:2; Vb = 44 m/s; k1 = 1.0; k 2 = 0.91; k3 = 1.0; h = 91 m; b = 7.337 m; Cy = 10; Cz = 12; f0 = 1.47 Hz; β = 0.02 Note: T.C.: Terrain category 152

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Japan code13 were obtained as 1.85, 1.989, 1.617, 1.927, 1.902, and 1.646, respectively. The percentage variation of these values as compared to IS code14 value were obtained as +1.2%, +8.8%, −11.5%, +5.4%, +4.0% and −10.0%, respectively. The gust effectiveness factor as per IS code14 for the 91 m tower was obtained as 1.858. The gust effectiveness factors for the 91 m tower as per AS code (with gu )8 , BS code9 , ASCE code (over hourly mean wind speed)10 , China code11 , Canada code12 and Japan code13 were obtained as 1.879, 2.015, 1.634, 1.937, 1.929, and 1.666, respectively. The percentage variation of these values as compared to IS code14 value were obtained as +1.1%, +8.4%, −12.1%, +4.3%, +3.8% and −10.3%, respectively. The gust effectiveness factor values as per these international codes were observed to be within and about ±10% of variation as compared to the gust effectiveness factor values as per IS code14 . Hence in the present investigation, the expression for gust effectiveness factor as per IS code14 is considered for the detailed failure investigation. FAILURE ANALYSIS OF TOWERS Under Normal Wind Conditions

In most damage surveys, collection of measured wind data at the failure site is difficult to ascertain. Exact failure triggering mechanism and progressive failure sequences during cyclones is not well understood due to lack of documented information. However internationally adopted design procedures have similar basic concepts to account for the dynamic effects of wind on open latticed structures. In the square towers, it is obvious that quartering wind (diagonal) will be governing for the design of leg members, since only two leg members will be actively resisting the moment due to lateral loads. While for the design of bracing members, wind normal to the face will be governing. Hence, the static analysis of 101 m and 91 m tall towers were carried out for wind loads calculated using basic wind speed of 44 m/s (as per IS code14 ), and the corresponding mean hourly wind speed of 29.48 m/s considering terrain category 2 and gust effectiveness factors of 1.828 and 1.858, respectively. The basic wind speed is defined as highest 3-sec gust wind speed at 10 m above ground level in a category 2 terrain (having mean surface roughness of 0.02 m). Figures 2 and 7 show the identified critical leg and bracing members for 101 m and 91 m tall steel lattice towers, respectively. Using limit state design approach, the leg and bracing members having the least margin of allowable to calculated stresses were identified in both the towers. For both the towers, the critical leg member was noted to be in the first panel from bottom and the critical bracing member was noted to be in the second panel from bottom initiating a condition for first passage failure leading to collapse of the tower. The expected buckling mode of failure as per analysis involving the critical leg member and the critical bracing member seems to compare well with the failed towers

as shown in Figs. 4 and 9. The limiting basic wind speeds causing the failure of the critical leg and bracing members with respective critical wind directions (diagonal/normal to face) were obtained as 67 m/s and 48 m/s, respectively for 101 m tall tower, and as 66 m/s and 54 m/s, for 91 m tall tower. Failure is expected to be initiated when the limiting basic wind speed for the critical bracing member is exceeded. Since the reported maximum wind speeds were sustained over 3-minutes of duration, the corresponding basic wind speed (3-sec gust) can be obtained by applying a gust factor over the 3-minute (180 sec) sustained wind speed. The gust factor can be obtained using the following expression as reported elsewhere19 for terrain category 214 and at 10 m level.   t 1.13 (10) Gv (t) = 1 − 0.59(0.15) ln 3600 where t is the averaging period in seconds. The gust factor for converting the 3-minute sustained wind speed to 3-sec gust speed is obtained as 1.23{= Gv (3)/Gv (180)}. For 101 m tall steel lattice tower, the site specific maximum wind speed was 35.46 m/s sustained over 3-minutes. The corresponding site specific cyclone basic wind speed (3-sec gust) is obtained by applying the gust factor as 43.61 m/s (= 1.23 × 35.46) which is less than the limiting basic wind speed of 48 m/s for the critical bracing member as obtained by the analysis. For 91 m tall steel lattice tower, the site specific maximum wind speed was 39.74 m/s sustained over 3-minutes. The corresponding site specific cyclone basic wind speed (3-sec gust) is obtained by applying the gust factor as 48.90 m/s (= 1.23 × 39.74) which is less than the limiting basic wind speed of 54 m/s for the critical bracing member as obtained by the analysis. In both the cases, the failures are not envisaged using the GEF values evaluated as per the existing codal provisions14 . Since the measured cyclone wind characteristics were different from normal wind characteristics, the GEF values under the cyclone wind conditions have to be evaluated with the modifications to various parameters as discussed in the following section to obtain the modified GEF values. Parameters for Cyclone Wind Conditions

Two full scale field experiments on a 101 m and a 52 m (at Structural Engineering Research Centre, Chennai) tall steel lattice towers were carried out to measure the wind, terrain and structural characteristics during normal and extreme wind conditions. The details of the structure, instrumentation system, collection of data, analysis of data and results of analysis are explained elsewhere4,5,20,21 . Based on the measurements during normal and cyclonic wind conditions it was observed that some of the specifications recommended in the IS code14 needed modifications for the evaluation of the gust effectiveness factor4,5 . Based on these observations, the following modifications to the parameters φ, gf , r, L, E and β, given in IS

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code14 for the evaluation of gust effectiveness factor for cyclone prone regions based on measured spectral characteristics of cyclone winds are considered15 . a) The value of background enhancement factor, φ, has to be considered for all terrain categories irrespective of the height of the tower5 . b) In cyclone prone areas, the structures should be designed for a pseudo rougher terrain category (category 3 instead of category 2), with the same mean wind velocity, V 10 , since the turbulence intensities are bound to be more than those during normal winds, thus increasing the dynamic loads. Hence, the values of gf , r and L have to be taken for the next rougher terrain category15 . c) The gust energy content in the cyclonic wind spectrum was observed to be constant between 1 and 10 Hz4 , whereas in normal wind, the gust energy decays beyond 1 Hz. Hence in cyclone prone areas, the modified turbulence spectrum may be used, and the structures with natural frequency more than 1 Hz also be considered dynamically wind sensitive. Hence the gust energy factor, E, used in Eq. (9), has to be modified taking into account of the measured cyclonic wind spectrum4 as given in Fig. 11. d) Since the aerodynamic damping increases with mean wind speed, the total damping ratio should include the aerodynamic damping in addition to structural damping. Hence a damping ratio of 0.0415 can be considered instead of 0.02 for bolted steel structures as per the current code of practice14 .

FIG. 11. GUST ENERGY FACTOR, E

Under Cyclone Wind Conditions

Under the post-collapse failure investigation of the two towers, the sensitivity of the GEF, which is inline with various international codes of practice, to parameters, with suggested modifications under cyclone wind conditions, has been studied. The GEF values with suggested modifications15 under cyclone wind conditions can be 154

written as,




Gc = 1 + gf rc B(1 + φc )2 +



 S(Ec1 + Ec2 + Ec3 ) βc (11)

where, Gc = GEF with cyclone wind characteristics; gf rc = peak factor and roughness factor due to cyclone for the next rougher terrain category (i.e. √ terrain category 3); B = background factor; φc = (gf rc B)/4); S = size reduction factor; Ec1 , Ec2 , Ec3 = measure of available energy in the cyclone wind storm at first (f01 ), second (f02 ) and third (f03 ) natural frequencies of the tower; and βc = increased total damping coefficient of the tower. The gust effectiveness factors were evaluated with these modified parameters for both towers. Tables 2 and 3 show the individual and combined effect of modifications as given in Eq. (10) for the evaluation of GEF for 101 m and 91 m tall steel lattice towers, respectively. Further, the contribution of two more higher modes was also included along with modified damping coefficient βc of 0.04 instead of usual 0.02, due to possible increase in aerodynamic damping. The Gc under cyclone wind conditions after considering the modifications discussed above, were obtained as 2.28 and 2.38 for 101 m and 91 m tall steel lattice towers, respectively. The ratios (Rg ) of Gc under cyclone wind condition (Eq. (11)) to G as per codal provisions14 (Eq. (9)) are obtained as 1.25 and 1.27 for 101 m and 91 m tall steel lattice towers, respectively. With the member capacity remaining the same, increase in gust effectiveness factor due to higher turbulence levels causes a reduction in limiting basic wind speed for the critical member. Since the dynamic wind loads are proportional to square of wind speed, the limiting basic wind speeds of the critical √ bracing members would be reduced by a factor of (1/ R g ).   1 (12) Vb Vlb = √ Rg where, Vlb = Limiting basic wind speed. For 101 m tall steel lattice tower, considering the enhanced gust factor, the limiting basic wind speed√ (as per Eq. (12)) at failure is estimated as 43 m/s (= 48/ 1.25) which is close to the reported site specific cyclonic basic wind speed of 43.61 m/s. For 91 m tall steel lattice tower, considering the enhanced gust factor, the limiting basic wind speed (as√ per Eq. (12)) at the failure is estimated as 48 m/s (= 54/ 1.27) which is close to the reported site specific cyclonic basic wind speed of 48.90 m/s. The closeness of the limiting wind speeds derived based on strength of bracings including the effects of increased turbulence to the reported cyclonic wind speed validates the design recommendations15 as discussed in the previous section. The critical bracing members in the 101 m and 91 m tall steel lattice microwave towers would have initiated the failure during the respective cyclonic storms. The failure of the critical bracing member causes the loss of triangulation and load path in the second panel from bottom, leading

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TABLE 2 CALCULATION OF DYNAMIC WIND PRESSURE FOR 101 m TALL STEEL LATTICE TOWER

Parameters f0 , (Hz) V h , (m/s) L(h), (m) gf r B φ S E β (SE/β) GEF/GRF % of increase

‘Gc ’ with modified parameters for cyclonic wind conditions φc , gf rc , L(h)c , φc , gf rc , L(h)c , ‘G’ IS code14 φc , gf rc , Ec1 , Ec2 , Ec3 , Ec1 , Ec2 , Ec3 , with φ = 0 Ec φ gf rc , L(h)c L(h)c , Ec (f01 + f02 + f03 ) (f01 + f02 + f03 ), βc 1.38 1.38 1.38 1.38, 2.83, 4.69 1.38, 2.83, 4.69 1.38 1.38 40.52 40.52 40.52 40.52 40.52 40.52 40.52 1687.5 1687.5 1687.5 1450.0 1450.0 1450 1450 1.1875 1.1875 1.1875 1.1875 0.9333 0.9333 0.9333 0.7 0.7 0.7 0.72 0.72 0.72 0.7 0.0 0.0 0.197983 0.0 0.248383 0.248383 0.248383 0.036 0.036 0.036 0.036 0.036 0.036,0.0215, 0.0086 0.036,0.0215, 0.0086 0.037 0.054 0.037 0.037 0.063 0.063,0.043, 0.043 0.063, 0.043, 0.043 0.02 0.02 0.02 0.02 0.02 0.04 0.02 0.0972 0.0666 0.0666 0.1134 0.1435 0.07355 0.0666 1.844 1.979 2.040 2.303 2.319 2.281 1.828 24.82 0.87 8.27 11.66 26.01 26.90

TABLE 3 CALCULATION OF DYNAMIC WIND PRESSURE FOR 91 m TALL STEEL LATTICE TOWER

Parameters f0 , (Hz) V h , (m/s) L(h), (m) gf r B φ S E β (SE/β) GEF/GRF % of increase

‘G’ IS code14 with φ = 0 Ec 1.47 1.47 40.04 40.04 1625.0 1625.0 0.97 0.97 0.72 0.72 0.0 0.0 0.037 0.037 0.034 0.054 0.02 0.02 0.0629 0.0999 1.858 1.878 1.08

‘Gc ’ with modified parameters for cyclonic wind conditions φc , gf rc , L(h)c , φc , gf rc , L(h)c , φc , gf rc , Ec1 , Ec2 , Ec3 , Ec1 , Ec2 , Ec3 , gf rc , L(h)c L(h)c , Ec (f01 + f02 + f03 ) (f01 + f02 + f03 ), βc φ 1.47, 2.89, 4.80 1.47 1.47, 2.89, 4.80 1.47 1.47 40.04 40.04 40.04 40.04 40.04 1420.0 1420 1420 1625.0 1420.0 1.25 1.25 1.25 1.25 0.97 0.72 0.7 0.7 0.7 0.7 0.205768 0 0.261456 0.261456 0.261456 0.037 0.037 0.037 0.037, 0.011, 0.0044 0.037, 0.011, 0.0044 0.034 0.058 0.058, 0.043, 0.043 0.058, 0.043, 0.043 0.034 0.04 0.02 0.02 0.02 0.02 0.0629 0.0629 0.1073 0.1404 0.0702 2.022 2.092 2.381 2.400 2.360 8.57 12.57 28.15 29.15 27.01

to large deflections in both the towers. The second order effects would have induced combined axial and bending stresses in the critical leg members of the bottom panel as can be seen in Figs. 3 and 8, for which the leg members were not designed. Hence the total collapse of the towers. CONCLUSIONS

Based on the failure analysis, the first passage failure is observed to be initiated always in the critical bracing members of both the towers. After the failure of the critical bracing members, the second order effects would have induced combined axial and bending stresses in the critical leg members of the bottom panel for which the leg members were not designed. This is ascertained by the observed failures (Figs. 3 and 8) which clearly indicate exceedance of combined bending and axial tension/compression in the leg members. The basic wind speeds at both sites corresponding to reported maximum wind speeds did not envisage the failure initiation in both the towers using the GEF values of G as given in Eq. (9), evaluated as per existing practice14 .

The dynamic wind loads acting on a lattice tower are increased greatly for the same mean wind speeds due to enhanced turbulence intensities and higher energy at high frequency regime of the wind spectra during a cyclone. Based on the earlier measurements, it was reported4,5,15 that during cyclones, there are significant changes in the practiced values of gf , r, φ, E, and β, which are used for evaluating dynamic wind loads on the tower. By considering the suggested modifications in evaluating Gc (Eq. (11)), the dynamic wind loads increase by 25% for 101 m and 27% for 91 m tall lattice tower. From the reported information, the estimated cyclonic basic wind speeds have been 43.61 and 48.90 m/s at the sites of 101 m tall steel lattice tower and 91 m tall steel lattice tower, respectively. The minimum limiting basic wind speeds reworked based on the failure triggering mechanism of brace members of both the towers have been 48 m/s (101 m tall steel lattice tower) and of 54 m/s (91 m tall steel lattice tower). With the member capacities remaining the same, increase in GEF (Gc > G) values due to higher turbulence would cause the member to fail even when the cyclonic basic wind speeds are lower than the limiting basic wind speeds, as has

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been observed in both the towers under respective cyclone conditions. While the simplicity in design procedure using GEF cannot be over looked, due to possible deviations of the parameters used for the calculation of Gc under cyclone wind conditions, the conservatism in designs using G may not be guaranteed. Hence in cyclone prone regions, it is preferable to consider Gc for the design of these towers along with the wind speed profiles corresponding to local terrain conditions to have an extra margin of safety against the additional turbulence induced dynamic peak loads. ACKNOWLEDGEMENTS

This paper is being published with the kind permission of the Director, Structural Engineering Research Centre, Madras. The authors wish to pay the deep sense of gratitude to their colleague (Late) Shri. J. Shanmugasundaram, Deputy Director (Field Experiments Laboratory), for his valuable suggestions during the course of the investigation. NOTATION

B B B B0 BH Ce H Cy Cz Eg F H H H H K KR L Md Mt M (z,cat) T T1 U0 V V V VH Vb W ZG 156

horizontal dimension of building measured normal to wind direction10 background turbulence factor12 projected breadth13 windward side bottom width of the structure windward side top width of the structure exposure factor at the top of the building, H lateral correlation constant longitudinal correlation constant topography factor gust energy ratio at the natural frequency of the structure overall tower height9 total height11 height of windward face of the building12 reference height13 a factor related to the surface roughness terrain roughness factor horizontal dimension of building measured parallel to the wind direction wind direction multiplier topographic multiplier for mean wind speeds mean wind speed multiplier for a terrain category at height z averaging time fundamental natural period of vibration basic wind speed basic wind speed8 basic wind speed10 reference wind speed mean wind speed (m/s) at the top of the structure, H regional basic wind speed width of windward face of the building gradient height

b b c f0 gv h h h k1 k2 k3 l n n0 n0 n1 nf r s s v0 w0 ws z z z z0 z0 zm α α α β β δ ε ζ

breadth of the structure normal to the wind stream mean hourly wind speed factor turbulence intensity factor natural frequency of the structure peak factor for wind response height of the structure above ground8 mean roof height of a building10 height of a structure14 probability factor terrain and height factor topographic factor integral length scale factor first mode natural sway frequency natural frequency of vibration12 natural frequency for the first translation mode in along-wind direction13 building natural frequency critical damping ratio for the first translation mode in along-wind direction design return period height of the design peak shear force and design bending moment above ground level size reduction factor12 maximum mean wind velocity average width of the structure between h/2 and h average width of the structure between h and s height above the ground at which bending moments or shear force is required9 height above the ground level10 reference height11 roughness length8 terrain roughness parameter9 height above the ground at which bending moment is required power law index of wind speed variation with height9 exponent of the power law in the wind speed profile13 mean hourly wind speed power law exponent critical damping ratio12 damping coefficient14 decay coefficient17 integral length scale power law exponent critical damping ratio

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