Analysis of communication tower with different heights subjected to wind loads using TIA-222-G and TIA-222-H standards

ABSTRACT Due to advancements in telecommunications, towers need special attention in terms of the analysis and design under wind loads. The Telecommunications Industry Association (TIA) in 2005 released a standard “TIA-222-G” which has gained a widespread reference for the analysis and design of communication towers. In 2018, TIA released the latest standard TIA-222-H. The latest TIA-222-H standard has some additional features, e.g. limit states for analysis of mounting systems, enhanced climber safety requirements, construction-related loading, etc. To date, not many studies are available describing how much change in member axial forces occurs with the tower height while using the latest standard for analysis. This study’s main objective is to provide guidelines for wind load calculation on tower body, appurtenances, and other structures and compare the member axial forces induced by the wind loads on different tower heights (40, 60, and 80 m) as per TIA-222-G and TIA-222-H standards. The procedure presented in the paper about the design calculations of wind load is a useful guide for structural engineers involved in the analysis and design of communication towers. The analysis results showed that the member axial forces increased by 22% to 37%, which can assist the practitioner in more optimized design.


Introduction
The best type of communication towers is selfsupporting towers with large face widths greater than or equal to the diameter of the mounted dishes (Albermani, Kitipornchai, and Chan 2009). The tower's design is an interactive compromise between many factors, which must ultimately satisfy basic strength requirements (Alam and Santhakumar 1996;Albermani, Mahendran, and Kitipornchai 2004;Zhang et al. 2009;Huang et al. 2020). The Telecommunications Industry Association (TIA) accredited by the American National Standards Institute (ANSI) develops TIA-222 standards with an objective to recognize literature about (a) minimum load requirements as derived from ASCE 7-16 (ASCE 7 2016), (b) design criteria as derived from AISC 360-16 (AISC 360 2016) and building code requirements for structural concrete (ACI 318 2019). TIA in 2005 released a standard TIA-222-G, namely "Structural Standard for Antenna Supporting Structures and Antennas" (TIA-222-G 2005), which became effective on 1 January 2016. The TIA-222-G Standard is highly appreciated and commonly used to analyze and design both structures and antennas under both wind and seismic loadings by local and international experts. In 2018, TIA released the latest standard TIA-222-H, namely "Structural Standard for Antenna Supporting Structures and Antennas and Small Wind Turbine Support Structures" (TIA-222-H 2018). This Standard provides the requirements for the structural design and fabrication of new and the modification of existing structural antennas, antenna-supporting structures, mounts, structural components, guy assemblies, insulators, and foundations. Structures like towers are sensitive to dynamic wind loads, and there is a need to design a lattice tower considering a dynamic response to wind loads considering its height (Jie 2006;Xie et al. 2009). The latest TIA-222-H standard uses ultimate wind, ice, and earthquake criteria, etc. To date, a lot of research has been done on tower members and complete towers. Zhang and Young (2012) performed compression tests of coldformed steel I-shaped open sections with edge and web stiffeners. The appropriateness and reliability of the direct strength method for I-shaped open sections with edge and web stiffeners was evaluated. It was found that the direct strength method can be used for cold-formed steel I-shaped open sections with edge and web stiffeners (Zhang and Young 2012). Some researchers studied the non-linear behavior of cold-formed steel sections (Dabaon, Ellobody, and Ramzy 2015;Roy et al. 2018aRoy et al. , 2018b, whereas, others performed experimental and numerical investigations on cold-formed steel sections (Roy et al. 2018c;Roy, Mohammadjani, andLim 2019a, 2019b). Ting et al. (2018) studied the effect of screw spacing on behavior of axially loaded back-to-back cold-formed steel builtup channel sections. The authors also prepared finite element model which showed good agreement with the experimental test results (Ting et al. 2018). Yaghoobi and Shooshtari (2018) performed a study that is useful to help in designing of wind turbine towers with a higher level of accuracy and safety (Yaghoobi and Shooshtari 2018). Gao and Wang (2018) studied the progressive collapse analysis of latticed telecommunication towers under wind loads. A progressive collapse fragile curve based on collapse probability of telecommunication tower under wind loads was proposed to assess the anticollapse performance of the towers (Gao and Wang 2018). Gao et al. (2020) performed a study that aimed at the dynamic response analysis and collapse simulation of wine-cup shape power transmission tower-line system under iceshedding. The study suggested that special attention should be paid on the structural members connecting the middle V-part and other parts of the tower, whose fracture would easily trigger the collapse of the whole tower (Gao et al. 2020). Rasool, Qureshi, and Ahmad (2021) a comparative study on the calculation of wind load and analysis of communication tower as per TIA-222-G and TIA-222-H standards and showed the effect of wind load on tower leg members (Rasool, Qureshi, and Ahmad 2021). James Butts (2021) stated that TIA-222-H Code uses ultimate wind speeds instead of basic wind speeds. The difference is ultimate wind speeds were calculated using much longer return periods. The return periods are now function of the Risk Category. The higher the Risk Category, the longer the return period, and the higher the wind speed (Butts 2021). Rosenberger (2020), in his report, performed wind tunnel tests and found higher drag coefficients on wind load using the TIA-222-H standard (Rosenberger 2021). As most of the existing towers around the world have been constructed as per guidelines of TIA-222-G, the latest standard also states, "Existing structures originally designed following a previous revision of this Standard are exempt from the provisions of this Standard". However, to date, not many studies are available describing how much change in member axial forces occurs with the tower height while using the latest standard for analysis of towers.
The main objective of this study is to provide guidelines for wind load calculation on tower body, appurtenances, and other structures and to compare the member axial forces induced by the wind loads on different tower heights as per TIA-222-G & TIA-222-H standards.
The paper first describes the analysis data in form of tower configuration, analysis software used and analysis assumptions, followed by the wind load calculations and analysis as per TIA-222-G and TIA-222-H standards. In the end, the design of the tower is also presented followed by the comparison of analysis & design results and concluding remarks.

Analysis data
To perform the simplified comparative study, computer models were prepared and analyzed by considering the dead and wind loads only. Details of tower configuration, analysis software used, and dead and wind loads are given as follows, load applied/acting on the tower.

Tower configuration
This study gives the analysis of the 40, 60, and 80 m high four-legged self-supporting towers. The general arrangement of the towers, including member sizes, is shown in Figure 1.

Analysis software
3D Line models of 40, 60, and 80 m towers were prepared to perform analysis on STAAD Pro V8i structural analysis software (STAAD V8i 2014). The towers were modeled in STAAD as a space frame, which takes all joints as pin joints. The tower support was also modeled as pin support. A typical mathematical and 3D model of the tower is shown in Figure 2. A standard constant E = 200 × 106 kN/ m 2 is assumed. Four load combinations that have been considered for analysis are; Where D is "Dead Load of Structure", W 0 is "Wind Load on Structure at 0°", and W 45 is "Wind Load on Structure at 45°"

Dead loads on tower
The tower's dead loads include the self-weight of the tower, the weight of the antenna and other equipment, and the weight of the ladder and feeders. Other lateral loads include ice, earthquake, and wind loads. Loads calculations other than wind loads are the same for both standards.

Wind load calculations and analysis
The wind force on 40, 60 & 80 m high tower, ladder, cables, linear appurtenances & antenna, and discrete appurtenances were computed at basic wind speeds of 125 kph & 225 kph for Structure Class I, II & Risk Category I, II as per both standards, respectively. Due to space limitations, detailed calculations of 80 m high tower at a wind speed of 125 kph for Structure Class I & Risk Category I as per relations given in TIA-222-G & TIA-222-H respectively are given here. In TIA-222-G, Structure classification (I, II, or III) is used to classify the structure. Structure Class I defines "Structures that due to height, use or location represent a low hazard to human life and damage to property in the event of a failure and/or used for services that are optional and/or where a delay in returning the services would be acceptable". Structure Class II defines "Structures that due to height, use or location, represent a significant hazard to human life and/or damage to property in the event of failure and/or used for services that may be provided by other means". In TIA-222-H, the Risk Category explains the function of risk to human life, potential damage to the facility, and the structure's primary use. Risk Category I defines "Structures that due to use or location represent a low risk to human life and/or damage to surrounding facilities in the event of failure". Risk Category II defines "Structures that due to use or location represent a moderate risk to human life and/or damage to surrounding facilities in the event of failure". The towers were analyzed using STAAD Pro V8i (STAAD V8i 2014). Finally, the analysis results are discussed and compared.

Analysis assumptions
The following assumptions are made in tower analysis as per both standards; (1) The height of the ladder is the same as the tower and has been divided into similar four sections as the tower (Assumption-1).
(2) 32-5 GHz-MW dishes (420 x 420 × 230) and 1-24 GHz-MW dish (768 x 593 × 370) will be installed on the tower. The force coefficients for dishes have been taken from TIA-222-G and TIA-222-G H, Annexure-C, Table C1. To give maximum force coefficients half dish area is placed at 0° and half at 180° azimuth about the tower (Assumption-2). (3) 3-LTE antennas (2570 x 270 × 136) and 3-RRU antennas (150 x 190 × 280) will be installed on the tower with one antenna facing the wind and two at 120° to wind (Assumption-3). (4) The antenna/dish supplier normally provides a layout for the location of dishes, which is beyond the scope of the current study, and their weight was distributed on the top 10 m of the tower in the form of a nodal load.

Wind load calculations
Wind calculations similar for both standards and specific for TIA-222-G and TIA-222-H will be discussed in this section.

Similar for both (TIA-222-G & TIA-222-H) standards
The factors similar in both standards are The solidity ratio (ε) for each segment of 40, 60, and 80 m high tower is calculated using relation (A f + A r )/A g , as shown in Table 1.
where, A f is the projected area of flat structural components taken from AISC A g is the gross area of one tower face calculated from AutoCAD A r is the projected area of round structural components The design wind force on the tower is calculated as, The effective projected area (EPA) S of each tower segment is calculated in Table 2 using the following relations, where, C f = Force Coefficient for a Structure D f = Wind Direction Factor for Flat Structural Components D r = Wind Direction Factor for Round Structural Components R r = Reduction Factor for a Round Element in a Tower Face The wind force is assumed to be applied at an angle of 0° and 45°, therefore, The effective projected area (EPA) A of the ladder (Assumption-1) and appurtenances are calculated in Table 3 using the relation below by employing K a = 0.6;

Using TIA-222-G standard
Wind load in TIA-222-G is based on 3-sec gust (50-yr return). The factors include basic wind speed V = 125 kph = 34.7 m/sec, importance factor, I = 0.87 (Structure Class-I) and I = 1 (Structure Class-II) and exposure category = C. Based on (EPA) A & (EPA) S calculated in Tables 2 and 3, design wind force on each tower segment, ladder (Assumption-1), cables & appurtenances is calculated in Table 4. Worst forces & moments on dishes (Assumption-2) are calculated in Table 5.
The wind force and moment on MW dishes are (4.51 kN, 2.25 kN.m). Whereas, wind force on LTE and RRU These forces are then applied to the tower model by dividing it by the number of nodes. Figure 3 illustrates one leg of 40 m high tower from the STAAD model showing the design wind on the structure, ladder, cables, appurtenances, antennas, and discrete  appurtenances calculated and applied at the nodes for the wind speed of 125 kph. The same pattern of the load has been applied on other legs, towers with different heights and wind speed of 225 kph, etc.  Figure 4(a). It is evident that for all tower heights, the forces in the leg members are decreased as the leg member distance from the tower base is increased. Leg members at the base of the tower showed maximum axial force and for the same member near the base of the tower, axial forces are increased with the increase in basic wind speed. Whereas, axial force in members is decreased for different wind speeds as the leg member distance from the tower base is increased. It can be seen from  wind speed of 125 and 225 kph is shown in Figure 6(a). This shows that change in the Structure Class in the TIA-222-G standard may affect the tower design by inducing more forces in members.
The solidity ratio (ε), effective projected area (EPA) S of each tower segment, and the effective projected area (EPA) A of the ladder (Assumption-1), and appurtenances are the same as calculated in Tables 1, 2, and 3, respectively. Based on (EPA) A & (EPA) S calculated in Tables 2 and 3, the design wind force on each tower segment, ladder (Assumption-1), cables & appurtenances are calculated in Table 6. The worst forces and moments on dishes (Assumption-2) are calculated in Table 7.
The wind force and moment on MW dishes are (7.21 kN, 3.61 kN.m). Whereas wind forces on LTE and RRU antennas (calculated using relation q z G h C a (EPA) A ) are (1.90 kN), and (0.12 kN) respectively. Hence, the total is 9.24 kN and 3.61 kN.m. The same calculations are also performed for the basic wind speed of 125 kph Risk Category-II and 225 kph Risk Category-I, II. These forces are then applied to the tower model by dividing it by the number of nodes.   Figure 4(b). It is evident that the forces in the leg members decrease as the height of the tower increases. Leg members at the base of the tower showed maximum axial force and for the same members near the base of the tower axial forces increased with an increase in basic wind speed. Whereas, axial force in members decreased for different wind speeds as the leg member distance from the tower base increased. It can be seen from Figure 4    is shown in Figure 6(b). This shows that change in the Risk Category in TIA-222-H may affect the tower design by decreasing forces in members.

Design of tower. After analyzing the tower
for different wind forces, the structural design of the tower was carried out in accordance with TIA-222-G, TIA-222-H, and ASIC standards (TIA-222-G 2005;AISC 360 2016;TIA-222-H 2018). Some aspects of tower design are listed as follows,

Design of members
(i) The design axial strength of the compression member was taken as ϕ c P n where, (ϕ c = 0.9 for TIA-222-G and 1.0 for TIA-222-H); P n = A g F cr (ii) The design axial tensile strength, ϕ t P n , of member was taken the lesser of yielding in the gross section, rupture in the net effective section or block shear rupture. However, block shear failure was not applicable in this case. • For tension yielding in the gross section: (ϕ t = 0.9 for TIA-222-G and 0.97 for TIA-222-H), P n = A g F y • For rupture in the effective net section: ϕ t = 0.75; P n = A en F u • For rupture in the effective net section: A en = A n U; 0.75 ≤ U ≤ 0.9 (use U = 0.75 for diagonal and 0.9 for leg members for TIA-222-G) (use U = 0.75 for diagonal and 0.8 for leg members for TIA-222-H)    Figures 7 and 8 respectively. It was observed that for all tower heights the member axial forces are increased by a percentage of 37% when analyzed for wind speed of 125 and 225 kph and Risk Category-I as per TIA-222-H. Similarly, member forces are observed to be increased by a percentage of 22% when analyzed for the wind speed of 125 and 225 kph and Risk Category-II as per TIA-222-H. Thus, analysis results show that the percentage difference in member forces remains the same with the tower height, it can also be noted that wind calculations as per TIA-222-H standard resulted in more member forces; however, change in Risk Category can also affect the member forces. It is also important to mention here that this study is limited to the comparison of member axial forces under dead and wind load  combination. The addition of any other forces might affect the analysis results.

Concluding remarks
Except the methods used in the paper, some of the most representative computational intelligence algorithms can be used to solve the problems, like monarch butterfly optimization (MBO) (Feng et al. 2021), earthworm optimization algorithm (EWA) (Pasupuleti and Balaswamy 2021), elephant herding optimization (EHO) (Wang, Deb, and Coelho 2015), moth search (MS) algorithm (Wang 2018), Slime mould algorithm (SMA) (Li et al. 2020), hunger games search (HGS) (Yang et al. 2021), Runge Kutta optimizer (RUN) (Ahmadianfar et al. 2021), colony predation algorithm (CPA) (Tu et al. 2021), and Harris hawks optimization (HHO) (Heidari et al. 2019). This study gives a comparative analysis of two ANSI/TIA standards (222-G & H) that are commonly used for the analysis and design of communication towers, poles, antennas, and supporting structures for antennas and small wind turbines.
• The procedure presented in the paper about the design calculations of wind load is a useful guide for structural engineers involved in the analysis and design of communication towers. The analysis results showed that the member axial forces increased by a percentage of 37% when analyzed for a wind speed of 125 & 225 kph (Risk Category-I) and 22% when analyzed for Risk Category-II, which can assist the practitioner in more optimized design. • The wind speed in TIA-222-H is significantly higher than in TIA-222-G. This is due to the change from using nominal wind speeds to ultimate wind speeds, which essentially have load factors and importance factors already builtin. • Ground elevation factor (Ke) is also an important factor. The density of air decreases as its distance from ground level increases. This means that at the same wind speed air produces more pressure on an object at sea level than it does at a higher elevation. TIA-222-H establishes a ground elevation factor (Ke) to take advantage of this, whereas, TIA-222-G conservatively calculated the wind pressure by assuming the site was located at sea level. • The ice thicknesses in TIA-222-H generally appear twice as big as in TIA-222-G. This is because a factor of two (02) was moved from the design ice thickness equation in TIA-222-G to the values in the ice thickness map in TIA-222-H. Essentially, a 1-inch thickness on the TIA-222-G map is equivalent to a 2-inch thickness on the TIA-222-H map. Incorporating the load factors into the mapped values makes the ice maps more consistent with the wind and seismic maps. • The latest TIA-222-H standard has some additional features including seismic analysis requirements for all risk categories (except Category-I), limit states for analysis of mounting systems, enhanced climber safety requirements, construction-related loading, etc.