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Wind Loads on Lattice Towers to IS 17740:2022 and IS 875 (Part 3)

The design wind load on a self-supporting lattice tower, step by step as IS 17740 sets it out — risk category, design wind speed and pressure from IS 875, the force on the structure, the force on the appurtenances, the wind directions and the dynamic check — with one section of a 40 m tower worked by hand.

By Sutranet · A guide for practising structural engineers

How IS 17740 sets up the wind load
Step 1: risk category and design life
Step 2: design wind speed (IS 875)
Step 3: design wind pressure P_d
Step 4: wind force on the structure, F_ST
Round members: which speed sets the flow regime
Step 5: wind force on appurtenances, F_A
Step 6: wind directions (Fig. 3)
Step 7: dynamic effects and the gust factor method
Worked example: one section by hand
Doing it in practice

On almost every lattice communication tower, wind is the load that sizes the steel. For a tower in India the calculation is governed by two standards working together: IS 17740:2022, the code of practice for isolated towers, masts and poles, sets out what the design wind load is made of and how it is applied, and IS 875 (Part 3):2015 supplies the wind speed, the pressure and the force coefficients. This guide follows IS 17740's order and brings in IS 875 where IS 17740 refers to it.

1. How IS 17740 sets up the wind load

IS 17740 Cl. 7.3.1 sends you to IS 875 (Part 3) for the basic wind pressure and the wind loads on the structure, with one tower-specific input: the design life, set by the tower's risk category. Cl. 7.3.2 then defines the design wind load as the sum of the horizontal forces on the structure and on everything attached to it:

F_w = F_ST + F_A + F_G

Term	What it is	Where
F_ST	Design wind force on the structure	IS 875 (Part 3) and IS 17740 Cl. 7.4
F_A	Design wind force on appurtenances: antennas, mounts, ladders, feed lines	IS 17740 Cl. 7.5
F_G	Design wind force on guys (guyed masts only; zero for a self-supporting tower)	IS 17740 Cl. 7.10

The same clause adds two rules that matter on every tower: all appurtenances — antennas, mounts and lines — are assumed to stay intact and attached, and the design forces are based on the wind directions that give the maximum responses.

Both standards have been amended, and the amendments are separate documents:

IS 17740 Amendment No. 1 (May 2026) deletes the Note to Cl. 7.3.1, writes out the limit state combinations (wind at 1.4 with dead load at 1.1 or 0.9), and lowers the leg slenderness limit from 150 to 120.
IS 875 (Part 3) Amendment No. 1 (2016) re-issues the headers of the force coefficient tables, including Tables 34 and 35 for round members, and corrects the captions of the two topography figures in Annex C.
IS 875 (Part 3) Amendment No. 2 (2020) replaces the wind map with the one in the National Building Code of India 2016 (Part 6, Section 1, Fig. 1), and replaces the list of city speeds in Annex A.

2. Step 1: risk category and design life

IS 17740 Cl. 6 classifies every tower into a risk category using Table 1, and the category is used for the wind and ice loads and the load factors. In short:

Category	Typical towers (IS 17740 Table 1)	Design life (Cl. 7.3.1)
III	Towers in urban locations or where failure could cause injury or loss of life; vital telecommunication facilities; defence, disaster-response and navigation networks	100 years
II	All towers that are not Category I or III	50 years
I	Unmanned sites in open countryside where failure is unlikely to injure people; optional or redundant services such as small cells, TV and radio reception, amateur radio	25 years

The design life then gives k₁, the probability factor, from IS 875 Table 1 for the site's basic wind speed:

Design life	33	39	44	47	50	55 m/s
25 years (Cat. I)	0.94	0.92	0.91	0.90	0.90	0.89
50 years (Cat. II)	1.00	1.00	1.00	1.00	1.00	1.00
100 years (Cat. III)	1.05	1.06	1.07	1.07	1.08	1.08

So the risk category is decided first. The 2022 text had a Note under Cl. 7.3.1 tying Categories I and II to towers up to 50 m; Amendment No. 1 deletes it, so the category now follows Table 1 alone.

3. Step 2: design wind speed (IS 875)

The design wind speed at height z is (IS 875 Cl. 6.3):

V_z = V_b · k₁ · k₂ · k₃ · k₄

Basic wind speed V_b

A 3-second gust at 10 m above ground in terrain Category 2, with a 50-year return period (IS 875 Cl. 6.2). The wind map places every site in one of six zones: 33, 39, 44, 47, 50 or 55 m/s. Since IS 875 Amendment No. 2 the map is the one in the National Building Code 2016.

k₂: terrain and height

k₂ (IS 875 Cl. 6.3.2) increases the speed with height, and by less over rougher ground. The terrain around the site is placed in one of four categories:

Category	Terrain	Roughness height z₀
1	Exposed open terrain with few or no obstructions	0.002 m
2	Open terrain with well-scattered obstructions 1.5 m to 10 m high	0.02 m
3	Numerous closely spaced obstructions up to about 10 m: towns, industrial areas, wooded areas	0.2 m
4	Numerous large, high, closely spaced obstructions: large city centres, generally above 25 m	2.0 m

IS 875 Table 2 gives k₂ by height for each category, with linear interpolation between rows. Take it at the height of each section; the pressure changes all the way up the tower.

A rougher terrain only slows the wind up to a certain height, which depends on how far that terrain extends upwind. IS 875 Cl. 6.3.2.4 and Table 3 give this developed height — for Category 4, 60 m with a 0.2 km fetch and 300 m with a 5 km fetch — and Annex B gives the rules for combining the two profiles. For a tall tower at the edge of a town, taking the town's category for the full height can under-estimate the speed at the top.

k₃: topography

k₃ (IS 875 Cl. 6.3.3) is 1.0 on level ground or where the upwind slope is under about 3°, and can rise to 1.36 near the crest of a hill, ridge, cliff or escarpment. Annex C calculates it from the height and length of the feature and the tower's position on it. Telecom towers are often placed on high ground, so k₃ is worth checking rather than assuming. IS 875 Amendment No. 1 swaps the captions of the two Annex C charts: Fig. 14 is for cliffs and escarpments, Fig. 15 for ridges and hills.

k₄: cyclonic regions

k₄ (IS 875 Cl. 6.3.4) applies in the 60 km coastal belt of the east coast and Gujarat. It is 1.30 for structures of post-cyclone importance, a class that includes communication towers, and 1.0 elsewhere.

4. Step 3: design wind pressure P_d

IS 17740 uses the design wind pressure P_d in both its force equations. From IS 875 Cl. 7.2:

p_z = 0.6 · V_z² (N/m², V_z in m/s)
P_d = K_d · K_a · K_c · p_z, and not less than 0.70 · p_z

K_d, the directionality factor (IS 875 Cl. 7.2.1), is 0.90 for lattice frameworks and trussed towers, whether triangular, square or rectangular. It is 1.0 for circular forms and 1.0 in cyclone-affected regions.
K_a, the area averaging factor (IS 875 Cl. 7.2.2, Table 4), is 1.0 for a tributary area up to 10 m², 0.9 at 25 m² and 0.8 from 100 m². The clause introduces it for the building pressure coefficients of Cl. 7.3; it reaches a tower through the definition of p_d. A tower panel's face is usually under 10 m², so K_a is 1.0 or close to it.
K_c, the combination factor, refers to IS 875 Cl. 7.3.3.13, which deals with frames of clad buildings combining wall and roof pressures. It has no application to a lattice tower, so take K_c = 1.0.

The 0.70 p_z floor does not bind at these values, but it limits any reduction you take. For a linear appurtenance and for each section of the tower, P_d is taken at that section's height.

5. Step 4: wind force on the structure, F_ST

The solidity ratio of each panel

Every force coefficient for a lattice tower is read against the solidity ratio φ of the face. IS 17740 Cl. 7.4 states it for a panel: the total covered area — the exposed areas of all the members, round and flat — divided by the panel area. IS 875 Cl. 4.20 adds that the panel area is the area enclosed by the boundary of the frame, calculated for individual frames, that is, one face.

The boundary is the outer edge of the legs. The panel area is the panel height times the face width measured out to out of the legs, not the width between leg centrelines. Using the centreline width makes the area a few percent too small and the solidity too large.
Count every member in the face: the two legs, the diagonals, horizontals and redundant members, each by its projected width times its length in the face.

Force coefficients from IS 875

The force coefficient of a whole tower is in IS 875 Cl. 7.4.3.5:

IS 875 table	Members	Tower	Direction
33	Flat-sided (angles)	Square and triangular	Square: corner wind is 1.2 × face wind. Triangular: constant for any direction
34	Circular (tubes)	Square	Separate values for wind onto a face and onto a corner
35	Circular (tubes)	Triangular	One value for any direction

For flat members, and for round members in subcritical flow, the coefficient falls as the solidity rises. Tables 34 and 35 also have a supercritical column for round members, which is much lower and behaves differently with solidity (next section).

Towers with both tubes and angles: IS 17740 Cl. 7.4

Many towers combine tube legs with angle bracing. IS 17740 Cl. 7.4 deals with this "hybrid" tower directly: the coefficient for the round members and the coefficient for the flat members are each read at the same solidity ratio of the panel, and

F = (C_f,flat · A_flat + C_f,cir · A_cir) · P_d

acting horizontally, where A_flat and A_cir are the exposed areas of the flat and round members. On a tower of one member type it reduces to F = C_f · A · P_d. The clause is written for equilateral triangular towers; the same split is the natural way to treat a square one.

Individual members and shielding

Two more clauses apply to members and attachments considered on their own rather than as part of the tower face. Cl. 7.7: the IS 875 force coefficients for individual members are for infinite length, and a member of finite length may take the reduction factor K for its length-to-width ratio. Cl. 7.9: an element shielded by another in front of it in the wind gets full shielding when the clear distance between them is no more than 2.0 times the element's smallest projected dimension, none beyond 4.0 times, and a straight-line transition between (Fig. 4). The unshielded element is taken as flat unless both are round.

6. Round members: which speed sets the flow regime

Round members take a lower force coefficient once the flow around them turns supercritical. IS 875 Tables 34 and 35 decide this on D·V̄_d: below 6 m²/s a member is subcritical, from 6 upwards supercritical.

The bar over the V matters. V̄_d is the design hourly-mean wind speed (IS 875 Cl. 6.4), not the 3-second gust V_z that gives the pressure:

V̄_z,d = V_b · k₁ · k̄_2,i · k₃ · k₄, with k̄_2,i = 0.1423 · ln(z / z_0,i) · (z_0,i)^0.0706

In a PDF's text layer, or when the tables are retyped, the bar is easily lost and D·V_d starts to look like D·V_z. The gust speed is roughly 1.3 to 1.8 times the hourly mean in terrain Categories 1 to 3 at 10 m to 100 m, so the wrong speed moves many tower tubes across the limit, onto the lower supercritical coefficient. The worked example below shows one.

The pressure stays on the gust speed: p_z = 0.6 V_z². Only the regime check uses the hourly mean. In an ice case, IS 17740 Cl. 7.2.4.2 lets the area added by ice take round-member coefficients; see Ice loads on lattice towers. If you also design to the US standard, ANSI/TIA-222-I answers the regime question differently; see our TIA-222-I guide.

7. Step 5: wind force on appurtenances, F_A

On a loaded telecom tower the antennas, mounts, ladder and feed lines can carry as much wind as the steel. IS 17740 Cl. 7.5 gives the force on each one:

F_A = C_fa · P_d · A_a

P_d is the design pressure at the centreline height of the appurtenance. For a linear appurtenance such as a ladder or a feed line, the pressure of each tower section or panel it passes is used.
A_a is the effective projected area, including ice in the ice combinations, with the appurtenance treated as made of flat and round components.
C_fa is the drag coefficient for that shape, from IS 875.
The force is applied at the centroid of the projected area, in the direction of the wind. An antenna standing off a face therefore also twists the tower.

Three further points from the same part of the standard:

Mounting pipes (Cl. 7.6): the projected area of the pipe above and below an antenna is included in A_a, and a mounting frame's area is taken on its windward face normal to its azimuth.
Microwave dishes are excluded from the formula above and, in the absence of better data, use Annex L: an axial force, a side force and a twisting moment, each with its own coefficient from Table 7 by the wind angle onto the dish and the dish type — without radome, with radome, with a cylindrical shroud, or grid.
The Note to Cl. 7.5: members that occur on only one face — ladders, conduits, antennas, feed lines — take the wind once, normal to their exposed projected area, including any ice.

8. Step 6: wind directions (Fig. 3)

Cl. 7.3.2 asks for each of the wind directions shown in Fig. 3 to be considered for each face of a lattice tower. For a bare tower, symmetry keeps the number small: on a triangular tower the force coefficient is the same for any direction, and on a square tower the face and corner cases bound the result.

The two notes to Fig. 3 change this in practice:

Note 1: the directions shown apply only when the antenna and equipment area is symmetrical on all faces. Otherwise all critical directions around the tower have to be investigated. In practice that means running the full compass — for example every 30° on a triangular tower and every 15° on a square one — and letting each member find its own governing direction. The direction that gives the largest base shear is often not the one that governs a particular diagonal.
Note 2: F_ST + F_A need not exceed the force on the structure taken as solid-faced (solidity 1.0) plus the wind on appurtenances mounted outside the structure's projected area. It is a permitted upper limit, useful on a very heavily loaded face.

9. Step 7: dynamic effects and the gust factor method

Everything above is the static method: a 3-second gust pressure applied as a steady load. IS 17740 Cl. 8.1 points to IS 875 Cl. 9 and 10 for the dynamic wind pressure, and Cl. 8.2 says the dynamic effects of wind-induced oscillations are, in general, to be considered when the tower's first natural frequency f₁ is below 1.0 Hz, with the along-wind load found by the Gust Factor method of IS 875 Cl. 10.2.

Cl. 8.4(a) gives f₁ for a self-supporting lattice tower from its height, its average and base face widths, and its weight, with the weight in the top 5 % of the height counted separately. That weight includes the antennas, mounts, ladder and feed lines, not only the steel, and a heavy head load lowers f₁.

The gust factor method works on the hourly-mean speed rather than the gust:

F_z = C_f,z · A_z · p̄_d · G, with p̄_d = 0.6 · V̄_z,d²

G, the gust factor, combines a background part and a resonant part that depends on the natural frequency, the damping and the turbulence at the site, and it varies with height. The hourly-mean pressure is well below the gust pressure, and G brings it back up. As written, IS 875's force equation for this method carries no K_d, K_a or K_c.

Cl. 8.2 also mentions across-wind oscillations. For the tower as a whole, IS 875 Cl. 10.3 states that across-wind response is not required for lattice towers. Many light lattice towers are stiff enough to stay above 1 Hz, but tall, slender or heavily loaded towers may not be.

10. Worked example: one section by hand

The tower is the demo tower from the SutranetTower wind-loading video: a 40 m triangular tower with steel tube legs and X-bracing, tapering from 4.2 m at the base to 1.5 m at 30 m, then straight to the top. We work out F_ST for the section from 20 m to 25 m, wind normal to a face.

Inputs

Item	Value
Risk category (IS 17740 Table 1)	II, so 50 years and k₁ = 1.0
Basic wind speed V_b	44 m/s
Terrain	Category 2 (z₀ = 0.02 m)
Topography, cyclone	Level ground, not a cyclone region: k₃ = k₄ = 1.0
Section	20 m to 25 m, mid-height z = 22.5 m, three panels
Face width (leg centres)	2.40 m at the bottom, 1.95 m at the top
Legs	CHS 168.3 × 4.5
Diagonals	CHS 60.3 × 2.9

Step 1: risk category

Category II gives a 50-year design life (IS 17740 Cl. 7.3.1), so k₁ = 1.0 from IS 875 Table 1 at 44 m/s.

Step 2: design wind speed

IS 875 Table 2, Category 2: k₂ = 1.07 at 20 m and 1.12 at 30 m. At 22.5 m: k₂ = 1.07 + 0.25 × 0.05 = 1.0825.

V_z = 44 × 1.0 × 1.0825 × 1.0 × 1.0 = 47.63 m/s

Step 3: design wind pressure

p_z = 0.6 × 47.63² = 1361.2 N/m²

K_d = 0.90 (lattice tower). One panel's face is 11.72 m² / 3 = 3.9 m², under 10 m², so K_a = 1.0. K_c = 1.0.

P_d = 0.90 × 1.0 × 1.0 × 1361.2 = 1225.0 N/m², above the floor of 0.70 × 1361.2 = 952.8 N/m².

Step 4a: solidity ratio (IS 17740 Cl. 7.4)

Panel area, out to out of the legs: mean width 2.175 m plus one leg diameter 0.168 m, times 5.0 m: 2.343 × 5.0 = 11.72 m².

Exposed area of the members in the face: the two legs, 0.1683 m wide and about 5.01 m long each, give 1.686 m²; the diagonals give 0.992 m². In all, A_cir = 2.677 m², and there are no flat members.

φ = 2.677 / 11.72 = 0.229

Step 4b: flow regime of the round members

Hourly-mean factor at 22.5 m, Category 2: k̄_2,i = 0.1423 × ln(22.5 / 0.02) × 0.02^0.0706 = 0.758.

V̄_d = 44 × 1.0 × 0.758 × 1.0 × 1.0 = 33.37 m/s

Member	D	D·V̄_d	Regime
Legs	0.1683 m	5.62 m²/s	Subcritical (< 6)
Diagonals	0.0603 m	2.01 m²/s	Subcritical

Had the gust speed been used, the legs would give 0.1683 × 47.63 = 8.02 m²/s and be taken as supercritical, which is not what the table says.

Step 4c: force coefficient

Triangular tower, round members, so IS 875 Table 35, subcritical column: 1.6 at φ = 0.2 and 1.5 at φ = 0.3. At φ = 0.229: C_f,cir = 1.6 − 0.285 × 0.1 = 1.571.

Step 4d: wind force on the section (IS 17740 Cl. 7.4)

With no flat members the Cl. 7.4 sum has one term:

F_ST = C_f,cir · A_cir · P_d = 1.571 × 2.677 × 1225.0 = 5154 N = 5.15 kN

SutranetTower's report for this tower gives 5.154 kN for the same section. With the gust speed in the regime check, the legs would have taken the supercritical coefficient of 1.1 while the diagonals (0.0603 × 47.63 = 2.87, still under 6) kept 1.571. The force would have been (1.1 × 1.686 + 1.571 × 0.992) × 1225.0 = 4.18 kN, about 19 % lower.

The rest of F_w

The demo tower also carries sector antennas, a dish, a ladder and a run of feed lines, so F_w for this direction adds F_A for each of them at its own height (Cl. 7.5, with Annex L for the dish). Because those items are not the same on every face, Fig. 3 Note 1 applies and the tower is checked for wind from all round. Its first natural frequency by Cl. 8.4(a) is about 2.3 Hz, above 1 Hz, so the static method used here is sufficient under Cl. 8.2.

11. Doing it in practice

One section by hand takes a few minutes. A whole tower is that calculation for every section and every wind direction, plus F_A for each antenna, mount, dish, ladder and feed line at its own height and on its own face — and then the forces have to go into a 3D model and come back out as member forces. That is where errors creep in, and where the details above tend to be simplified away.

In SutranetTower you enter the site data — basic wind speed, risk category, terrain category, k₃ and whether the site is in a cyclone region — and the program works out every section's k factors, pressure, solidity, flow regime and force coefficient, adds the appurtenance forces, applies them in every wind direction, switches to the gust factor method when f₁ is below 1 Hz, and shows each step in the report's Wind tab, as in the example above. The wind-loading video goes through it on screen.

Run the wind calculation on your own tower.

Model the tower once and get the wind load on every section and appurtenance, the 3D analysis and the member checks to IS 17740, with the working shown — free to start.

Try it free →

This guide is an engineering overview and is not a substitute for the standards. Clause numbers refer to IS 17740:2022 with Amendment No. 1, and IS 875 (Part 3):2015 with Amendments No. 1 and 2. Always design against the current published text and confirm the site data — basic wind speed, terrain, topography and cyclone exposure — for the actual location.