Ice Loads on Lattice Towers to IS 17740:2022 (Cl. 7.2.4)

The atmospheric ice load on a self-supporting lattice tower, step by step — the ice thickness at each height, the out-to-out dimension D_c of each member shape, the weight of the ice, the wind on the iced members and appurtenances, and the ICE1 combination of Amendment No. 1 — with one tower section worked by hand.

Over most of India atmospheric icing does not occur, and most tower designs never need an ice case. Where it does occur, it changes the tower in two ways at once: the ice adds weight, and it widens every member, antenna and cable, so the same wind pushes harder on a frame that has also become more solid. On small bracing the ice can weigh more than the steel. This guide follows IS 17740 Cl. 7.2.4 and shows how SutranetTower applies it.

1. When ice has to be considered

IS 17740:2022 Cl. 7.2.4 covers atmospheric icing of towers. The design ice thickness for the site has to come from meteorological data for the location; it is an input, not something the standard or the program derives. The Foreword adds that ice may be considered both with and without wind, from that data.

In SutranetTower the ice case is off by default. It runs only when the Ice & Seismic Loads panel has ice switched on and a design ice thickness above zero is entered. The program runs ice with wind (the ICE1 combination, Step 6); there is no separate ice-without-wind case.

A separate clause, Cl. 7.2.3, deals with snow by IS 875 (Part 4), or a uniform 30 mm of ice where no data exist. The program does not apply snow or that uniform 30 mm (Section 8 explains why the ice input is not a substitute for it).

2. Step 1: design ice thickness t_iz at each height

Cl. 7.2.4.1 turns the site's radial ice thickness t_i into a factored design thickness at height Z:

tiz = 2.0 · ti · k1² · kiz · k30.7, with kiz = (Z / 10)0.1 ≤ 1.4  (Z in metres)

• 2.0 is the limit state factor. It is built into the thickness, which is why the ice weight and the wind on ice later take a load factor of 1.0.
• k₁ is the IS 875 probability factor for the tower's risk category and design life, the same value as in the wind calculation, and it enters squared.
• k₃ is the IS 875 topography factor, entering to the power 0.7. A tower on a hill gets thicker ice as well as faster wind.
• k_iz is the height factor. As written it is below 1.0 under 10 m, and reaches its cap of 1.4 at about 289 m.

Height Z	5 m	10 m	20 m	40 m	60 m	100 m	289 m+
kiz	0.933	1.000	1.072	1.149	1.196	1.259	1.400
tiz for ti = 10 mm, k1 = k3 = 1.0	18.7	20.0	21.4	23.0	23.9	25.2	28.0

The height factor increases slowly; k₁ and k₃ matter more. At 40 m with t_i = 10 mm, a Category III tower (k₁ = 1.08 at V_b = 50 m/s) on a hill with k₃ = 1.2 gets t_iz = 30.4 mm against 23.0 mm on level ground at k₁ = 1.0.

A reading trap. In the PDF's text layer the exponent and the cap of k_iz come out as loose digits, which can be read as an exponent of 0.14 with no cap. The page itself gives 0.1 and a cap of 1.4. With 0.14, k_iz at 100 m comes out at 1.38 instead of 1.26, and the ice about 10 % too thick.

The program takes one t_iz per tower section, at the section's mid-height, and uses it for both the weight and the wind on that section.

3. Step 2: the out-to-out dimension D_c of each member

The ice weight is worked out on D_c, the largest out-to-out dimension of the member, which IS 17740 Fig. 2 defines by shape:

Member	Dc	Example
Round (tube or solid bar)	The diameter	CHS 60.3: 60.3 mm
Single equal angle	The toe-to-toe diagonal, a·√2	ISA 75×75: 106.1 mm, not 75 mm
Two angles back to back	The larger of the overall width across both angles and the gap, and the leg depth	2 × ISA 65 with a 10 mm gap: 140 mm

Fig. 2 also covers a rectangular hollow section on its diagonal; the program has no such section. Using the angle's leg width instead of the toe-to-toe diagonal under-estimates the ice on an angle by about 25 %.

4. Step 3: weight of ice on the members

Cl. 7.2.4.3 gives the cross-sectional area of the ice and its weight:

Aiz = π · tiz · (Dc + tiz), ice density 8.83 kN/m³

The weight per metre is A_iz × 8.83 kN/m³. The program applies it to every member of the built lattice — legs, diagonals, horizontals and redundant members, including plan and hip bracing across the faces — each with its own D_c, its own length and the t_iz of its section. That matters on a tube-leg tower with small bracing: weighing the bracing at the leg diameter over-states its ice several times, and leaving the redundants out under-states the total.

Member	Dc (mm)	Aiz at tiz = 23.0 mm (mm²)	Ice (N/m)
CHS 168.3	168.3	13 805	121.9
CHS 60.3	60.3	6 010	53.1
ISA 75×75×6	106.1	9 313	82.2
2 × ISA 65, 10 mm gap	140.0	11 763	103.9

t_iz = 23.0 mm is the value at 40 m for t_i = 10 mm and k₁ = k₃ = 1.0.

5. Step 4: wind on the iced tower

Cl. 7.2.4.2 takes the ice as a uniform layer over the exposed surfaces of the structure and its appurtenances, and lets the area added by the ice use round-member force coefficients. In the program:

• Every member's projected width grows by 2·t_iz, redundant members included.
• The added area takes the round-member coefficient of the tower's own table: IS 875 Table 34 on a square tower, Table 35 on a triangular one. The bare members keep their own coefficients, so the face force is the sum of the bare members at their coefficients and the ice strips at the round coefficient.

Two points on which the clause is silent, and the reading the program takes:

• Solidity. The solidity ratio of an ice case is that of the iced frame — bare area plus ice area, over the out-to-out panel area including the ice on the legs — and both the bare and the ice areas read their coefficients at that solidity.
• Flow regime. The D·V̄_d test that decides whether a round member is subcritical or supercritical (IS 875 Tables 34 and 35) is made on the iced width, D + 2·t_iz, for the member's bare area and its ice area alike. A wider member reaches the supercritical limit at a lower speed; the worked example below shows a tube leg changing regime.

The wind speed with ice

The wind that acts with the ice may be lower than the design wind. On an IS tower the program takes an optional wind speed with ice: if you enter one, it replaces V_b in the IS 875 calculation, and k₁, k₂, k₃, k₄ and the design pressure follow as in the wind cases. If you enter none, the ice case runs at the full V_b, and the builder lets you state that choice explicitly. A speed of zero or below is refused, as is a file that gives a speed and also says to use V_b. (On a TIA-222-I tower the wind speed with ice is required; see our TIA-222-I guide.)

6. Step 5: ice on antennas, ladder, feed lines and mounts

Cl. 7.2.4.2 puts ice on the appurtenances too. On a loaded telecom tower they can carry as much iced wind as the steel, so the program treats each one:

Item	Iced wind area	Ice weight
Panel antenna	Every projected dimension + 2·tiz; the added area at Cf = 1.2 (IS 875 Table 30, round), the bare antenna at its own coefficient	tiz × outer surface (all six faces) × 8.83 kN/m³
Dish	Diameter + 2·tiz; the added area at Cf = 1.2	tiz × front and back surface × 8.83 kN/m³
Antenna mount pipe	Width + 2·tiz	Aiz as for a member
Climbing ladder	1.2 × 2·tiz × total length of rails and rungs, shielded as the ladder is	Aiz on the declared rail/rung size × that length
Feed lines	From each cable's own size; an iced waveguide's ice rim takes the round coefficient 1.2
Mounts, platforms, attachments	From the area and mass tables you declare by ice thickness (manufacturer's data), interpolated at the item's own tiz

The antenna ice weight is a reading: A_iz is written for a member's cross-section, and an antenna is a box or a dish, so the program takes ice of thickness t_iz over its outer surface and says so on the report. Three inputs are needed that a no-ice run does not ask for, and the run is refused before the solve if they are missing:

• for the ladder, the total length of rails and rungs in 1 m of ladder (about 3.3 m for two rails with rungs 0.4 m long at 300 mm) and their size;
• for an antenna, its dimensions: a generic antenna entered only as an area cannot be iced, so an ice run with one is refused;
• for a mount or attachment, an iced table that reaches the item's t_iz; the table is not extrapolated.

7. Step 6: the ICE1 combination

IS 17740 Amendment No. 1 (May 2026) adds Note 5 to Table 2, which writes out the ice combination:

ICE1 = 1.1 DL + 1.0 Di + 1.0 Wi

D_i is the weight of ice and W_i the wind on the iced structure, both at the factored thickness t_iz. Their 1.0 factors go with the 2.0 already inside t_iz; only the dry dead load takes 1.1.

The note also lists a temperature term; it is not modelled, and Table 2 Note 4 says temperature need not be considered for self-supporting structures. ICE1 is run in every analysed wind direction, and it goes into the member capacity envelope with the wind and earthquake cases. The program applies this combination whichever set the tower runs on, IS 17740 Table 2 or IS 800 Table 4.

8. What the ice case does not cover

• Ice without wind is not a separate case. The dead-load-only combinations do not include the ice weight, so they do not stand in for it.
• Snow and the uniform 30 mm of Cl. 7.2.3 are not applied, and the report says so. Entering 30 mm as the design ice thickness is not the same thing: the ice input goes through Cl. 7.2.4.1, which doubles it and scales it with height, k₁ and k₃.
• One thickness per section, at its mid-height, rather than per member height.
• Guys are outside the program's scope (self-supporting towers only).
• Unbalanced ice (ice shed from some faces or levels and not others) is not modelled; Cl. 7.2.4.2 treats the ice as uniform.

9. Worked example: one section by hand

The section is the 20 m to 25 m section of the 40 m triangular tube tower worked in the wind guide, now in an icing area.

Step 1: ice thickness

kiz = (22.5 / 10)0.1 = 1.0845 (under the 1.4 cap)

tiz = 2.0 × 10 × 1.0² × 1.0845 × 1.00.7 = 21.69 mm

Steps 2 and 3: ice weight

Member	Dc	Aiz = π·tiz·(Dc + tiz)	Ice	Steel
Leg	168.3 mm	π × 21.69 × 189.99 = 12 946 mm²	114.3 N/m	178 N/m
Diagonal	60.3 mm	π × 21.69 × 81.99 = 5 587 mm²	49.3 N/m	40 N/m

On the diagonals the ice outweighs the steel. For the section:

Di = 15 m × 114.3 + 48 m × 49.3 = 1.71 + 2.37 = 4.08 kN

Weighing the diagonals at the leg diameter instead would have given 48 × 114.3 = 5.49 kN for them alone, 2.3 times the correct value.

Step 4: wind on ice

Each member's projected width grows by 2 × 21.69 = 43.4 mm: the legs to 211.7 mm, the diagonals to 103.7 mm. The 43.4 mm strip takes the Table 35 round coefficient, the face solidity is recalculated with the ice, and the flow regime is checked on the iced widths.

From the wind guide, the design hourly-mean speed at 22.5 m is V̄_d = 33.37 m/s at V_b = 44 m/s. If the ice case runs at the full V_b:

Member	Bare D·V̄d	Iced (D + 2tiz)·V̄d	Regime in the ice case
Leg	0.1683 × 33.37 = 5.62	0.2117 × 33.37 = 7.06	Supercritical (≥ 6)
Diagonal	0.0603 × 33.37 = 2.01	0.1037 × 33.37 = 3.46	Subcritical

With a wind speed with ice of 30 m/s entered instead, V̄_d scales to 33.37 × 30/44 = 22.75 m/s, the iced leg gives 0.2117 × 22.75 = 4.82, and it stays subcritical — with a higher coefficient on a lower pressure. Which of the two ice cases is larger depends on the tower; the point is that the wind speed with ice is a design input worth getting from the site data rather than defaulting.

Step 6: ICE1

For this section ICE1 applies 1.1 × the dry dead load, plus 1.0 × 4.08 kN of ice, plus 1.0 × the iced wind, in each wind direction, with every other section, antenna and cable iced in the same way.

10. Doing it in practice

The thickness and weight of one member are quick by hand. A whole tower is that calculation for every member of every section, for every antenna, cable and mount, with the iced solidity and regime recalculated for each face and direction — and then the case has to be solved and enveloped. In SutranetTower you switch on ice in the Ice & Seismic Loads panel, enter the design ice thickness and the wind speed with ice (or state that V_b applies), and the program does the rest, listing t_iz, D_c and the ice weight by section, and the ice on each antenna and on the ladder, in the report.

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 take the design ice thickness and the wind speed with ice from meteorological data for the actual location.