Seismic Loads on Lattice Towers to IS 1893 (Part 4):2024 and IS 17740

How the earthquake load on a self-supporting lattice tower is built up — zone, structure category and importance, response reduction, the design spectrum at the damping of steel, the Rayleigh period, the equivalent static base shear and the EQ1/EQ2 combinations — with a four-section example worked by hand, and a plain list of what the method leaves out.

On most lattice communication towers wind is the load that sizes the steel. A tower is light, so the inertia force from an earthquake is small next to the wind on its members and antennas. That does not remove the need to check it: in the higher seismic zones, on soft soil, or for a tower with a heavy head load, the earthquake case has to be calculated and enveloped with the wind cases before you can say it does not govern. This guide shows how that calculation is set up for a tower in India, and how SutranetTower carries it out.

1. Where the seismic rules for a tower come from

IS 17740:2022, the code for lattice towers, masts and poles, gives no earthquake provisions of its own. Its Foreword refers the designer to IS 1893 (Part 4), the part of the earthquake code for industrial and stack-like structures. The Foreword names the 2005 edition; the current edition is IS 1893 (Part 4):2024, and its method differs from the 2005 text. This guide, and the program, follow the 2024 edition.

Two clauses of IS 17740 still matter here:

• Table 2, as amended by Amendment No. 1 (May 2026), sets the load factors for the earthquake combinations (Step 8), and its Note 2 says the earthquake load is worked out from the unfactored dead load.
• Cl. 8.3 lets the fundamental period be found by observation or by any rational method, and otherwise takes it as 1/f₁. Rayleigh's method (Step 6) is such a rational method.

The seismic case is optional. In SutranetTower it is off until you switch it on in the Ice & Seismic Loads panel, and a report without it says nothing about earthquake.

2. Step 1: seismic zone and Z

The site's seismic zone gives the zone factor Z from IS 1893 (Part 4):2024 Table 13:

Zone	II	III	IV	V
Z	0.10	0.16	0.24	0.36

There is no sensible default for a zone, so the program has none: a seismic case with no zone (or no Z entered directly) is refused before the analysis runs. If you enter both a zone and a Z, they must agree.

3. Step 2: structure category and importance factor I

IS 1893 (Part 4) places each structure in one of four categories, and Table 3 gives the importance factor for each:

Category	1	2	3	4
Importance factor I (Table 3)	2.0	1.5	1.2	1.0

Table 6 lists typical structures by category, and its row for microwave towers places them in Category 2, so I = 1.5 is the usual starting point for a communication tower. The program takes Category 2 when none is entered, and says on the report that it assumed it.

The standard allows a higher importance factor than the table value (Cl. 8.5), not a lower one. The program follows that: an importance factor above the Table 3 value for the category is used and reported; one below it is refused, with a message to correct the factor or the category.

This seismic category is separate from the risk category of IS 17740 Table 1, which sets the design life for wind and ice. A tower has both, and they are entered separately.

4. Step 3: response reduction factor R, and the I/R limit

The response reduction factor R allows for the ductility and redundancy of the structure. IS 1893 (Part 4):2024 Table 4 has no row for lattice towers. Its Note 5 leaves a structure that is not in the table to the designer's judgement of strength, redundancy and ductility. The program uses R = 2.0 when none is entered — Table 4's value for steel chimneys and steel stacks — and the report says that this is an assumed value. If you have grounds for another value, enter it.

Annex C Note 2 limits the ratio I/R to 1.0. With the default R = 2.0 the limit binds only for Category 1 (I = 2.0, I/R = 1.0 exactly); with a lower R it can bind for other categories too. When it binds, the program uses 1.0 and records that on the report.

5. Step 4: soil type

The spectrum depends on the foundation soil, in three types (Table 15): Type A rock or hard soil, Type B medium soil, Type C soft soil. The program takes Type B when none is entered and says so. Type D — unstable, collapsible or liquefiable soil — needs a site-specific study and special treatment (Table 1B); the standard spectrum cannot be used for it, and the program refuses it rather than analysing it as one of the other types.

6. Step 5: the design spectrum at 2 % damping

IS 1893 (Part 4):2024 Annex C gives the spectral acceleration coefficient S_a/g at 5 % damping, by period T and soil type:

Period T	Sa/g at 5 % damping
0 to 0.01 s	1.0
0.01 to 0.1 s	1 + (50/3)(T − 0.01)
0.1 s to Tc	2.5
Tc to 6.0 s	c / T
6.0 s to 10 s	6c / T²

Soil	Tc (end of the plateau)	c
Type A, rock or hard	0.4 s	1.0
Type B, medium	0.6 s	1.5
Type C, soft	0.8 s	2.0

Annex C stops at 10 s. The program refuses a longer period rather than extending the curve.

Damping of a steel tower

A steel structure is analysed at 2 % damping (Table 5), not the 5 % the spectrum is drawn for. Table 14 gives the multiplying factor for another damping ratio: 1.4 at 2 %. Its note makes the factor 1.0 at T = 0, rising linearly to the tabulated value at 0.1 s, and the tabulated value above that. So for a steel tower:

T (s)	Type A	Type B	Type C
0.1 to 0.4	3.50	3.50	3.50
0.6	2.33	3.50	3.50
0.8	1.75	2.63	3.50
1.0	1.40	2.10	2.80
2.0	0.70	1.05	1.40

S_a/g at 2 % damping. Two points about this table. The plateau is 3.5, not 2.5: leaving out the damping factor under-estimates the load by 29 % on the plateau. And the Part 4 spectrum is not the IS 1893 (Part 1) building spectrum. Past the plateau, the Part 4 values at 2 % are 1.40, 1.54 and 1.68 times the Part 1 values at 5 % for rock, medium and soft soil. A spreadsheet carried over from building design gives a low answer here.

The design horizontal seismic coefficient

Ah = (Z / 2) · (Sa/g) · (I / R)  (IS 1893 Part 4:2024 Cl. 7.3.2)

Site-specific spectrum

Cl. 7.1 asks for a site-specific spectrum for Category 1 and 2 structures, with the standard spectrum as the minimum. With one, A_h = (S_a/g)_SS · (I/R) (Cl. 7.3.1, with no Z/2), and never less than the Annex C value.

The program accepts a site spectrum as a list of (T, S_a/g) points. It must be stated at 2 % damping: Table 14's factors are given for the standard spectrum, so a site spectrum at another damping ratio is refused rather than converted. It is interpolated linearly between points and never extrapolated. A Category 1 or 2 tower analysed without one gets a compliance warning on the report saying that the Annex C spectrum was used as the minimum that Cl. 7.1 allows. Because Category 2 is the default for a microwave tower, most towers will show this warning until a site spectrum is supplied.

7. Step 6: the fundamental period by Rayleigh's method

The period sets where the tower sits on the spectrum. IS 1893 (Part 4):2024 Cl. 14.2 gives Rayleigh's approximation:

T = 2π · √( Σ Wi · δi² / ( g · Σ Fi · δi ) )

W_i is the weight at level i, F_i a lateral force applied at that level, and δ_i the lateral deflection it produces. If the forces are taken in proportion to the weights, the result does not depend on how large they are, so one linear static solve is enough.

That is what SutranetTower does: it applies each section's seismic weight as a horizontal force (at 1 % of its value, so the model stays linear) to the leg nodes of the full 3D lattice, solves once, and reads the deflections. The weights are the same ones used for the base shear in Step 7, so the period and the force cannot be worked out on different masses.

The alternative is 1/f₁, with f₁ from the empirical formula of IS 17740 Cl. 8.4(a). The two can agree well or not: on a 40 m example tower they gave 0.447 s by Rayleigh and 0.436 s by the formula; on a 10 m tower, 0.061 s and 0.034 s. Cl. 8.4(a) is still used for the 1 Hz test that decides the wind method; only the seismic period comes from Rayleigh. A period you enter yourself (from a modal analysis or a measurement) overrides both.

If you also design to the US standard: ANSI/TIA-222-I gives the same Rayleigh formula as its primary route, and the program's TIA path uses the empirical formula that standard also permits. See our TIA-222-I guide.

8. Step 7: seismic weight, base shear and storey forces

Seismic weight

Following IS 17740 Table 2 Note 2, the seismic weight is the unfactored dead load. In the program that is, section by section, the steel (with the 10 % allowance for connections the dead load carries) plus every appurtenance the dead load carries: antennas with their mount pipes, the climbing ladder, feed lines, mounts and other attachments. A mass at or above the top of the tower — where antennas usually are — is lumped into the top section.

Base shear and its distribution

VB = Ah · W
Qi = VB · Wi hi² / Σ Wj hj²

W_i is the weight of section i and h_i the height of its mid-point. Each section's force is split equally between the section's bottom and top levels, and equally among the leg nodes at each level.

When the equivalent static method may be used

IS 1893 (Part 4):2024 Cl. 17 permits the equivalent static method when the first mode governs the base shear — Cl. 17.1 puts that at over 80 %. The program does not run a modal analysis, so it does not verify this condition, and the report says so. For a slender tower with a heavy head load, or one with a stiffness change partway up, a modal check is worth doing separately.

Two further choices to be aware of. The forces are distributed in proportion to W h², the familiar rule from building design; Cl. 17.1's own formulas are written for chimney shells (a shear coefficient by the ratio of height to shell radius) and are not used, because applying them to a lattice would need an interpretation the standard does not give. The US standard takes the same route: ANSI/TIA-222-I's equivalent lateral force procedure (§2.7.8.1) is also an equivalent static method.

9. Step 8: directions and load combinations EQ1 / EQ2

IS 1893 (Part 4):2024 Cl. 16 has the earthquake applied in one horizontal direction at a time. The program applies it once in each analysed wind direction, using the same directions as the wind cases. On a triangular tower with a full compass of directions, that is twelve earthquake directions, not just the two axes. On a 10 m test tower, moving from two axes to all directions raised the largest leg reaction under the earthquake case from 5.70 kN to 6.15 kN.

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

Case	Combination	Purpose
EQ1	1.1 DL + 1.0 E	Dead load adverse: largest leg compression
EQ2	0.9 DL + 1.0 E	Dead load favourable: largest uplift

The program uses these factors whichever combination set the tower runs on (IS 17740 Table 2, the default, or IS 800 Table 4); IS 800 Table 4's own earthquake factors are not used. Both cases, in every direction, go into the member capacity envelope with the wind cases, so each member is checked for whichever case governs it.

10. What this method does not cover

• Vertical shaking is not included. Section 2 of IS 1893 (Part 4):2024, for stack-like structures, gives the equivalent static method a horizontal force only. The general clauses (Part 4 Cl. 6.1.7, and IS 1893 Part 1 Cl. 6.3.3.1) ask for vertical effects in Zones IV and V and on soft soil. Every seismic report says vertical shaking was left out: as a compliance warning when the tower's zone or soil meets one of those conditions, as a note otherwise.
• The first-mode condition of Cl. 17.1 is not verified (Step 7).
• No response spectrum or time-history analysis is run; the method is equivalent static.
• Site-specific spectra can be imported in the tower file and are used as described in Step 5, but the builder has no editor for them.
• Type D soil and periods over 10 s are refused, not approximated.
• Foundations are not designed. The report gives the support reactions for each case, EQ1 and EQ2 included.

11. Worked example: a four-section tower

A 40 m triangular tower in four 10 m sections. The weights below are unfactored dead load per section, the top one including the antennas and their mounts. The period is taken as 0.45 s, of the order the Rayleigh solve gives for a 40 m tower.

Spectrum and A_h

Type B, T = 0.45 s lies on the plateau (0.1 to 0.6 s): S_a/g = 2.5 at 5 % damping. At 2 % damping and T above 0.1 s the Table 14 factor is 1.4: Sa/g = 2.5 × 1.4 = 3.5.

I/R = 1.5 / 2.0 = 0.75, within the 1.0 limit.

Ah = (0.24 / 2) × 3.5 × 0.75 = 0.315

Base shear

VB = 0.315 × 80 = 25.2 kN

Storey forces

Section	Wi (kN)	hi (m)	Wihi²	Share	Qi (kN)
0 to 10 m	28	5	700	0.019	0.49
10 to 20 m	20	15	4 500	0.125	3.15
20 to 30 m	14	25	8 750	0.243	6.13
30 to 40 m	18	35	22 050	0.613	15.44
Total	80		36 000	1.000	25.2

The h² weighting puts 61 % of the base shear into the top section, because that is where the antenna mass is. The base overturning moment from these forces is 0.49×5 + 3.15×15 + 6.13×25 + 15.44×35 = 743 kN·m before factoring; EQ1 and EQ2 apply it with 1.1 and 0.9 times the dead load.

What the report would add

For this input the report would list the assumed values (R = 2.0, and the category and soil type if they were not entered), the Cl. 7.1 warning that no site-specific spectrum was supplied for a Category 2 structure, and a warning that vertical shaking was not assessed, since Zone IV is one of the conditions where IS 1893 asks for it. Compare V_B with the wind base shear of the same tower: the capacity envelope will show which case governs each member.

12. Doing it in practice

By hand, the arithmetic above is short; the period is the hard part, and so is applying the forces in every direction and enveloping EQ1 and EQ2 with a dozen wind cases for every member. In SutranetTower you switch on the seismic case in the Ice & Seismic Loads panel and enter the zone; the category (default 2), soil type (default medium) and R (default 2.0) are there to change. The program computes the Rayleigh period on the 3D lattice, the spectrum at 2 % damping, A_h, the base shear and storey forces, runs EQ1 and EQ2 in every analysed direction, and shows each step — with every assumed value, warning and note — in the report.

This guide is an engineering overview and is not a substitute for the standards. Clause and table numbers refer to IS 1893 (Part 4):2024 unless another standard is named, and to IS 17740:2022 with Amendment No. 1. Always design against the current published text and confirm the site data — seismic zone, soil type and any site-specific spectrum — for the actual location.