Bolted Connection Design for Lattice Tower Members to IS 800 and IS 17740

How a bolted bracing end or leg splice is checked — bolt grade by IS 800 Table 1 with both amendments, bolt shear and bearing, the edge, end and pitch limits of Cl. 10.2, IS 17740's 12 mm minimum bolt, and the member's own net-section rupture and block shear — with one diagonal end worked by hand.

Connections are where many tower designs fail a review. The member checks may pass, but a bolt is too close to an edge, the bolt grade is assumed rather than stated, or the end of an angle bolted through one leg was never checked for net-section rupture. IS 17740:2022 Cl. 13.1.1 sends bolted connections to IS 800:2007, and adds one rule of its own: the 12 mm minimum bolt of Cl. 13.1.2. This guide goes through the checks for a bolted bracing end in the order a checker would, and shows how SutranetTower carries them out.

1. What is checked at a bolted tower joint

A bolted bracing end has two sides, and both have to be checked:

• The connection: the bolts in shear, the plies in bearing, the connected plate's net section and block shear, and the detailing limits that the strength equations assume.
• The member end: IS 17740 Cl. 11.1 makes the design tension strength of a member the lowest of gross-section yield, net-section rupture and block shear. The last two depend on the bolt holes, so they belong with the connection.

In SutranetTower both are driven by one input: an optional connections block per section, with an entry for the diagonals, the horizontals, each redundant member tag, and the leg splice. The bolt line you declare there (hole diameter, number of bolts in line, pitch, end and edge distances) is the one source of hole geometry for the member check too. A section with no entry has no connection check, and its members' rupture and block shear are reported as not checked rather than passed.

The demand is the factored axial force from the envelope of all strength combinations, with tension and compression enveloped separately, so a tension-only limit state such as block shear is checked against the largest tension, not against whichever sign governs the member.

Partial safety factors are those of IS 800 Table 5, and IS 17740 Table 4 gives the same values: γ_m0 = 1.10 for yielding, γ_m1 = 1.25 for rupture, γ_mb = 1.25 for bearing-type bolts and γ_mw = 1.25 for shop welds. The field-weld value of 1.50 is not selectable.

2. Step 1: bolt grade and diameter

IS 800 Table 1 gives the minimum yield and ultimate strength of bolts by property class. Two amendments changed what it says:

Class	fyb (MPa)	fub (MPa)	Note
4.6	240	400	Yield values added by Amendment No. 2 (2024); the 2007 table had a dash
4.8	320	420
5.6	300	500
5.8	400	520
6.8	480	600
8.8, d ≤ 16 mm	640	800	8.8 is split at 16 mm: an M20 8.8 bolt is 660 / 830, not 640 / 800
8.8, d > 16 mm	660	830
9.8	720	900
10.9	940	1040
12.9	1100	1220

In the program you state the class ("bolt_grade": "8.8") and both strengths are read from this table at the bolt's diameter. You can instead state f_ub and f_yb directly, but not both ways at once: two values for one strength would leave the program to pick one, so that is refused. If you state neither, 800 MPa is assumed and the report says so. A yield strength the check needs is never assumed: it comes from the class your f_ub names, or the run is refused.

IS 17740 Cl. 13.1.2 sets the bolt diameter at not less than 12 mm, and names 12, 16 and 20 mm as preferred sizes. The program checks the minimum on every connection type; the preferred sizes are a preference and are not checked.

3. Step 2: bolt shear (IS 800 Cl. 10.3.3)

The nominal shear capacity of one bolt is f_ub/√3 times its area in each shear plane. The program takes the area per plane as:

• the net tensile area at the threads, taken as Anb = 0.78 · Asb, when the threads are in the shear plane (the default), or
• the shank area Asb = πd²/4 when you state that they are not.

Vnsb = (fub / √3) · nplanes · A,   Vdsb = βlj · Vnsb / γmb

β_lj is the long-joint reduction. When the length of the joint, l_j = pitch × (n − 1), is more than 15d, βlj = 1.075 − 0.005 lj/d, kept between 0.75 and 1.0; otherwise it is 1.0. No other reduction on the bolt shear is applied.

The number of shear planes defaults to two (double shear). A single angle bolted to one gusset is in single shear, so state it. The fabrication one-liner on the report (for example 2 × M16 8.8 @ 50 c/c · single shear · 6 mm ply) is built from the values actually used, defaults included, so an unintended double shear shows there.

4. Step 3: bolt bearing (IS 800 Cl. 10.3.4)

Vnpb = 2.5 · kb · d · t · fu,   Vdpb = Vnpb / γmb
kb = min( e / 3d0,  p / 3d0 − 0.25,  fub / fu,  1.0 )

t is the ply thickness that governs bearing, f_u its ultimate strength, e the end distance, p the pitch and d₀ the hole diameter. The design value of one bolt is the lower of V_dsb and V_dpb, and the bolt group carries that times the number of bolts.

On tower bracing, with thin plies and short end distances, bearing usually governs, and the end distance is what sets it. If no end distance is given, the e/3d₀ term cannot be evaluated; the program then leaves it out of k_b, which over-states the bearing capacity, and so it reports the connection as failing detailing (Step 4) until the end distance is entered.

5. Step 4: detailing limits (IS 800 Cl. 10.2, IS 17740 Cl. 13.1.2)

The capacity equations assume a minimum geometry. The program checks:

Rule	Limit	Clause
Minimum bolt diameter	12 mm	IS 17740 Cl. 13.1.2
Minimum end and edge distance	1.7 d0 for sheared or hand-flame-cut edges; 1.5 d0 for rolled, machine-flame-cut, sawn or planed edges	IS 800 Cl. 10.2.4.2
Maximum edge distance	12 t ε, ε = √(250/fy), t the thinner outer ply	IS 800 Cl. 10.2.4.3
Minimum pitch	2.5 d (nominal bolt diameter)	IS 800 Cl. 10.2.2

The minimum end and edge distance depends on how the edge is made, and guessing it is unsafe either way: assuming 1.5 on a sheared edge passes a joint that does not comply, and assuming 1.7 on a rolled edge fails one that does. So the program has no default for the edge preparation. If you give distances without it, the minimum cannot be established and the joint reads DETAILING until the edge preparation is stated; an unrecognised value stops the run.

A detailing failure is reported as DETAILING, separately from OVERSTRESSED. A joint at 40 % utilisation with a bolt too close to an edge is not overstressed, and the fix is to move the bolt or change the plate, not to resize the member. Both verdicts fail the connection.

Two IS 800 limits are not checked: the maximum pitch of Cl. 10.2.3.1 (it differs between tension and compression members) and the reduced maximum edge distance for corrosive exposure.

6. Step 5: tension in the member (IS 17740 Cl. 11.1, IS 800 Cl. 6.3 and 6.4)

The design tension strength T_d is the lowest of three values.

Gross-section yield

Tdg = Ag · fy / γm0

Net-section rupture of an angle bolted through one leg (IS 800 Cl. 6.3.3)

Tdn = 0.9 · Anc · fu / γm1 + β · Ago · fy / γm0
β = 1.4 − 0.076 · (w/t) · (fy/fu) · (bs/Lc),   0.7 ≤ β ≤ 0.9 · fuγm0 / (fyγm1)

• A_nc is the net area of the connected leg (one hole deducted per bolt line), A_go the gross area of the outstanding leg.
• w is the full outstanding leg; b_s = w + w₁ − t, with w₁ the distance from the heel to the bolt line (IS 800 Fig. 6).
• L_c is the distance between the outermost bolts along the force. With one bolt, L_c = 0 and β takes its lower bound, 0.7.
• The 0.9 in the upper limit comes from IS 800 Amendment No. 1 (2012). Without it the limit never binds on E250 steel, and on E350 it is up to about 11 % high where it binds — for example a 65 mm leg, 6 mm thick, with five or more bolts in line at 50 mm pitch.

Other end types, as the program treats them:

• Angle leg splice gripping both legs: one hole from each leg and no shear lag. Whether the splice grips both legs has no default and must be stated.
• Welded angle through one leg: β = 0.7 (the weld length is not an input, and 0.7 holds for any length).
• Round member, bolted (flattened end or tongue plate): gross yield in the member; the holes are in the connected element, whose net section and block shear are checked in the connection.
• Round member, welded: rupture on the full section, 0.9·A_g·f_u/γ_m1.
• Nothing declared: gross yield only, marked not checked on the report, with a warning naming the member groups.

Block shear (IS 800 Cl. 6.4.1)

Tdb1 = Avg fy / (√3 γm0) + 0.9 Atn fu / γm1
Tdb2 = 0.9 Avn fu / (√3 γm1) + Atg fy / γm0
Tdb = min(Tdb1, Tdb2)

For one line of n bolts, with end distance e, pitch p, edge distance e' (bolt line to the free edge) and hole d₀, the program takes the block as shear along the bolt line and tension across to the edge:

• Avg = t · (e + (n − 1)p),  Avn = t · (e + (n − 1)p − (n − 0.5)d0)
• Atg = t · e',  Atn = t · (e' − 0.5 d0)

Block shear is checked on the member (an angle bolted through one leg) and on the connected element in the connection check. Without an end distance, an edge distance and (for two or more bolts) a pitch, it cannot be set up, and the report says it was not checked.

Net section of the connected plate

If you give the gross width of the connected plate, the connection check also takes its net section, 0.9 · (b − d0) · t · fu / γm1.

Eccentric bracing ends

A single angle whose bolt group lies beyond the centreline of the connected leg loses compression capacity under IS 17740 Cl. 10.4.4.2. That is a member check; it is covered in the member capacity guide, and its one input, the distance g from the heel to the bolt group, is entered in the same connections entry.

7. Leg splices: angle cover plates and pipe ring flanges

Angle legs

An angle leg is usually spliced with bolted lap or cover plates. The program checks it with the same limit states as a bracing end — bolt shear, bearing, plate net section, block shear and detailing — fed with the leg's own tension and compression, and in double shear unless you state otherwise. Where the angle ends are prepared for full contact in bearing, you may state that, and the compression fed to the splice is then halved, on the reading that IS 800 Cl. 10.8 requires the splice itself to carry at least 50 % of the compression; tension always takes the full force. Confirm that reading for your project before relying on it for a governing splice.

Pipe legs: the ring flange

A tube leg is usually spliced with a pair of welded ring flanges bolted on a pitch circle. The program checks:

• Bolt tension (IS 800 Cl. 10.3.5): Tnb = 0.90 fub An, not more than fyb Asb · γmb/γm0, and Tdb = Tnb / γmb. The ratio is γ_mb/γ_m0 = 1.136, as printed. On 8.8 and 10.9 bolts the first term governs anyway; on 4.6 to 6.8 bolts, now that Table 1 gives their yield, the cap can govern.
• Prying (IS 800 Cl. 10.4.7), always added to the bolt tension — Amendment No. 2 removed the wording that made it conditional.
• The flange-to-pipe fillet weld (IS 800 Cl. 10.5), on the larger of the tension and the compression.
• The flange plate in bending. Neither IS 800 nor IS 17740 gives the flexural mechanism of an annular plate or its effective width per bolt, which the prying formula needs. The program takes both from the EN 1993-1-8 T-stub method, evaluated with IS partial factors. The T-stub equations are written for a pair of bolts, one each side of a web, and are applied here per bolt on the circle. Treat this part as a scaffold that needs checking against a ring-flange method before you rely on it.

8. What is not checked

• Welded bracing ends are not designed. A welded end only changes the member's slenderness and net section.
• Block shear around a weld (IS 800 Cl. 6.4.2) is not checked, and the report says so.
• Friction-grip (slip-critical) bolts: there is no slip check. Bolt tightening (IS 17740 Cl. 13.1.3) is an erection requirement, not a design check.
• Base plates and anchor bolts (IS 17740 Cl. 13.3 and 13.4) are not designed; the report gives the support reactions.
• Maximum pitch and the corrosive-exposure edge limit (Step 4).
• Gusset plates are checked only as the connected ply you describe (thickness, width, distances); their buckling and the Whitmore width are not checked.

9. Worked example: a diagonal end by hand

A single-angle diagonal bolted through one leg to a gusset, in single shear.

Step 1: bolts

M16 is above the 12 mm minimum. Class 8.8 at 16 mm: f_ub = 800 MPa.

Step 2: bolt shear

Anb = 0.78 × π × 16² / 4 = 156.8 mm²

Vnsb = 800 / √3 × 1 × 156.8 = 72.4 kN. l_j = 50 mm < 15d = 240 mm, so β_lj = 1.0. Vdsb = 72.4 / 1.25 = 57.9 kN per bolt.

Step 3: bearing

kb = min(30/54, 50/54 − 0.25, 800/410, 1.0) = min(0.556, 0.676, 1.95, 1.0) = 0.556

Vnpb = 2.5 × 0.556 × 16 × 6 × 410 = 54.7 kN, Vdpb = 54.7 / 1.25 = 43.7 kN per bolt.

Bearing governs: the bolt group gives 2 × 43.7 = 87.5 kN.

Step 4: detailing

Rule	Limit	Provided
Bolt diameter	≥ 12 mm	16 mm	OK
End distance, rolled	≥ 1.5 × 18 = 27 mm	30 mm	OK
Edge distance, rolled	≥ 27 mm	28 mm	OK
Maximum edge distance	≤ 12 × 6 × 1.0 = 72 mm	28 mm	OK
Pitch	≥ 2.5 × 16 = 40 mm	50 mm	OK

Step 5: tension in the angle

Gross yield: Tdg = 753 × 250 / 1.10 = 171.1 kN.

Rupture. The program splits the gross area into the connected leg, a·t = 65 × 6 = 390 mm², and the outstanding rest, A_go = 363 mm². Anc = 390 − 18 × 6 = 282 mm². w = 65 mm, w₁ = 65 − 28 = 37 mm, b_s = 65 + 37 − 6 = 96 mm, L_c = 50 mm.

β = 1.4 − 0.076 × (65/6) × (250/410) × (96/50) = 1.4 − 0.964 = 0.436, below the lower bound, so β = 0.7.

Tdn = 0.9 × 282 × 410 / 1.25 + 0.7 × 363 × 250 / 1.10 = 83.2 + 57.8 = 141.0 kN

Block shear: Avg = 6 × (30 + 50) = 480, Avn = 6 × (80 − 1.5 × 18) = 318, Atg = 6 × 28 = 168, Atn = 6 × (28 − 9) = 114 mm².

Tdb1 = 480 × 250 / (√3 × 1.10) + 0.9 × 114 × 410 / 1.25 = 63.0 + 33.7 = 96.6 kN
Tdb2 = 0.9 × 318 × 410 / (√3 × 1.25) + 168 × 250 / 1.10 = 54.2 + 38.2 = 92.4 kN

Td = min(171.1, 141.0, 92.4) = 92.4 kN, block shear governs the member.

Result

The end is limited by the bolts in bearing, 87.5 kN, just below the member's block shear of 92.4 kN. At 40 kN the bolt utilisation is 40 / 87.5 = 0.46, and the joint is OK. SutranetTower's connection and member checks give the same values.

Three changes show how sensitive this is:

• End distance 40 mm instead of 30 mm. k_b moves to the pitch term, 0.676; bearing rises to 53.2 kN per bolt, 106.4 kN for the group, and block shear to 102.6 kN, which now governs. The end is 17 % stronger for 10 mm of material.
• Sheared edges instead of rolled. The minimum becomes 1.7 × 18 = 30.6 mm; the 30 mm end and the 28 mm edge both fail, and the joint reads DETAILING at the same 0.46 utilisation.
• M10 bolts. Below the 12 mm minimum of IS 17740 Cl. 13.1.2: DETAILING, whatever the strength.

10. Doing it in practice

A tower has a bracing end for every diagonal, horizontal and redundant group in every section, and the demand at each is a different envelope of a dozen combinations. In SutranetTower you describe each joint once in the section's connections block — bolt class and diameter, hole, number in line, pitch, end and edge distances, edge preparation, ply thickness and shear planes — and the program checks the bolts, the plies, the detailing and the member's rupture and block shear against the enveloped forces, reports OK, OVERSTRESSED or DETAILING for each, and prints the working and a fabrication one-liner for the member schedule.

This guide is an engineering overview and is not a substitute for the standards. Clause numbers refer to IS 800:2007 with Amendments No. 1 (2012) and No. 2 (2024), and IS 17740:2022 with Amendment No. 1. Always design against the current published text and have connection details reviewed by a qualified structural engineer.