If you are a contractor, site engineer, or project manager working on Indian Railways bridge projects, this guide covers everything you need to know about HSFG bolt torquing — drawn directly from **RDSO Report BS-111**, the official guideline published by the Research Design and Standards Organisation, Lucknow, which forms the technical foundation of the Indian Railways mandatory torquing requirement.
Bookmark this page. Share it with your site team. This is the only comprehensive, technically accurate reference on HSFG torquing for Indian Railways available from a practising torquing contractor.
Why Indian Railways Moved From Rivets to HSFG Bolts
For over 150 years, rivets were the standard field connection method for steel bridges on Indian Railways. They worked — but they came with serious practical problems that RDSO formally identified.
Making quality rivets in difficult or inaccessible locations is unreliable. Longer rivets for large grip lengths are hard to heat uniformly, and differential cooling means they often fail to completely fill the annular space in the hole. Most critically, **the skilled labour required for riveting is disappearing** — Indian Railways had become virtually the only major organisation in India still using structural rivets, and finding trained riveters was becoming increasingly difficult and expensive.
RDSO evaluated alternatives and determined that **High Strength Friction Grip bolts were the most suitable replacement for rivets** in railway bridge field connections. The benefits are significant — faster erection, less scaffolding required, lower skilled manpower needs, fewer rejections, easier supervision, and comparable or lower cost than rivets with better fatigue performance.
RDSO BS-111 was published in June 2012 to provide comprehensive guidelines covering the design, installation, and maintenance of HSFG bolts on Indian Railways bridges. The mandatory use of calibrated electric torque wrenches flows directly from this document.
What Makes HSFG Bolts Different — The Structural Mechanism
Understanding why HSFG bolts require precise torquing starts with understanding how they work — and it is fundamentally different from how ordinary bolts or rivets work.
An ordinary bolt resists load through two mechanisms — the bolt shank bears against the hole edge (bearing action) and the bolt shank itself resists shearing. If the load becomes large enough, the bolt shank shears or the hole edge crushes.
**An HSFG bolt works entirely differently.** When correctly torqued, the bolt clamps the connected steel plates together with such high force that friction between the plate surfaces carries the entire load. The bolt shank never bears against the hole edge and never experiences shear — as long as the friction grip is maintained.
The structural implication is profound. The clamping force — and therefore the entire load-carrying capacity of the joint — depends completely on achieving the correct bolt tension during installation. Under-torque the bolt and the friction grip does not fully develop. The joint is weaker than designed.
This is the technical reason why precise torquing using calibrated electric torque wrenches is not optional — **it is the mechanism by which the joint works**.
HSFG Bolt Grades Used on Indian Railways
RDSO BS-111 specifies two property classes of HSFG bolts for use on Indian Railways bridges, both conforming to IS 3757.
Property Class 8.8
Nominal tensile strength of 800 N/mm². Yield stress of 640 N/mm² (80% of tensile strength). RDSO recommends property class 8.8 as the **preferred choice for most applications** because of its ductility and good reserve strength. Bolts are marked **8S or 8.8S** on the bolt head along with the manufacturer's identification symbol.
Property Class 10.9
Higher strength than 8.8. Used where joints need to be made smaller or for better detailing. Because property class 10.9 imparts very high tension — high enough to potentially damage mild steel plate members — **two washers are mandatory**, one against the bolt head and one against the nut. Bolts are marked **10S or 10.9S**.
Both property classes are supplied with zinc phosphate coating in conjunction with rust preventive oil as per IS 1367 Part XII.
Standard diameters for bridge work per IS 4000: M16, M20, M22, M24, M30, and M36. For most structural bridge connections RDSO recommends 20mm or 22mm diameter bolts.
Mandatory Minimum Bolt Tensions — The Exact Figures
The HSFG bolt must be tightened to achieve a minimum clamping force (proof load) specified in IS 4000 Table 3. These are the exact minimum bolt tensions that Electorc Engineering's calibrated electric torque wrenches are set to achieve:
Bolt Size | Class 8.8 (kN) | Class 10.9 (kN)
These values represent the minimum acceptable bolt tension. Achieving less than these values means the friction grip is not fully developed and the joint does not meet the design specification.
This is why calibration matters — a torque wrench that is out of calibration by more than the allowable ±10% tolerance can produce bolt tensions outside the acceptable range.
Surface Preparation — The Step Most Contractors Get Wrong
Before a single HSFG bolt is installed, the steel interface between the plates being connected must be correctly prepared. This is one of the most commonly overlooked aspects of HSFG bolt compliance on Indian Railways projects — and one of the most consequential.
The friction that carries the structural load in an HSFG joint depends on two things — the clamping force (controlled by torquing) and the slip factor of the steel interface (controlled by surface preparation). Get the surface preparation wrong and the joint's load capacity is compromised regardless of how well the bolts are torqued.
For new construction
RDSO mandates that the interface between connected plates must be **aluminium metallised to a nominal thickness of 150 microns with no overcoating whatsoever**. Painting the interface is not permitted. If the surface has been painted inadvertently, it must be sand or grit blast cleaned and re-metallised — even if it was already metallised before the paint was applied. This gives a slip factor of 0.40.
For existing structures
The interface must be cleaned by wire brushing or flame cleaning to remove all loose rust and paint layers. Isolated patches of coating or rust may remain, but the surface must be substantially clean. This gives a slip factor of 0.25.
If adequate surface preparation cannot be achieved on an existing structure, RDSO is explicit — **HSFG bolts shall not be used** and the existing rivets must be replaced by appropriate close tolerance turned bolts instead.
The Two-Stage Tightening Procedure — Step by Step
The mandatory procedure for installing HSFG bolts on Indian Railways projects, as specified in RDSO BS-111, consists of two distinct stages. Both stages are non-negotiable.
Pre-installation checks
Stage 1 — Snug tightening
All bolts in the joint are inserted and tightened to **snug tight condition** — meaning tightened using an ordinary wrench by an average worker applying maximum hand force. This stage brings all the plates into close contact and eliminates gaps. Every bolt in the joint must be snug tightened before any bolt proceeds to Stage 2.
After Stage 1 the joint is inspected. The residual gap at plate edges must be less than 2mm. There must be no gap in the central portion of the joint.
Stage 2 — Full torquing
**Critically, Stage 2 tightening must always begin from the stiffest part of the joint — generally the centre — and proceed outward toward the free edges.** Never tighten randomly or from the edges inward.
The electric torque wrench is set to the required torque value for the bolt grade and diameter and applied to each bolt in the correct sequence.
For DTI method (RDSO preferred): Torquing continues until the DTI projections indicate full tightening. After Stage 2, a 0.40mm feeler gauge is used to check 100% of bolts.
For plain washer method: The Stage 1 torque wrench is set to 75% of the full torque value. After Stage 1 checks, Stage 2 applies a further rotation to each bolt based on grip length.
Torque Wrench Requirements and Calibration
RDSO BS-111 is specific about the type of torque wrench required. For all but minor works, **only mechanical torque wrenches — pneumatic, hydraulic, or electronic — shall be used**. Manual torque wrenches are permitted only for small quantum of work.
Calibration requirements:
An uncalibrated torque wrench does not meet the RDSO requirement regardless of how recently it was purchased or how experienced the operator is.
**Electorc Engineering's electric torque wrenches are calibrated within the last 12 months to ±10% accuracy** and calibration certificates are available for inspection at any project site.
Direct Tension Indicators — Why RDSO Prefers Them
The Direct Tension Indicator (DTI) is a precision-engineered washer with multiple raised projections on one face. When the bolt is tensioned, the projections compress. The degree of compression — measured using a feeler gauge — indicates whether the correct bolt tension has been achieved.
RDSO describes DTIs as **the most reliable method of verifying correct HSFG bolt tension** and specifies them as the preferred method over plain washer installation.
The reason is straightforward — DTIs measure the actual clamping force in the bolt directly, rather than inferring it from the torque applied. Torque and tension are related but not identical — thread friction, lubrication condition, and surface condition all affect the relationship between applied torque and actual bolt tension. DTIs eliminate this uncertainty.
The Critical Rule — Never Reuse a Fully Tightened HSFG Bolt
This is the rule most often missed on site — and it is absolute.
An HSFG bolt is tightened beyond its yield point during installation. This causes permanent plastic deformation — the bolt elongates permanently and the first few threads suffer irreversible damage.
**If a fully tightened HSFG bolt is removed for any reason, it must be permanently rejected and removed from site.** The nut, all washers, and any DTI used with that bolt must also be rejected and removed. None of these components may be reused.
A bolt that has been snug tightened (Stage 1 only) and then removed before Stage 2 has not been tightened beyond yield and may be reused. However, a bolt that has completed Stage 2 full torquing must never be reused under any circumstances as per RDSO BS-111.
Compliance Documentation — What Indian Railways Inspectors Check
Based on the requirements of RDSO BS-111 and IS 4000, the following documentation is typically required for HSFG bolt torquing operations on Indian Railways projects:
**Electorc Engineering produces complete documentation for every torquing operation as a standard deliverable** — all records are traceable and formatted for submission to Railways inspection.
Why Engage a Specialist HSFG Torquing Contractor
The technical requirements detailed in this guide make clear that HSFG bolt torquing on Indian Railways bridges is a specialist operation — not a task that can be assigned to general site labour with a borrowed wrench.
The equipment requirement alone — calibrated electric torque wrenches with valid calibration certificates — represents a significant investment that most contractors do not carry as standing equipment.
Electorc Engineering brings all of this as a complete package. Our teams are trained specifically in the RDSO BS-111 requirements and have executed HSFG bolt torquing operations on Indian Railways bridge projects across India for over five years.
Engaging Electorc Engineering for your HSFG torquing is not just a convenience — it is the most reliable way to ensure your project passes inspection first time, without rework, without delays, and without compliance risk.
Download the Official RDSO Guidelines
You can download RDSO Report BS-111 — Guidelines for use of High Strength Friction Grip (HSFG) bolts on bridges on Indian Railways — from our resources page:
[Download RDSO BS-111 PDF](/resources/rdso-bs-111.pdf)
This document is the official technical reference for all HSFG bolt applications on Indian Railways bridges.
Contact Electorc Engineering for Your Project
Electorc Engineering is one of India's oldest and most experienced HSFG bolt torquing companies, serving Indian Railways contractors pan-India for over 5 years.
📞 +91 91213 93878 | +91 70009 60470
✉️ info@electorc.com
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📍 Thane West, Maharashtra | Pan-India Deployment
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Related Reading
*Reference codes cited: IS 4000:1992, IS 3757:1985, IS 6623:2004, IS 6649:1985, RDSO Report BS-111 June 2012, IRS Steel Bridge Code*