Steel girders fabrication- Part 1 (Steel Bridge)

SITE VISIT (KKB FABRICATION YARD) In the morning on 23th Feb 2011, a factory visited with JKR representative and contractor side to KKB fabrication yard to check on the progress of the steel girders fabrication. The flow of the work sequence for the steelworks: 1. Surface preparation 2. Prime coating 3. Cutting 4. Jointing (transverse weld-flange to flange) 5. Fitting/ fit up (Assembly)

6. Jointing (flange & web) 7. Painting & coating application During the visiting on 23th & 29 Feb 2011, the overall progress is covering the item 1 to item 4, which jointing / welding got the flange to flange connection. SURFACE PREPARATION: Degreasing – Any grease and dirt shall be removed by emulsion cleaner prior to start blasting work. Blasting Cleaning-

All rough welds, burns, weld spatter, indentation and all sharp surface project shall be ground smooth and steel surface must be free from oil and grease. All basted surface shall be coated with in 4 hours. Oxidized surface shall be sweep blasted.

Shot blast machines by OMSG include an integral abrasive recovery and cleaning system. After passing through the blast wheel, the abrasive is collected, separated from the resulting dust and other contaminants, and then stored in a hopper, ready for reuse. Collected contaminants are stored in a separate hopper for periodic disposal.

Before

After

t Ou

In

The surface was clean and free of rust after steel shot blasting

PRIME COATING The priming coat shall be applied to the surface immediately after cleaning is completed. The primer shall be capable of “welding” the steel surface and forming an adherent film on it to provide a base for the following coats.

International – Intergard 251 A two component epoxy anticorrostive premer pigmented with zinc phophate

Primer coat on the steel plate after steel shot blasting CUTTING Prior of cutting activities been carried out, proper cutting plan was submitted to minimize the wastage of the steel plates. As plates are cut up, each piece is marked for identification and material traceability according to the heat no. shown on the parent plate. During the factory visiting, we observed that all plate was cut by the Mechanized or automatic cutting which is widely used in production work where a large number of works. The portable flame cutting is more suitable for using in camber cutting. FLAME CUTTING MAECHINE A machine cutting tourch, sometimes refered to as a blowpipe. Operates in a similar manner to hand a cutting torch but it is high consistantcy on the speed and the supply of the gas. It may require 2 oxygen regulators, one for perheat oxygeb and another for sutting oxygen stream. The addition of separator cutting supply allows the flame to be more accurately adjusted and allows the pressures to be adjusted during a cut without disturbing the other parts of the flame.

FLAME CUTTING The steel girders will be cut from plate by flame cutting equipment mounted on gantries. These closely controlled techniques are generally accurate and efficiency is dependent largely on the process. Other components such as bracings should be detailed for simplicity with the aim of minimizing the number of pieces to cut. Ends of bracings should be cut square rather than mitred and single members should be used in preference to back-to-back members.

Variations in camber can arise in practice: •

Fabrication pre-camber to allow for flame cutting and welding effects is difficult to achieve with precision due to the number of imponderables which are involved, including the residual stresses which exist in the material

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The per-camber was marked / offset with the interval of 2.0m in order to obtain the desired camber profile.

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Mechanized or automatic cutting will cut along the per-setup gantries which match the camber profile

Portable flame cutting machine

Pre-set gantrie s

Machine cutting torch

The flame cutting was used in the preparation of the jointing with the ability of adjust the angle of cutting. The photo above shown the cutting of double bevel for the joint flange to flange

PLATE EDGE PREPARATION FOR BUTT WELDS The illustrations show standard terminology for the various features of plate edge preparations for the flange to flange joints

WELDING The jointing of the bottom flange of 25mm steel plate was carried on. The transverse welds with the runoff plate provided were prepared to weld.

Transverse weld

Runof f plate

Submerged arc welding (SAW)

This is probably the most widely used process for welding bridge web to flange fillet welds and in-line butts in thick plate to make up flange and web lengths. The process feeds a continuous wire into a contact tip where it makes electrical contact with the power from the rectifier. The wire feeds into the weld area, where it arcs and forms a molten pool. The weld pool is submerged by flux fed from a hopper. The flux, immediately covering the molten weld pool, melts forming a slag protecting the weld during solidification; surplus flux is re-cycled. As the weld cools the slag freezes and peels away leaving high quality, good profile welds. Solid wires of diameters from 1.6 to 4.0 mm are commonly used with granular fluxes. Mechanical properties of the joint and the chemistry of the weld are influenced by careful selection of the wire/flux combination. Fed Wire

Hopper

Flux fed Molten pool

Runoff plate

Mechanize d on gantries

The process is inherently safer than other processes as the arc is completely covered during welding, hence the term submerged; this also means that personal protection requirements are limited. High deposition rates are a feature of the process because it is normally mechanized on gantries, tractors or other purpose built equipment. This maintains control of parameters and provides guidance for accurate placement of welds.

GOUING Back-gouging is the process to remove the weld metal and base metal from the side opposite of a partially welded joint to facilitate complete joint penetration. It can ensure 100% joint fusion at the roof and remove the discontinuities of the root pass.

Gouging processes Gouging operations can be carried out using the following thermal processes: • • • •

oxyfuel gas flame manual metal arc air carbon arc plasma arc

It is also an essential part of welding fabrication. Used for rapid removal of unwanted metal, the material is locally heated and molten metal ejected - usually by blowing it away. Normal oxyfuel gas or arc processes can be used to produce rapid melting and metal removal.

WELD MARKING The complete weld flange o flange joint are ready for testing purpose. Each joint were marked with the following

Jt No (Joint No)

: BG1/3 (member mark)

Visual check

: Acc (Accepted)

NDT(non destructive test)

:UT(ultrasonic test) & MPI (magnetic particle inspection)

VISUAL CHECK (SURFACE CHECK) Defects, which can be detected by visual inspection, can be grouped under five headings. 1. Cracks. 2. Surface irregularities. 3. Contour defects. 4. Root defects. SURFACE CRACKS A crack is a linear discontinuity produced by fracture. Cracks may be longitudinal, transverse, edge, crater, centreline, fusion zone, underbead, weld metal or parent metal.

2. SURFACE IRREGULARITIES Undercut, overlap, carter pipe, spatter and stray flash. 3. CONTOUR DEFECTS The profile of a finished weld may considerably affect performance of the joint under load bearing conditions. Specifications normally include details of acceptable weld profiles to be used as a guide. •

Excess weld metal

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Lack of fusion

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Incompletely filled groove

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Bulbous contour

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Unequal legs

4. ROOF DEFECTS •

Incomplete roof penetration.

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Lack of root fusion

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Excess penetration bead

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Root concavity (underwashing)

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Shrinkage groove

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Burnthrough (melt through)