Online Estimating Guide - Fabrication
General
This estimating guide uses a fabrication process based approach to developing an estimate. Therefore the fabrication process needs to be understood and measured as part of building an estimate.
While no workshop is exactly the same as another, there many things that are common to most. For the purpose of describing a market based estimate it is only necessary to ensure that the workshop modelled reflects industry norms rather than industry leaders or laggards.
Where products are being traded competitively and intelligently, with good value measurement criteria, in an open market, a fairly consistent market level of pricing becomes established for a given measure of customer product value. Suppliers who can produce more efficiently will achieve better returns than those who don't and will tend to draw the overall market price downwards over time.
Workstations
A typical structural steel fabrication workshop consists of a number of major work stations at which particular fabrication activities occur. Often a piece of machinery will designate the workstation, such as a saw, drill, hole punch or guillotine. A structural member will pass from the storage yard into the workshop and through a number of these workstations and finally out of the shop upon completion.
Workstation Types
- Yard
- Shafts
- Saw
- Drill
- Preparation: Gas Cutting & Weld Preparation
- Welding
- Fittings
- Cut: Guillotine & Profile Cutter
- Hole: Punch & Drill
- Load Out
Workflow & Handling
In addition to the time spent actually performing workstation operations such as cutting, drilling or welding, a significant amount of time is involved in handling the steelwork in the workshop. All this time needs to be assessed in developing a work process based estimate.
Various handling systems such as overhead cranes and trolley systems are used to move the items from station to station. At each workstation time is also spent positioning and rotating the section. For lighter sections this can be done manually with the assistance of levers and drifts. For heavier items the assistance of an overhead crane is needed.
For each of the common connections used in structural steel construction the workstations required are determined and handling times assessed per end connection.
Assessing handling times on the basis of end connection type allows the estimating process to be simplified. The following rules have been used to develop a consistent method of assessing handling times through the shop and at work stations.
Work Piece or Shaft Handling
The movement of a member through the shop can be assessed on the basis of end connection type.
Therefore determine the number of workstations and transit or temporary storage piles, the connection must be passed through. Add one to allow for fittings handling related to the shaft, then divide by two to get the number of shaft movements per end connection.
End to end splices include two member ends in the one connection so the number of shaft movements per splice connection is twice that for other end connections.
The shaft of the member may pass through any of the workstations except the guillotine and the hole punch, which are used for processing the fittings.
Each shaft has two ends.
All shaft ends have some form of connection or end cut, except some pre-cut items.
All shafts are cut to length, except Steltech beam sections. However Steltech sections do need to be end saw cut to square when they are used as column sections requiring full contact end bearing splices.
Most shafts are either drilled or welded or both.
Some shafts are coped or prepared for welding.
Handling of fittings between workstations is combined into the workstation times as fitting plates and flats are often moved in large batches, resulting in small through shop handling times per fitting.
Through Shop Handling
Most shafts are not light enough to be carried by a single man, so it is assumed that an overhead crane or a trolley is used to make any through shop handling movements.
The time allocated for each movement is 8 minutes / shaft or 4 minutes / end connection.
Temporary Piles (TP)
Allowance needs to be made for the fact that each Workstation will form a process bottleneck and therefore will require temporary storage piles for work in progress. The Temporary Piles are used for stacking items immediately prior or following processing at a work station. Two through-shop handling movements will occur at a workstation as shafts are moved from the pre-worked pile to the work bench and then to the processed pile.
Work station Handling
At each station the shaft may need to be rotated or shifted to allow work to be done on various faces of the section. This is described as workstation handling.
For shafts under 60.5 kg/m it is assumed that a single man can rotate and position the element using levers and drifts. Time per movement is 4 minutes / shaft.
For shafts over 60.5 kg/m an overhead crane is considered necessary. Time per movement is 6 minutes / shaft. For the typical two end connections / shaft this becomes 3 minutes / movement/ end connection.
The shaft is rotated at the workstation so that wherever possible all work operations are performed in a down hand position.
All welding is assumed to be laid in multiples of 6mm legs for fillet welds or 6mm depths for butt welds. The shaft is rotated in a single direction so that all welds around a connection are built up symmetrically. Therefore the number of rotations required at the welding station is governed by the weld size.
Each 90 degree rotation is considered one movement.
For stiffeners assume that a stiffener will occur both sides of a web for rotation assessment.
Drilling and cutting rates include an allowance for positioning of the shaft.
Workshop Process and Handling Rates
A simplified list of workshop handling and process rates for various fabrication activities are listed based on estimated real-time values including allowances for average productivity levels in New Zealand workshops. The productivity level reflects an allowance for non-process work activities such as shop floor work planning, machinery servicing, idle time, rework, equipment set up and coordination with other staff that must be built into production schedules.
The process times are calculated from data published in AISC Steel Construction Vol.30 No.2, June 1996.
Welding times are based on analysis using PROWELD software for a typical FCAW semi-automatic welding process on a variety of short length weld preparations, in various thicknesses of plate.
