FCAW Versus SAW on an 850 ft Bridge Girder Bid
Halden Fabrication is pricing 850 linear ft of 1 in. thick full-penetration groove welds on A36 for a pedestrian bridge girder, 100% UT per AWS D1.1, all welds made flat. Estimating has two routes. Route A is FCAW-G with E71T-1 on 90% Ar/10% CO2, deposition 8.5 lb/hr at a 45% operating factor, filler at $2.10/lb deposited, a 5% reject rate, and $210/ft average repair cost including re-test. Route B is SAW with F7A2-EM12K, deposition 14 lb/hr at a 55% operating factor, filler and flux at $1.35/lb deposited, a 1.5% reject rate, and $240/ft repair. Route B also needs back-gouging, a $9,000 tractor setup and fixture, and a new procedure qualification, because the shop holds no SAW WPS; PQR and testing run about $12,000. Deposited cross-section is 0.62 in2 per foot of weld, steel weighs 0.283 lb/in3, and the loaded shop rate is $62/hr. Management wants a recommendation with the arithmetic shown.
1. Compute the cost per foot for each route and compare.
Read the worked answer
Deposited weight: 0.62 in2 x 12 in = 7.44 in3/ft x 0.283 lb/in3 = 2.11 lb/ft.
Route A - FCAW-G
- Arc hours = 2.11 / 8.5 = 0.248 hr/ft. At a 45% operating factor, labor = 0.248 / 0.45 = 0.551 hr/ft x $62 = $34.16.
- Filler = 2.11 x $2.10 = $4.43.
- Repair = 5% x $210 = $10.50.
- Total = about $49.09/ft.
Route B - SAW
- Arc hours = 2.11 / 14 = 0.151 hr/ft. At 55%, labor = 0.151 / 0.55 = 0.274 hr/ft x $62 = $16.99.
- Filler and flux = 2.11 x $1.35 = $2.85.
- Repair = 1.5% x $240 = $3.60.
- Subtotal = $23.44/ft.
- Setup $9,000 / 850 ft = $10.59/ft. Qualification $12,000 / 850 ft = $14.12/ft.
- Total = about $48.15/ft.
So SAW is under a dollar per foot cheaper - roughly $800 on the whole job. On this job alone it is a tie, not a win, and a tie decided by an unproven reject rate.
2. Which assumption is carrying the decision, and how much room does it have?
Read the worked answer
The SAW reject rate is the fragile number, and it has almost no margin.
The fixed cost is $21,000, or $24.71/ft over 850 ft. Route A's variable cost is $49.09/ft, so SAW's variable cost must stay below $24.38/ft to break even. Strip out repair, SAW runs $19.84/ft, leaving $4.54/ft for repair - which at $240/ft allows a reject rate of only 1.9%.
That is 0.4 percentage points of room above the 1.5% assumption. A shop with no SAW WPS and no recent SAW operating history rarely hits a textbook reject rate in the first weeks; a 5% rate pushes SAW variable cost to $31.84/ft, and the route loses about $14,600 on the job even before learning-curve losses.
The second assumption is the 55% operating factor. SAW is fast under the arc but slow around it - flux handling, back-gouging, slag removal, tractor setup between passes. If the true factor is 45%, labor alone rises $4.15/ft.
Deposition rate is the one number that is genuinely reliable, because it comes from the consumable data sheet.
3. What is your bid recommendation, and what would change it?
Read the worked answer
Bid Route A on this job; keep Route B as a capital decision, not a job decision.
- Route A uses procedures, consumables and welders Halden already has, and $49/ft with a single-pass SAW trial behind it is a defensible number. Route B's apparent advantage is $800 on an $800k-scale job - well inside the estimating error on either route.
- Treat the $12,000 qualification as an investment against the pipeline, not against this girder. Break-even on the fixed cost is $21,000 / $25.65 variable advantage per foot, about 820 ft of future work. So the question is not "is SAW cheaper on this job" but "will we run SAW again in the next 12 months?" If the answer is yes on two or more comparable jobs, qualify the procedure now, absorb the cost over the pipeline, and bid SAW where the volume justifies it.
- What would flip the recommendation: a firm pipeline of 2,000+ ft, a customer specification that favors SAW, or a demonstrated 1.5% SAW reject rate from a trial on 50 ft of production weld with UT results.
- Present both routes to management with the fixed and variable costs separated, so the decision is made on the pipeline question rather than on a $0.94/ft mirage.