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Economical Guidelines for Design of Structural Steel. By Gary Glenn 07/30/13. Structural Steel Costs (Where is the Money?). Conclusions from Steel Costs. Fabrication and Erection costs are same magnitude as Material.
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Economical Guidelines for Design of Structural Steel By Gary Glenn 07/30/13
Conclusions from Steel Costs • Fabrication and Erection costs are same magnitude as Material. In addition to Material costs, Fabrication and Erection costs should be a consideration in design. • Detailing costs are significantly lower than the cost of Fabrication and Erection (by a factor of 10). Efforts to minimize Detailing costs at the expense of Fabrication and Erection costs should be avoided.
Material Availability Factors • Domestic vs Offshore (or Single Source) Material • A36 (Flatbar) vs A572 Gr 50 (Sheet) • A529 Gr 50/55 (Flatbar) vs A572 Gr 50 (Sheet) • Anchor Bolt Grades • A325 vs A490 Bolts and Diameters • Sole Source Product
Domestic WF Mills 1. Nucor Yamato – WF mill located in Arkansas. • W6 to W44s. W14s up to W14x730 (A992) (50 yield). • W14x550s and larger may be rolled based on tonnage accumulation. • Check w/ Fabricator for availability. 2. Steel Dynamics Inc. (SDI) `– WF mills located in Indiana. • Lighter WF sections from W6 to W36. 3. Chapparel – WF mill located in Texas. • Smaller, lighter WF sections. 4. Warehouses (PDM, Gerdue Ameristeel, R&S, etc) – • Lighter WF sections. • Premium in cost to purchase through warehouses. • Some fabricators do you have credit with mills, thus purchase thru warehouse. 5. Mill material is normally shipped via rail to fabricator yard. Warehouse is normally shipped via truck.
Offshore WF Mills 1. Arcelor / Mittal – Mills located in Luxemburg. • W14x550 thru 730s, in A992 (50 ksi yield) that may not be domestically produced. • Also, various WF sections in A913 Gr 65. 2. Nippon - Mills located in Japan. • WF sections including W14x550 thru 730s in A992 (50 ksi yield) that may not be domestically produced. • Currently developing equivalent to A913 Gr 65, but not yet approved by ASTM. 3. Pros of Offshore Material • Availability of larger sections not domestically produced in lieu of builtup WF or box columns. • Availability of A913 Gr 65 that is not domestically produced. 4. Cons of Offshore Material • Prone to schedule problems (no commitment is literally written into their Purchase Agreements; Fabricators have little influence over offshore mills). (Rolls are based on tonnage accumulation; mills are shutdown for maintenance; vessels depart once a month; vessels can offload at wrong port; delays in customs/offloading, etc). • Cost premium. For example, W14x730 A913 Gr 65 at $.75/#. W14 A992 domestic sizes might be around $.39/#. • Sole source material. • Welding of A913 Gr 65 requires E80 electrodes or qualification of welding procuedures to achieve high yield at welds. Only applies when welding Gr 65 to Gr 65 material (splicing).
Specifying WF • Desirable to specify domestic (not offshore), unless domestic size will not work. • In general, you can check Nucor Yamato website for produced sections. For heavier sections, suggest checking w/ fabricator to confirm. • Avoid offshore material, except when domestic sizes will not work. Suggest reviewing w/ fabricator when considering offshore steel.
FB Availability • Available in std widths of ½” or 1” increments, and thickness increments of 1/8” • A36 FB readily available from warehouse. (A36 also available in plate (sheet)). • A529 Gr 50/55 • FB available from mills and usually must be ordered in bundle qnty or min tonnage. • Available widths and thickness is more limited than what is available for A36. • A529 Gr 50/55 is a material now specified in AISC Steel Construction Manual (13th Ed Black). • A572 Gr 50 is not generally produced in FB. Instead, it is available in Plate or Sheet (5’ x 12’, 10’ x 12’, 10’ x 20’, etc).
Advantages of FB over PL (Sheet) • By multing shear PLs, Stiffeners, etc. into lengths of FB, only one edge of part must be cut. 2. FB is easier to handle than PL (sheet), and takes up less floor space. 3. There is less drop in producing parts from FB over PL (mult end to end out of FB where as parts nested in PL) 4. There is slight premium in Gr 50 PL over A36. Probably $.01/# premium.
Specifying PL and FB Material • Spec A36 where possible. Allows fabricator to produce parts out of A36 FB widths. • Spec that A529 Gr 50/55 or A572 Gr 50 may be utilized in lieu of A36 as fabricator option. Allows fabricator max flexibility to utilize materials on hand and to minimize drop. • Where 50 ksi Fy is necessary, spec A529 Gr 50/55 and A572 Gr 50. Allows fabricator flexibility to use FB as an option in lieu of having to cut from PL where possible. • Make it very clear where 50 ksi Fy is required so fabricator need not have to clarify extent in RFI (specify it right at details??).
Anchor Bolt (AB) Availability 1. A36 Rod or F1554 A36 (all thread) – • Most readily available and cost effective. • Specify either, giving fabricator flexibility. 2. High Strength ABs - • Heat treated, increasing cost and leadtime. • F1554 Gr 55 or 105 or A354 BD are common and available. • A307, A325, A490 are generally bolt (fastener) specifications and should not be used at anchor bolts (due to lack of availability). • A449 and A572 are less commonly available and should be avoided. 3. Double Nut End is desirable over a Headed or J End - • Head or J end must be fabricated by supplier and has leadtime. • Double Nut End allows straight bolts to be used with all thread or threaded shaft, which is more economical. • Exception is short anchor bolts at ledger angles. In this situation, headed A307 bolts may be desirable.
Anchor Bolt Availability & Issues 1. Nut Types – • ASTM defines exactly which types of nuts are to be provided for grade of anchor bolt, such that the shaft fails prior to the nut. The supplier must follow ASTM. • So, it should not be necessary to specify the exact nut grade and type (such as A563 Gr A for example), as this simply creates confusion and additional cost to fabricator. 2. Welding at Anchor Bolts – Should be avoided. Cannot occur at high strength bolts as it destroys the heat treatment properties. 3. Spoiling Threads – Double Nuts jambed together is preferable over spoiling threads.
Field Bolts - A325 vs A490 Bolts 1. Bolt Cost (based on 2” long TC bolts) • 7/8” A325 $0.14 • 7/8” A490 $0.16 • 1” A325 $0.18 • 1” A490 $0.22 • 1 1/8” A325 $0.22 • 1 1/8” A490 $0.28 2. Installation Cost of Bolts • 7/8” and 1” cost about same to install. • 1 1/8” costs about 15% more than 7/8” or 1” to install. • 1 ¼” costs about 30% more than 7/8” or 1” to install. 3. 1 ½” A325 or A490 bolts should be avoided due to tightening issues (if fully torqued, requires hydraulic gun vs TC gun or traditional impact wrench). 4. A325s tend to be more readily available than A490s. Minor risk in switching. 5. Conclusion – 1” dia. A325 may be most cost effective for typical connections. 6. Minimize Different Diameters/Grades used on Project (use all 1” bolts). This allows: • Erector to utilize one dia set of guns onsite. • Allows fabricator/erector to consolidate bolts together. • Simplifies detailing, fabrication, erection. • Reduces errors.
Field Bolts – Other Consideration 1. TC vs Std Hex Head • Spec both giving fabricator/erector flexibility to use either. • Considerations such as length, access, equipment, thru bolts at HSS dictate which will be used. 2. Bearing Type Bolts • Generally, there is no cost difference in X over N in the bolt or installation. • So, suggest specifying X so as to take advantage of higher bearing capacity. 3. Slip Critical Type Bolts • There is a slight cost difference in specifying SC, primarily due to inspection. • Suggest only specifying SC at SLRS (collectors, frames, etc) locations. 4. LIWs (Load Indicator Washers) • Can cost as much as the bolt itself. • Also, costly to install. • TC, Calibrated Wrench, or Turn of the Nut methods for achieving SC are cost effective and reliable methods, so LIWs should be avoided. 5. Lock Washers – Not applicable to high strength bolts and should be avoided.
Tolerance and Deflection Issues • Variation in WF – Depth and out of squareness. • Anchor Bolt Tolerance & Oversize Holes – • Steel vs Finish System Tolerances – • Lack of Camber and Floor Deflection -
Oversizing AB Holes 1. Anchor Bolt Tolerance • Patterns can be ¼” off from grid. • Patterns can be ¼” off relative to each other. • Bolts w/in pattern can be 1/8” off from each other. 2. Erection Tolerance – Column base can be ¼” off in location. 3. Oversized Holes in Base PLs (Necessary for 1 and 2 above) • Recommend oversized holes per AISC ASD 9th Ed (Green Manual pg 4-130). • Size is adequate for AB tolerances, yet not too large to where std washers begin cupping. • Oversized holes per AISC 13th Ed (Black Manual pg 14-21) requires use of PL washers in lieu of std washers, increasing costs. 4. DSA – We have seen DSA try to limit AB hole diameters to 3/16” oversized, creating substantial issues.
Steel vs Finish System Tolerance • Allowable Steel Tolerance Exceeds Finish System Tolerances significantly. • Suggest designing 1” to 1 ½” gap between studwall/window wall system and edge of steel horizontally and vertically. • Applies to perimeter of building, canopies, soffits, roof screens, etc.
Warpage and Distortion Considerations Some items contributing to warpage/distortion: • Continuous Welding (vs Stitch Welding) • Unsymmetrical Sections • Relatively Thin Material
Weld Types • Fillet Welds • PJP Welds • CJP Welds
Fillet Weld Guidelines • AWS D1.1 - Single pass fillet welds up to 3/8” in flat / horizontal positions allowed. • In practicality, some 3/8” fillet welds cannot be made in a single pass. • ½” fillet weld ≈ 3 passes. • 5/8” fillet weld ≈ 4 passes. • ¾” fillet weld ≈ 6 passes. • 1” fillet weld ≈ 10 passes. In summary, 5/16” or smaller fillet welds desirable.
Minimum Fillet Weld Size Per AWS D1.1 2008 Structural Welding Code: Base Metal ThicknessMin Fillet Weld Size T < ¼” 1/8” ¼” < T < ½” 3/16” ½” < T < ¾” ¼” ¾” < T 5/16”
PJP Weld vs Fillet Welds • PJP (vs Fillet) • Requires prepping. • Fitup is more costly. • Meeting inspection more costly. • 5/16” fillet weld generally desirable over PJP welds. • > 3/8” fillet welds require multipasses, so GMC starts reviewing as to whether a PJP or CJP may be more cost effective than the fillet weld. • Suggest reviewing fillet welds > 3/8” w/ fabricator as to cost effectiveness.
CJP vs PJP vs Fillet • CJP (vs PJP or Fillet) • Requires beveling. • Either requires backup bar & runnoff tabs, or back gouging (fitup is costly). • UT inspection is costly. • Again, this requires additional labor and process over fillet welding. • Again, GMC starts to consider PJP or CJP welding when fillet welds are > 3/8”.
SLRS Members, DC Welds, Charpy Electrode, and Preheat 1. SLRS Members • Clearly define which members are SLRS (show on plans, frame elevations, and column schedule). 2. DC Welds • Clearly show DC in weld tails in design details to signify DC welds. 3. Charpy Electrode • Avoid specing Charpy rated electrodes at all welds. • Charpy rated electrodes should only be needed at SLRS members and DC welds. 4. Preheat, Interpass Temperature, Postheat Treatment • Avoid calling out preheat or interpass temperature above and beyond typical AWS. Very costly to preheat. • Postheat treatment is not practical.
Bolted vs Welded Connections • Oftentimes, a field bolted connection is desirable over a field welded connection. • Expedites erection considerably. • Reduces inspection. • Less skill required to install bolts. • Improves safety.
Dimensional Concerns (Location of Framing) • Lack of dimensional information on Structural Drawings is costly to Fabricators. • When GMC has to release partial packages into production due to missing dimensions, efficiency dramatically drops. • Dbl handling of material due to beam multed in on mill stick • Can’t stage, fit, weld out all beams on floor at one time (must stage twice) • Lost efficiency in loss of numbers. • Increased time needed for fabrication, requiring lost efficiency in expediting work. • Disruption to flow of production of multiple projects through plants. • For example, releasing a 1,700 ton project for fabrication w/ 25% beams on hold may cost over $100 k in lost efficiency.
