Structural Fabrication

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H OW TO ESTIMATE the COST OF STRUCTURAL STEEL ROBERT R. BONENFANT, JR. - JULY 2008 CONTENTS 1. INTRODUCTION Main CSI Division Specific Sub-Division/Specification Section Brief Description of Structural Steel 2. TYPES AND METHODS OF MEASUREMENT Steel Beam Sections and Dimensions 3. FACTORS THAT MAY AFFECT TAKE-OFF, PRICING, ETC. Large Quantities Versus Small – Economies of Scale Union Labor Versus Non-Union Labor Geographical Location Supply and Demand of Steel Height of Building Site Conditions Experience of Erection Crew Renovation Project Versus New Construction Connection Details Moment Connections Beam Penetrations 4. OVERVIEW OF LABOR, EQUIPMENT, INDIRECT COSTS AND MARK-UPS 5. SAMPLE TAKE-OFF AND PRICING SHEETS 6. SPECIAL RISK CONSIDERATIONS Material Pricing Escalation Surcharges Fuel Costs Value of the Dollar Lead Times Estimating Today • January 09 15 7. RATIOS AND ANALYSIS 8. MISCELLANEOUS AND PERTINENT INFORMATION 9. GLOSSARY 10. REFERENCES

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how to fabricate structure members

Transcript of Structural Fabrication

Page 1: Structural Fabrication

HOW TOESTIMATEthe COSTO F S T R U C T U R A L S T E E L

ROBERT R. BONENFANT, JR. - JULY 2008

CONTENTS1. INTRODUCTION

Main CSI DivisionSpecific Sub-Division/Specification SectionBrief Description of Structural Steel

2. TYPES AND METHODS OF MEASUREMENTSteel Beam Sections and Dimensions

3. FACTORS THAT MAY AFFECT TAKE-OFF, PRICING, ETC.Large Quantities Versus Small – Economies of ScaleUnion Labor Versus Non-Union LaborGeographical LocationSupply and Demand of SteelHeight of BuildingSite ConditionsExperience of Erection CrewRenovation Project Versus New ConstructionConnection DetailsMoment ConnectionsBeam Penetrations

4. OVERVIEW OF LABOR, EQUIPMENT, INDIRECT COSTS AND MARK-UPS5. SAMPLE TAKE-OFF AND PRICING SHEETS6. SPECIAL RISK CONSIDERATIONS

Material PricingEscalationSurchargesFuel CostsValue of the DollarLead Times Estimating Today • January 09 15

7. RATIOS AND ANALYSIS8. MISCELLANEOUS AND PERTINENT INFORMATION9. GLOSSARY10. REFERENCES

MILAN GOWEN

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1. INTRODUCTION

The purpose of this technical paper is to provide a basic understanding of how to prepare an estimate for structural steel beams and columns. This paper will focus on how to do a quantity survey of steel members, bent plate (pour stops), shear studs, connections, steel floor and roof decking, and price accordingly. The paper will not address AESS, diago-nally braced frames, open web steel joists, trusses, steel tube, steel stairs, grating and other miscellaneous compo-nents that may be part of a building system. In addition, ancillary trades such as cementitious spray-fireproofing and intumescent fireproofing will not be considered in this technical paper. The intent of the paper is to isolate on a few key aspects of a structural steel framed building and not all possible components. Please note that CSI’s 2004 MasterFormat edition has been used.

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BRIEF DESCRIPTION A steel framed building is an alternative to a concrete framed building. The steel framed building is equally a good choice as a concrete framed building. They each have distinct building materials, practices and benefits that have to be carefully weighed, as timing and situation will dictate the best choice. However, steel is a material that has the high-est strength-to-weight ratio and is very ductile. Steel is also shop fabricated and therefore has a higher quality control as it’s produced in a controlled environment. Steel beams and steel columns are used to structurally frame a building so that it supports the exterior façade, roof, interior construc-tion, mechanical and electrical systems of the building and of course the personnel, furnishings and equipment that will occupy the building. The structural steel framed build-ing also includes components such as steel floor decking, steel roof decking, steel joists, bolted connections, welded connections, base plates, slab pour stops, braced framing, moment connections, and other miscellaneous cold-formed metals to complete the steel framing system. Again, for the purposes of this paper I will only focus on a few of these components.

In the construction of structural steel building some basic design criteria needs to be developed. One is the column spacing and an-other is the floor-to-floor height. This leads to the sizing of columns and beams to carry the building loads. A 30’ by 30’ column bay is typical and a 12’ deck height is typical in an office building. A 14’ deck height is normal in a laboratory building due to significant HVAC requirements that need to be placed in the plenum space above ceilings. Structural steel beams and columns are designed on the basis of their yield stress. The most common design stress is 36 KSI and referenced by ASTM designation A36. Structural steel is fabricated in many sections such as wide flange (W), American standard beam (S), miscellaneous beams (M), American standard channel (C), miscellaneous channel (MC), angle (L), and structural tees (T). A beam and column is designated in the following way; W18x35. The W references the section shape; 18 references the nominal height of the beam/column flange in inches; and 35 ref-erences the weight of the beam in pounds per linear foot of beam or column length. This description is interpreted as a wide flange beam with a nominal depth of 18 inches and a weight of 35 pounds per linear foot.

Main CSI DivisionsDivision 05 Metals

Subdivisions - 050000 Metals

050100 Maintenance of Metals050500 Common Work Results of Metals050600 Schedules for Metals 050800 Commissioning of Metals051000 Structural Metal Framing

051200 Structural Steel Framing053000 Metal Decking

053100 Steel Decking

2. TYPES AND METHODS OF MEASUREMENT

The standard unit of measurement for steel beams and columns is tons. The formula forcalculating tons of steel will be:

Wt. (Tons) = (L x Wt.) / 2,000L = Length of Beam of ColumnWt. = Unit Weight of Beam or Column in Pounds2,000 = Conversion Factor from Pounds to Tons

The unit measurement for floor and roof deck is square feet (SF).

Area (SF) = (L x W) x 1.15L = Length of Building in FeetW = Width of Building in Feet1.10 = Excess Factor for Seams (10%)

The unit measurement for pour stop is Tons.Wt. (tons) = (P x Wt.) / 2,000P = (L + W) x 2 if rectangular or square buildingand measured perimeter (P) if irregular shapedL = Length of Building in FeetW = Width of Building in FeetWt. = Unit Weight of Beam or Column in Pounds2,000 = Conversion Factor from Pounds to Tons

The unit measure of shear studs is Each. Shear Studs (Each) = Quantity Count from Structural Steel Drawings x 1.05 1.05 = Excess Factor for Defective or Broken Studs (5%)

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Structural steel drawings will be provided that shows plan views, column schedules, and connection details. The plan drawings will show beam lengths and sizes with shear stud counts and cambering and the column schedules will show column heights and sizes. Detail drawings will also show items such as beam penetrations, moment connec-tions, base plates, deck opening framing, HVAC support framing, and other special framing conditions. The follow-ing charts will show some typical steel sections, specifica-tions, dimensions and welding symbols.

(Diagrams Cont. page 18)

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3. FACTORS THAT MAY EFFECTTAKE-OFF AND PRICING

EFFECT OF SMALL QUANTITIES VERSUS LARGE QUANTITIES; ECONOMIES OF SCALE

If the project is small and there’s a minimal amount of tonnage the unit cost will be higher than a project with a significant amount of tonnage. The mobilization and crane costs that need to get absorbed into the small project with less efficiency in production will make for a higher unit cost than on a large project.

UNION VERSUS NON-UNION

If the project is based on union labor the project will have a higher cost than non-union labor. Unions negotiate higher wage rate and benefit packages than would be provided by open-shop contractors.

GEOGRAPHICAL LOCATIONThe location of the project may have an influence on cost due to regional material, labor, and equipment costs based on availability of resources and manpower. R.S. Means uses a coefficient factor to adjust for regional differences.

SUPPLY AND DEMAND OF STEEL

Currently the strong global demand of steel; particularly in China, India and Russia is driving up the cost of steel. Even though steel mills have become more efficient and there are additional mills coming online demand is outstripping sup-ply leading to higher prices. Foreign mills are also not ex-porting to the U.S. as they once did.

HEIGHT OF BUILDING

The height of the building will dictate the type of crane to be used on the project. Smaller projects may be able to use a mobile wheel-mounted or track-crawler crane. Tall build-ings will need a tower crane that will require concrete pad foundations and electrical power requirements.

SITE CONDITIONS

The site conditions and constraints will also dictate the type crane that will be utilized. Tight sites in city areas will need tower cranes versus the ability to use mobile cranes in open sites assuming heights aren’t an issue.

EXPERIENCE OF ERECTION CREW

The experience of the erection crew can affect the number of picks they make in day. The less experienced the crew the longer the erection duration will be resulting in higher erection costs.

RENOVATION PROJECT VERSUS NEW CONSTRUCTION

Sometimes an existing building has to be structurally rein-forced. The unit cost of structural steel will be higher on a renovation project than on a new building. On a renovation project the steel has to be fabricated in smaller sections to access the existing building and scissor lifts and other small hydraulic lift equipment has to be utilized in order to posi-tion the steel in place. The production rate is much slower working in an existing building, as you will be utilizing lighter equipment and probably using some handwork.

CONNECTION DETAILS

A project that utilizes welded connections takes longer to erect than with bolted connections. In addition, the field-testing of welded connections will also add time to the schedule. Curtainwall support connections will add to the cost as well as precast panel connections. Therefore, it’s im-portant to know what the façade of the building will be so connection detail costs can be accounted for.

MOMENT CONNECTIONS

Moment connections carry a premium cost to be installed, as they’re a special condition connection to make a rigid or semi-rigid connection between a column and a beam. They’re more labor intensive to install so the result is slow-er production leading to increased erection costs.

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BEAM PENETRATIONS

Beam penetrations are cutouts made in the steel beams so that mechanical piping and ductwork can pass through. There will be unreinforced and reinforced beam penetra-tions depending on the size of the opening. Sometimes the architectural, structural, and mechanical drawings are co-ordinated enough to allow the Engineer to show on the plans where a portion of these beam penetrations may occur. The beam penetrations shown on the plans that can be fabri-cated in the shop will have a much lower cost than the beam penetrations that will have to be provided for in the field. There will inevitably be beam penetrations that will need to be cutout and reinforced in the field at a higher unit cost.

4. OVERVIEW OF LABOR, MATERIAL, EQUIPMENT ANDINDIRECT COSTS

The following example is intended to demonstrate a simple take-off and pricing method for a structural steel frame sys-tem including beams, columns, decking, shear studs, bent plate (pour stop), and connections.

With today’s technology many companies perform take-off using OST and digitizers. OST allows you to take-off quantities on your computer screen while color coding the element you’re taking off and recording up to three (3) units of measure for that element at the same time.

However, for the purposes of this paper I will complete the take-off and pricing sheets on Excel to demonstrate the method being used.

MATERIAL

Take-off the lengths of each different size beam and col-umn. Next I’ll multiply the length of each beam and col-umn by its respective weight. From the sum of all these beams and columns I’ll then calculate the total tonnage. The shear studs will be counted for each beam and a to-tal tallied. The perimeter of the floor plates will be calcu-lated in order to figure the length of pour stop. The area of the floors will be calculated to determine the deck-ing quantity. And lastly, the connections will be figured by calculating percentages of the total steel tonnage.

LABOR

Since the steel is fabricated in the shop the labor por-tion refers to the erection crew. Generally, it’s assumed that for erection a crane can pick between 35 and 60 pieces a day. Assuming the average is 45 picks per day, and using common sizes of beams and columns that would probably equate to around 30 tons erected per day.

EQUIPMENT

The equipment utilized will be mobile cranes or tower cranes. Tower cranes will be predominantly utilized on high buildings in tight city quarters.

INDIRECT COSTS

Tower cranes also require concrete pad foundations to be set on and also require electrical power service. Safety costs for perimeter cabling needs to be considered along with other temporary construction costs to provide a secure and safe working environment.

Other indirect costs to consider are sales tax, payment and performance bonds and liability insurance.

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Office Building Parameters

L W P AStructure Floor Length (lf) Width (lf) Perimeter (lf) Area (sf)SOG 1st Floor 90 90 360 8,100SOMD 2nd Floor 90 90 360 8,100SOMD 3rd Floor 90 90 360 8,100SOMD 4th Floor 90 90 360 8,100

Total Interior Building SF: 32,400

SOMD Roof 90 90 360 8,100

Steel Columns Take-Off

Floor-to-Floor Height is 12'0"

Floor # of Col. Size Unit Wt. (plf) Height (vf) Len (lf) Total Wt. Un Total Wt. Un2nd Floor 16 W14x82 82 12 192 15,744 LB 7.87 TN3rd Floor 16 W14x61 61 12 192 11,712 LB 5.86 TN4th Floor 16 W14x61 61 12 192 11,712 LB 5.86 TNRoof 16 W14x53 53 12 192 10,176 LB 5.09 TN

Total Weight: 49,344 LB 24.67 TN

Steel Beams Take-Off

Typical for 2nd floor through the roof (4 levels)

2nd floor

Qty Size Length Unit Wt. Total Wt. (lbs.) Total Wt. (tns.)

8 W24x55 30 55 13,200 6.604 W36x135 30 135 16,200 8.108 W21x44 30 44 10,560 5.288 W8x10 10 10 800 0.4022 W16x31 30 31 20,460 10.23

Subtotal Weight: 61,220 30.61

3rd floor

Qty Size Length Unit Wt. Total Wt. (lbs.) Total Wt. (tns.)

8 W24x55 30 55 13,200 6.604 W36x135 30 135 16,200 8.108 W21x44 30 44 10,560 5.288 W8x10 10 10 800 0.4022 W16x31 30 31 20,460 10.23

Subtotal Weight: 61,220 30.61

4th floor

Qty Size Length Unit Wt. Total Wt. (lbs.) Total Wt. (tns.)

8 W24x55 30 55 13,200 6.604 W36x135 30 135 16,200 8.108 W21x44 30 44 10,560 5.288 W8x10 10 10 800 0.4022 W16x31 30 31 20,460 10.23

Subtotal Weight: 61,220 30.61

Roof

Qty Size Length Unit Wt. Total Wt. (lbs.) Total Wt. (tns.)

8 W24x55 30 55 13,200 6.604 W36x135 30 135 16,200 8.108 W21x44 30 44 10,560 5.288 W8x10 10 10 800 0.4022 W16x31 30 31 20,460 10.23

Subtotal Weight: 61,220 30.61

Total Weight: 244,880 122.44

Bent Plate Take-Off

Assume 6"x6"x7/16" bent plate at 18 pounds per linear foot (plf).

Floor Perimeter Un Unit Wt. Un Total Wt. Un Total Wt. Un2nd Floor SOMD 360 LF 18.00 PLF 6,480 LB 3.24 TN3rd Floor SOMD 360 LF 18.00 PLF 6,480 LB 3.24 TN4th Floor SOMD 360 LF 18.00 PLF 6,480 LB 3.24 TNRoof SOMD 360 LF 18.00 PLF 6,480 LB 3.24 TN

Total Weight: 25,920 LB 12.96 TN

Structural Steel Connections Take-Off

Subtotal of Steel Tonnage 160.07 TN

Steel Tonnage Min. % Max. % Min. Wt. Un Max. Wt. Un Total Wt. UnBase Plates 2.00% 3.00% 3.20 TN 4.80 TN 9,604 LBColumn Splices 4.00% 5.00% 6.40 TN 8.00 TN 16,007 LBMiscellaneous Details 4.00% 5.00% 6.40 TN 8.00 TN 16,007 LB

Total Weight: 20.81 TN 41,618 LB

Steel Deck

L W AFloor Length (lf) Width (lf) Area (sf) Waste (10%) Total SF2nd Floor 90.00 90.00 8,100 810 8,9103rd Floor 90.00 90.00 8,100 810 8,9104th Floor 90.00 90.00 8,100 810 8,910

Total Steel Floor Deck: 26,730 SF

Roof 90.00 90.00 8,100 810 8,910

Total Steel Roof Deck: 8,910 SF

Total Steel Decking: 35,640 SF

4- Story Office Building Estimate (32,400 GSF)

GSF 32,400

Item Description Qty Un Mat Un Pr Mat Total Lab Un Pr Lab Tot Equip Un Pr Equip Total Total Cost $/SF

1. Structural Steel Beams & Columns 147.11 tons 2,200.00$ 323,642$ 900.00$ 132,399$ 200.00$ 29,422.00$ 485,463$ 14.98$ 2. Bent Plate at Pour Stops 12.96 tons 2,200.00$ 28,512$ 1,100.00$ 14,256$ -$ -$ 42,768$ 1.32$ 3. Connection Steel 20.81 tons 3,350.00$ 69,714$ 1,650.00$ 34,337$ -$ -$ 104,050$ 3.21$ 4. Shear Studs - 3/4"x5" 5,124 each 2.68$ 13,732$ 1.32$ 6,764$ -$ -$ 20,496$ 0.63$ 5. Steel Floor Deck - 3"x18 Gauge 26,730 sf 2.18$ 58,271$ 1.07$ 28,601$ -$ -$ 86,873$ 2.68$ 6. Steel Roof Deck - 3"x20 Gauge 8,910 sf 2.01$ 17,909$ 0.99$ 8,821$ -$ -$ 26,730$ 0.83$

Direct Construction Cost 511,780$ 225,177$ 29,422$ 766,380$ 23.65$

Overhead & Profit 20.00% 153,276$ 4.73$

Total Construction Cost 919,655$ 28.38$

5. SPECIAL RISK CONSIDERATIONS

When estimating the cost of structural steel there is signifi-cant risk to mitigate. These risk items include the following:

MATERIAL COST

The cost of structural steel is increasing, as there’s a huge global appetite for steel currently underway. Contributing to this escalation is also the fact that steelmakers are pay-ing large premiums for raw materials from mining com-panies and passing this cost onto the consumer. Raw ma-terial costs have recently contributed to 50% of the steel cost where a short time ago they traditionally accounted for roughly 15% of the steel material cost. Steelmakers are now trying to acquire mines to provide raw materials for their own companies in order to be more self-sufficient.

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ESCALATION

Escalation is a significant cost factor when putting a steel bid together. Your project may not require the steel to be on-site for many months to a year, if not longer. Many steel contractors will not even try to guess what steel prices may be at that time because of price volatility. In order to level the playing field for the steel contractors during a bid you may have to forecast as best you can the projected future steel cost and carry an escalation allowance. This strategy should be discussed should be discussed with the Owner.

SURCHARGES

Surcharges are added to the steel material costs because the material cost is volatile and it gives the steel supplier more flexibility to lower and raise his price. This practice eliminates the need to constantly adjust a fixed price so that you can appear to remain competitive.

FUEL/ENERGY COSTS

The cost of a barrel of oil is hovering around $140/barrel and the average cost of gasoline in the U.S. right now is about $4.00/gallon. Fuel and energy costs are making steel more expensive to produce and ship steel.

VALUE OF THE DOLLAR

The value of U.S. dollar has been declining steadily for sev-eral years now and is playing a large part in the high cost of exported steel to the U.S. Steel is exported at a higher cost to the U.S. because of the weak dollar. And many foreign companies buying U.S. steelmakers are exporting to their own country because of strong demand instead of selling to the U.S. Therefore, we’re left still paying higher prices because many U.S. steel companies are foreign owned.

LEAD TIME OF MATERIAL

If the steel sections required for your project are not in stock then it becomes more critical to get your shop drawings ap-proved in a timely manner to secure a place on the mill fab-rication schedule. Depending on how many rolling jobs are ahead of you this lead-time will have to be taken into con-sideration relative to your construction schedule.

6 . RAT IOS AND ANALYS IS

There are a few ways to analyze whether your bid or esti-mate is in the “ballpark” and reflects whether or not you are providing a reasonable cost. One way is to look at historical data from similar projects and escalate those previous proj-ect costs to current day costs. Using past project costs can be a good benchmark to gauge your bid or estimate.

Another way is to use conceptual design information that has been acquired through experience with putting to-gether conceptual estimates and/or interaction with struc-tural engineers. For the office building example used, I

will take the total steel tonnage and divide by the total building square footage (assuming the ground is a slab-on-grade) and expect to get a unit weight of between 11 psf and 15 psf. If my tonnage is not in this range I will investigate further to determine why the structure is seemingly light or heavy. Having a structure with inadequate steel strength is one issue and having a structure with excessive weight may be inefficiency in beam usage leading to an unnecessarily higher cost.

In regards to shear studs I would also divide the total building square footage by the total quantity of shear studs and expect this number to be in the range of 6-8 sf per stud (or 0.13 – 0.17 studs per sf). If not, I’ll make adjustments to make sure that I carry enough shear studs in the estimate.The following logic chart below is a sample calculation used to check the validity of the estimate. I have shown how to complete a logic check on the structural steel beams, col-umns, bent plate, and connections to see if the unit weight of the structural steel system is in the projected range. I have also shown how to complete a logic check on the shear studs to make sure there are enough for the composite slab-on-deck system. I have performed a quick check on the structural steel erection cost to see if it is in-line with my estimate. As you will the erection estimate at $161,821 is close to what I calculated in the estimate at $176,077. This again is a logic check to make sure that the numbers in the estimate are a reasonable cost for the scope of work.

Steel Logic Check

WeightColumns 49,344 LBBeams 244,880 LBBent Plate 25,920 LBConnections 41,619 LB

Total Weight 361,763 LB

Building (SF) 32,400 SF

Unit Weight (PSF) 11.2 PSF

Okay: The unit weight of steel for the structure fallsin the range of 11-15 psf at 11.2 psf. Since this is anoffice building I would expect it to be closer to 11#.

Shear Stud Logic Check

Shear Studs 5,124 EA

Building (SF) 32,400 SF

Square Feet Per Stud 6.3 SF

Okay: The quantity of shear studs falls in the rangeof one (1) stud per 6-8 sf of floor area. We're at one (1)stud per 6.3 sf of floor area for the structure.

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7. MISCELLANEOUS PERTINENT INFORMATION

Recently, in order to evaluate the construction sequencing of erecting structural steel BIM modeling has been utilized. BIM stands for Building Information Modeling and is used to create a computer model of the building from the design drawings. This computer model is capable of detecting dimen-sional busts as well clashes with other building elements. It also has shown weaknesses in erection sequencing that can lead to a more efficient construction schedule. Another in-teresting development is the use of barcodes that are placed on the fabricated steel in the shop, scanned when shipped, scanned when delivered to the site, and scanned when erected in place. This real-time scanning process allows the BIM model to show the Construction Manager and Client when the steel has been fabricated, shipped, delivered to the site and erected. The computer model then displays that actual to-date work put in place showing a time sequence.

GLOSSARY

A36 – ASTM Steel DesignationAESS – Architecturally Exposed Structural SteelOST – On-Screen Take-offSOG – Slab on gradeSOMD – Slab on metal deckBIM – Building Information ModelingMoment Connection – A rigid or semi-rigid column tobeam connectionShear Stud – A metal stud welded to the top of a beamin a composite slab on deckIntumescent – Referring to fire retardant paint on architecturally exposed steelCamber – An arch fabricated in the beam so that designed loads acting on the beam limit the deflection ofthe beam. The intent is to have a level floor.

REFERENCES

Means Estimating HandbookAISC Manual of Steel Construction

Structural Steel Erection Cost

Item Qty UnSteel Columns 48 Each

Steel Beams 200 Each

Total # of Beams & Columns 248 Each

Total Weight of Beams & Columns 147.11 TonsAverage Weight of Steel Member 0.59 Tons

If Pick 30 Tons Per Day Production = 51 Pieces

Duration of Steel Erection 5 Days

Crew Unit Cost ($/Ton) 1,100.00$

Crew Daily Cost (30 Tons Per Day) 33,000$

Total Steel Erection Cost 161,821$

Check Cost From Estimate 176,077$