Seismic Design of Industrial Industrial Structures Craig Brinck, SE
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Published by ASCE
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General seismic design guidelines
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Tables for finding fundamental period of complex structures Recommendations on when to use dynamic analysis Available for purchase from ASCE’s website
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Document Highlight •
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Published in AIST A IST newsletter May May ‘07 The authors are experts in the industry Excellent reference for industrial structures in general Detailed recommendations for crane supporting structures Free download from AIST at: http://news.aist.org/newsletter/07_ may_282_298.pdf
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Document Highlight •
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Published in MSC October ‘13 Good aid for selecting appropriate bracing systems Available for free on AISC’s website
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Response Modification Factor, R •
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Accounts for system ductility Allows elastic analysis methods to be used to design inelastic systems Reduces seismic design forces R ≤ 3: Limited ductility – system is essentially elastic R > 3: System is inelastic – special detailing is required to ensure ductility
Elastic
Inelastic
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The Cost of Ductility “It is recognized that when the designer has the option to design a building to meet the AISC Specification with R=3, such a design will generally be more cost effective than the same structure designed in accordance with the AISC Seismic Provisions using a higher value of R. The extra fabrication, erection and inspection costs needed to achieve the high ductility commensurate with the higher R more than offset the additional steel tonnage required by the R=3 system.” -AISC Seismic Design Manual, 2 nd Edition “If the Seismic Provisions are required because of building type or usage, system choice, or because of owner preference, all parties should be aware of the cost involved. The requirements of the Seismic Provisions have been known to increase the structural steel cost by 30 to 40%.” - AISC Design Guide 29: Vertical Bracing Connections – Analysis and Design
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Response Modification Factor, R
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AISC Design Guide 29: Vertical Bracing Connections AISC Design Guide 29: Vertical Bracing Connections
Inelastic Steel Systems Provisions in AISC 341 that can be difficult to apply: •
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Field welding of connections Lateral bracing at points of expected inelasticity
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Corrosion protection for tube braces
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Special inspection requirements
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Loads based on member capacities – Overkill for LFRS’s with small tributary areas
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Terminology (per ASCE 7-10) Building: “Any structure whose intended use includes shelter of human occupants.” Nonbuilding Structure: “A structure, other than a building, constructed of a type included in Chapter 15 and within the limits of Section 15.1.1.” Nonbuilding Structure Similar to a Building: “A nonbuilding structure that is designed and constructed in a manner similar to buildings, will respond to strong ground motion in a fashion similar to buildings, and has a basic lateral and vertical seismic force-resisting system conforming to one of the types indicated in Tables 12.2-1 or 15.4-1.”
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Important Elastic Steel Systems Systems not Specifically Detailed for Seismic Resistance, Excluding Cantilever Column Systems •
Buildings (Chapter 12)
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R=3
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Restricted to Seismic Design Categories A-C
Ordinary Concentrically Braced Frames •
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No height limits •
Nonbuilding Structures (Chapter 15) R = 1.5 Permitted in all Seismic Design Categories No height limits
Ordinary Moment Frames
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Nonbuilding Structures (Chapter 15) R=1 Permitted in all Seismic Design Categories No height limits
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Seismic Weight, W •
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Dead load 20% of snow load above 30 psf (unless modified by the Utah Snow Load Study) Normal operating weight of permanent equipment •
Upset condition weight is unlikely during an earthquake
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Collateral load from piping, cable tray, chutework, etc.
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Weight of empty crane, parked in worst case position
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Seismic Accelerations (International Projects) •
UBC ‘97 provided seismic accelerations for countries outside the U.S.
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UBC accelerations were based on a 475 year event
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For international projects, 475 year quakes are often still used
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IBC accelerations are based on a 2500 year event
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USGS website now provides international values compatible with IBC
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Conversions from UBC accelerations to IBC can be roughly made •
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SDS ≈ 2.5Ca SD1 ≈ Cv
Sometimes international clients will give you PGA. PGA is not Ss
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Seismic Accelerations (International Projects) s n o i t a l u c l a C r a e h S e s a B
Seismic Coefficient, C s Nonbuilding Structures Similar to Buildings (Ch. 15)
Buildings (Ch. 12) •
≤ : ≤ For > : ≥ 0.044 ≥ 0.01 For ≥ 0.6: ≥ .5 For
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≤ : ≤ For > : ≥ 0.044 ≥ 0.01 For ≥ 0.6 : ≥ . For < 0.06 (Rigid) 0.3 For
Nonbuilding Structures Not Similar to Buildings (Ch. 15) •
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≤ : ≤ For > : ≥ 0.044 ≥ 0.03 For ≥ 0.6 : ≥ . For < 0.06 (Rigid) 0.3 For
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Fundamental Period - Buildings Approximate Equations for Buildings:
ℎ (Equation 12.8-7) 0.1 (Equation 12.8-8) 0.0019 ℎ (Equation 12.8-9)
Section 15.4.4: “Equations 12.8-7, 12.8-8, 12.8-9, and 12.8-10 shall not be used for determining the period of a nonbuilding structure.”
100 ℎ ℎ ℎ = 1+0.83
(Equation 12.8-10)
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Fundamental Period – Nonbuilding Structures •
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Modal/Eigenvalue Analysis Rayleigh Procedure:
= 2 = •
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Roark’s Formulas for Stress & Strain
Roarke’s Formulas for Stress & Strain (Simple Structures) Guidelines for Seismic Evaluation and Design of Petrochemical Facilities The Conservative Approach:
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ASCE Guidelines for Petrochemical FacilitiesASCE (2011) 7-10
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Nonbuilding Structures Supported by Other Structures (Case 1) •
Nonbuilding Portion: •
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Seismic Forces per Chapter 13 R value taken from Chapter 15 ap value taken from Chapter 13 (see tables w/ footnotes) Anchorage per Chapter 13
Support Structure: •
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Design per Chapter 12 or Chapter 15 (whichever applies) Include nonbuilding structure in seismic weight, W
ASCE Guidelines for Petrochemical Facilities (2011)
Nonbuilding Structures Supported by Other Structures (Case 2) •
Nonbuilding Portion: • • • •
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Seismic Forces per Chapter 13 R value taken from Chapter 15 ap value taken as 1.0 Anchorage per Chapter 13
Support Structure: •
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Design per Chapter 12 or Chapter 15 (whichever applies) Include nonbuilding structure in seismic weight, W Use R for the support structure ASCE Guidelines for Petrochemical Facilities (2011)
Nonbuilding Structures Supported by Other Structures (Case 3) •
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Both portions must be modeled together Use the lowest R value between the nonbuilding structure and the supporting structure Design both portions for the forces from the combined model
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Structural Irregularities
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Torsional Irregularity s e i t i r a l u g e r r I l a r u t c u r t S
2015 NEHRP Provisions
Soft Story Irregularity s e i t i r a l u g e r r I l a r u t c u r t S
2015 NEHRP Provisions
Mass Irregularity s e i t i r a l u g e r r I l a r u t c u r t S
2015 NEHRP Provisions
Geometric Irregularity s e i t i r a l u g e r r I l a r u t c u r t S
2015 NEHRP Provisions
When is Dynamic Analysis Required? •
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Provisions of ASCE 7, Ch. 12 are equally applicable to Ch. 15, and may not be stringent enough Mass irregularities
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Structures with heavy equipment on a flexible structure Coupled systems Torsional irregularities Soft stories Offset LFRS with one bay < 70% stiffness of adjacent bay Stacks & chimneys
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Irregular vertical vessels
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A Word of Caution s e i t i r a l u g e r r I l a r u t c u r t S
A Word of Caution s e i t i r a l u g e r r I l a r u t c u r t S
Orthogonal Effects •
100%/30% rule applies to: •
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Weak stories in SDC C or higher Columns/walls of intersecting lateral systems, in SDC D or higher, with axial force ≥ 20% of axial strength.
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Lateral systems systems frequently share a column in 2 directions
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Dynamic Analysis w/ Orthogonal Effects Effects •
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Software may give you the option of using SRSS to combine orthogonal effects rather than the 100%/30% rule. SRSS of orthogonal effects will cause all results to be positive (again). Check (+) and (-) SRSS combinations.
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Quality •
Foreign Work • • • • • •
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What materials are available? Are they ductile? Charpy V-Notch toughness of weld metal. How will items be inspected/tested? Explicitly call out important details on the drawings clearly. Poor welding is common in developing countries. Review the shop drawings thoroughly.
Special Inspections • • • •
Is the site remote? Are certified special inspectors available nearby? Will the local jurisdiction enforce special inspections? Keep it simple - avoid using components that require special inspection. Bolted connections preferred over welded connections.
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Structure Lifecycle •
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The structure always loses Plants make frequent upgrades and modifications. Braces often get in the way of new equipment. If you build it, they will hit it with a loader.
Redundancy is even more important for industrial structures.
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Redundancy, r •
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Members are frequently damaged by trucks or loaders Braces are often removed by plant personnel to make room for new equipment and walkways Redundant load paths are more important than ever for industrial structures
r = 1.0 is permitted for nonbuilding structures not similar to buildings
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Diaphragms (or Lack Thereof) • •
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Bar Grating PBR Panel & Standing Seam Roofs Checkered Plate (If Detailed Correctly)
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Horizontal Bracing Details
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Dowswell, Brice & Blain (2010)
Dowswell, Brice & Blain (2010)
Horizontal Bracing Details
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Dowswell, Brice & Blain (2010)
Dowswell, Brice & Blain (2010)
Horizontal Bracing Details
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Dowswell, Brice & Blain (2010)
Dowswell, Brice & Blain (2010)
Crane-Supporting Steel Structures
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Types of Crane Columns
Web-Plated Columns can be used too, although they are less common
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Unique Properties of Crane Buildings •
Mass & stiffness properties •
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The mass per unit volume is relatively low due to light framing systems and crane clearances. Heavy cranes create a mass irregularity, especially if the building frame is light. Crane buildings are usually large sway frames and tend to be very flexible compared to commercial/institutional structures of similar height. The upper limit on the fundamental period is probably not applicable here. The crane bridge itself could potentially act as a tie spanning between the columns.
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Members are typically sized for stiffness to control drift. Designing for inelastic yielding of such members becomes difficult. Slender members are commonly used in an effort to control drift. Using seismically compact members would affect the building’s cost much more than it would for a typical commercial/institutional building.
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Unique Properties of Crane Buildings •
Building geometry •
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High floor-to-roof heights and long roof spans. Height restrictions for OMF’s and OCBF’s are overly restrictive for crane structures considering how flexible they typically are.
Framing systems •
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Tributary areas to the lateral load system are usually very small (typically one bay width). Crane buildings often require truss moment frames due to the long roof spans. The stiffness of stepped, laced and battened columns changes abruptly at the crane elevation.
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Drift Limits Condition 10-year Wind Load or Crane Forces
Cab/Radio Operated Cranes
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Frame Drift < H/240 (AISC DG7) Drift @ T/Rail < H/400 < 2” (AIST)
Gravity Loads*
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Seismic Load
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Pendant Operated Cranes
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Frame Drift < H/100 (AISC DG7) Drift @ T/Rail < H/400 < 2” (AIST)
Rail Gauge Within +1” and -1/2”
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Rail Gauge Within +1” and -1/2”
0.025hx = H/40 (ASCE 7)
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0.025hx = H/40 (ASCE 7)
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*Reduction in Gravity Loads May be Permitted – See AIST Technical Report #13 and AISC Design Guide 7
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Selecting a Seismic System •
Special Truss Moment Frames (STMF, R = 7) •
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Limited to span lengths of 65 t Depth limited to 6 ft Special detailing requirements
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MacCrimmon & Kennedy (1997)
Selecting a Seismic System •
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Behavior is similar to a joist-girder moment frame per SJI Technical Digest 11 Strong beam/weak column behavior expected Ordinary Moment Frame System Max moment that can be delivered by the system = 1.1RyMp(column) System is limited to 1 story. Multiple bays are permitted SJI recommends designing chord splices and truss connections to column per Section 7 of the AISC Seismic Provisions for SDC D, E, or F, or R>3
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MacCrimmon & Kennedy (1997)
Traditional K Factors •
Fix the base •
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Stabilizes the columns (K = 2.0 vs K = 1.2) Cuts the moment in the columns down significantly Reduces sway significantly Anchor chairs are generally recommended Watch foundation overturning
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K-Factors for Frame Members
AISC 360-10 Commentary
Stepped Column K Factors •
Anderson-Woodward Equations •
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Fixed-pinned under crane loads Fixed-slider under wind & seismic s e n a r C
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K=1.0 for all members Stability issues are easy to spot
Anderson & Woodward (1972)
History of the AISC Interaction Equations 1936: Simple Interaction Check
P-d Effects Incorporated by “Moment Magnification” 1961: P-D Effects Still Ignored
Rewritten in Terms of Strength, 2005: But Where Did the Second-Order Effects Go?
What happens when we mix traditional linear analysis methods with new interaction equations?
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Practical Measures •
Even when using an OMF, the location of the column step becomes an obvious place for potential plastic hinging to occur. •
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Design this connection for the plastic moment of the upper column segment in high seismic areas. It may be wise to design column lacing/battens & roof trusses for overstrength (if using R=3).
Provide a stretch length in the anchor bolts by using anchor chairs. Don’t go cheap on the connections. Provide redundant load paths (e.g. the roof bracing connecting adjacent bays). If energy dissipation is needed soil-structure interaction could be considered.
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Bracing System Layout vs. Thermal Expansion
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Mueller (1965)
Mueller (1965)
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Pedestal Reinforcement
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Pedestal Reinforcement - Tension t n e e c r o f n i e R l a t s e d e P
ACI 318-08
ACI 318-08
Pedestal Reinforcement - Tension t n e e c r o f n i e R l a t s e d e P
PCA Notes on ACI 318-11
Pedestal Reinforcement - Shear t n e e c r o f n i e R l a t s e d e P
Pedestal Reinforcement - Shear t n e e c r o f n i e R l a t s e d e P
Pedestal Reinforcement - Shear t n e e c r o f n i e R l a t s e d e P
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Tank Seismic Loads
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Tank Seismic Loads Impulsive & Convective Weight 1 •
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Sum using SRSS – CQC may be required for closely spaced impulsive & convective periods As L/HL approaches zero, the load becomes fully impulsive, and acts at the liquid midheight Sum of impulsive & convective usually don’t add to exactly 100%
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0.7 ACI 350 Rect (I) ACI 350 Rect (C) ACI 350 Circ (I)
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W / c W &0.5 L
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ACI 350 Circ (C) API 650 Circ (I)
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Tank Seismic Loads
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Tank Anchorage (TanchorageTM) •
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Self-Anchored vs. Mechanically Anchored Anchor chairs with 8do stretch length required for SDC’s C, D, E & F (see ASCE 7-10 15.7.5) Do not include the weight of the liquid as ballast. References on circular bolt patterns: •
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“Tubular Steel Structures – Theory & Design” by M.S. TroitskyCircular bolt “Pressure Vessel Engineering Handbook”
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Cold Weather
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Cold Weather
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http://practicalmaintenance.net/?p=968 http://www.tms.org/pubs/journals/jom/9801/felkins-
Cold Weather •
ASTM A633 Grades A, C, D, & E
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Fy ranges from 42 ksi to 60 ksi depending on grade/thickness
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Suited for -50° F [-45° C]
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Sections built from plates
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S355NL outside the U.S.
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Limiting stress to some fraction of Fy – not recommended for seismic design
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Corrosion
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Atmospheric Corrosion Rates
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Atmospheric Corrosion Rates
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Mitigating Corrosion (Steel Structures) •
Corrosion Allowance • • •
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Painting • • • •
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Surfaces do not corrode uniformly The atmospheric corrosion rates presented do not include any factor of safety Commonly used for tanks and pressure vessels (depending on the content) Paint will need to be reapplied throughout the life of the structure Double angles are hard to paint between – intermediate spacers should be bolted Cannot paint inside pipes and tubes – cap and seal weld them Slip-critical connections – do not use bare steel faying surfaces
Galvanizing • •
Field welding requires cold galvanizing afterward – a process that creates toxic fumes Galvanized Bolts • •
A490 bolts cannot be galvanized Call for galvanized bolts in General Notes - bolts & nuts are an ass embly from a single manufacture
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Mitigating Corrosion (Steel Structures) •
“Weathering” Steels •
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Copper content inhibits oxidation A242 (Cor-Ten A) A588 (Cor-Ten B) Not for abrasive environments
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Albrecht, Hall (2003)
Mitigating Corrosion (Concrete Structures) •
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Higher compressive strength (f’c) Protective coatings Corrosion inhibitors Extra clear cover Epoxy coating FRP reinforcing bars Smaller bars at closer spacing to limit cracking Limit ‘z’ to 95 or 115 as you would for an environmental engineering structure:
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References •
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Guidelines for Seismic Evaluation and Design of Petrochemical Facilities (2nd Edition). (2011). Reston, VA: American Society of Civil Engineers. Industrial Building Design – Seismic Issues, AIST ASCE 7-10: Minimum Design Loads for Buildings and Other Structures. (2010). Reston, VA: American Society of Civil Engineers. NEHRP Recommended Seismic Provisions for New Building Structures (2015 Edition). (2015). Washington, D.C.: Building Seismic Safety Council. Technical Digest 11: Design of Lateral Load Resisting Frames Using Steel Joists and Joist Girders. (2007). Florence, SC: Steel Joist Institute. Rolfes, John A., & MacCrimmon, Robert A. (2007). Industrial Building Design – Seismic Issues. Iron & Steel Technology, May 2007. pp. 282-298. Walter, Robert J. (2013). Bracing for Nonbuilding Structures Similar to Buildings. Modern Steel Construction, October 2013. Dowswell, Bo, & Brice, Allen, & Blain, Brian. (2010). Horizontal Bracing. Modern Steel Construction, July 2010.
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