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Introduction

Introduction. Load Path Continuity ( not new! ) 1982 UBC Section 2303: “…shall result in a system that provides a complete load path capable of transferring all loads and forces from their point of origin to the load-resisting elements.” Load Path Design & Detailing Most Important!

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Introduction

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  1. Introduction • Load Path Continuity (not new!) • 1982 UBC Section 2303: “…shall result in a system that provides a complete load path capable of transferring all loads and forces from their point of origin to the load-resisting elements.” • Load Path Design & Detailing • Most Important! • Not Always Provided by Engineer • Not Always Enforced by Jurisdiction • Not understood by all players

  2. Quotes from the EERI White Paper Report: • “There is a lack of conceptual understanding of building performance in an earthquake.” • “The engineering profession is placing insufficient emphasis on developing education materials for and providing training to trades workers and inspectors” • “Encourage providers and producers of education materials that primarily target engineers to cooperate with existing providers serving the trades and inspectors.” • The goal is to improve the presentation of technical information to nontechnical audiences.”

  3. Sharing Common Goals • Building Owner • Architect/Engineer • Contractor/Trades Worker • Specialty Inspector • Building Department Staff (BO/PCE/BI) • Insurance Company • Our Common Goal: Protect the Public Who Occupy Buildings

  4. Pre-Quiz 1. What element provides support for the top of a wall which is subjected to out-of-plane wind or seismic loading? A[ ] The shear walls B[ ] The foundation C[ ] The horizontal diaphragm D[ ] The drag strut 2. What element carries and distributes the horizontal diaphragm shear to the shear walls? A[ ] The collector or drag strut B[ ] The shear walls C[ ] The subdiaphragm D[ ] The diaphragm chord 3. What element usually serves as the horizontal diaphragm chord in a typical single family dwelling with plywood shear walls? A[ ] The holdown device B[ ] The double top plates C[ ] The eave blocking D[ ] The sill anchor bolts 4. What elements transfer the in-plane shear (sliding) force of the shear wall to the shear wall footing? A[ ] The holdown device B[ ] The drag strut C[ ] The diaphragm chord D[ ] The sill anchor bolts

  5. Earthquake Basics • Pacific/N. American Plate • San Andreas Fault - Plate Boundary • 160 Known Active Faults • Ground Motion • Acceleration/Inertial Forces • EQ Strength • 1. Distance to Focus (hypocntr) • Shallow Stronger Shaking • 2. Magnitude Energy • Higher Magnitude Stronger • 3. Site Soil • Soft Soil Stronger Shaking

  6. California really has Earthquakes Date Fault Location Magnitude • 1940 Imperial 7.1 • 1947 Manix 6.4 • 1950 Fort Sage 5.6 • 1951 Superstition Hills 5.6 • 1952 White Wolf 7.7 • 1956 San Miguel 6.8 • 1966 Imperial 3.6 • 1966 San Andreas 5.5 • 1968 Coyote Creek 6.4 • 1971 San Fernando 6.6 • 1979 Imperial 6.6 • 1987 Whittier 6.1 • 1989 Loma Prieta 7.1 • 1990 Upland 5.5 • 1992 Yucca Valley 7.4 • 1992 Cape Mendocino 7.0 • 1994 Northridge 6.6 Date Fault Location Magnitude • 1836 Hayward 7.0* • 1838 San Andreas 7.0* • 1852 Big Pine --- • 1857 San Andreas 8.0* • 1861 Calveras --- • 1861 San Andreas --- • 1868 Hayward 7.0* • 1872 Owens Valley 8.3* • 1890 San Andreas --- • 1899 San Jacinto 6.6* • 1906 San Andreas 8.3 • 1922 San Andreas 6.5 • 1925 Mesa/Santa Ynez 6.3 • 1927 Santa Ynez 7.5 • 1933 Newport-Inglewood 6.3 • 1934 San Andreas 6.0 • 1934 San Jacinto 7.1

  7. Building Response • EQ Force on Depends: • Type of Structural System • Dynamic Properties: Building Period • Stiffness / Mass / Height • Design Response < Ground Acceleration • Damping / Ductility / Overstrength • TBLDG ~ TEQ • Resonance Amplifies Forces • Stiff Systems • Shorter Periods Higher Forces • Stiffer Systems “Attract” More Force

  8. Building Response • Building Shape & Configuration • Regular Buildings • Uniform Force Distribution • More Predictable Response • Irregular Buildings • Force Concentrations • Less Predictable Response • Building Offsets • Vertical & Horizontal • Stiffness & Strength Variations • Weak Story • Soft Story

  9. Key Elements in the LFRS • Roof (horiz. diaphragm) • Floors (horiz. diaphragm) • Shear Walls (vert. diaphragm) • Foundation • Load Distributing Elements: • Get the load FROM the point of origin TO the resisting element: Complete Load Path • Details, details, details • Connections, connections, connections

  10. Wood Frame Buildings • Horizontal Diaphragm: A large thin deep beam loaded in its plane which spans between and distributes loads to the supporting shear walls. The horizontal diaphragm supports the out-of-plane walls, and distributes loads to the supporting shear walls.

  11. Wood Frame Buildings • Factors affecting the allowable load of wood structural panels horizontal diaphragms: • Blocked or unblocked • Sheet thickness • Nail size and spacing • Sheet layout pattern • Framing member size • Orientation of load • New Terminology: Wood Structural Panel defined in Section 2302 and means all structural panel products (UBC Std. 23-2 or 23-3) includes plywood, OSB, waferboard…etc…but NOT particleboard

  12. Horizontal Diaphragm Boundaries Boundaries Boundaries Boundary Interior shear wall Boundaries Boundaries Boundaries Diaphragm boundaries may not just occur at the perimeter of the diaphragm. Interior shear walls and drag members create diaphragm boundaries.

  13. Horizontal Diaphragm Nailing

  14. Sub Diaphragms • Subdiaphragm - a smaller diaphragm within the main horizontal diaphragm used to transmit anchorage forces (from out-of-plane wall loads) to the main diaphragm cross-ties (Section 1633.2.9). • Sub-Chord - the boundary member which serves as the sub-diaphragm chord member. Sub-ties carry wall anchorage forces to sub-chord. • Subdiaphragms are primarily used in buildings with masonry or concrete walls with plywood roof or floors to minimize the number of continuous ties between diaphragm chords required by Section 1633.2.9. subdiaphragms shown dashed main diaphragm main cross ties Maximum width-to-depth ratio of subdiaphragms is limited to 2 1/2:1

  15. Roof Diaphragm Nailing Diagram

  16. Diaphragm Chord / Beam Analogy • Simple Beam Moment Resistance: • Compression in the Top Fiber • Tension in the Bottom Fiber • Horizontal Diaphragm Moment Resistance: • Tension Chord • Compression Chord

  17. Diaphragm Chord / Beam Analogy Load Compression shearwall shearwall Tension Load Compressive Stress Support Support Tensile Stress

  18. Wood Frame Shear Walls • Shear Wall: A cantilevered vertical diaphragm which supportsthe horizontal diaphragm and distributeslateral loads to the foundation. • Bearing Walls are not necessarily shear walls • Shear Walls are not necessarily bearing walls • Shear Walls are: • “Engineered” Designed & Detailed • Shear Walls are NOT: • Braced Wall Panels or Alternate BWP’s

  19. Wood Frame Shear Walls • Important Shear Wall Properties • Strength • Resists Lateral Forces • Stiffness! • Resists Deflection • Limits Building Drift which Limits Damage • Shear Wall Deflection • Height-to-Width Ratio • Sheathing Thickness • Fastener Slip

  20. Wood Frame Shear Walls • Some specific requirements for wood structural panel shear wall diaphragms: • Must be fully blocked. No unblocked edge allowed in wood structural panel shear walls • The holdown device (if required) must be connected to the edge (chord) members of the shear wall. • The shear wall sheathing must be edge nailed to the edge member that is connected to the holdown device. • The shear wall sheathing must be edge nailed to the top and bottom (perimeter) members of the shear wall.

  21. Wood Frame Shear Walls • Forces Acting on Shear Walls: • Sliding Force in Plane of Wall (Shear) • Resisted by Anchorage to Sill or Foundation Plate • 1997 UBC • 3X foundation sill & 3X framing at abutting panel joints where edge nail spacing less than 6” o.c. • Overturning Forces (Moment) • Resisted by Holdown Anchor Devices • Resisted by dead load (weight) of footing

  22. Shear Wall Holdowns • Not all shear walls require holdowns • Small lateral loads & large dead loads (stable) • Long shear walls - long resisting moment arm • Proper Holdown Installation • Proper size: • Stud size, Holdown, Stud anchor, anchor bolt • Properly Located • NO countersunk stud bolts • NO shims at holdown

  23. Foundations - 1997 UBC Chapter 18 • Anchor Bolts (Section 1806.6) • Seismic Zone 3 requires 1/2” @ 6’ o.c. • Seismic Zone 4 requires 5/8” @ 6’ o.c. • 7” embedment into concrete • 7 bolt diameters from end of sill • Square Plate Washer (Section 1806.6.1) • 2” x 2” x 3/16” square plate washer required • Reinforcing (Section 1806.7) • Seismic Zone 3 & 4: • 1-#4 Continuous top and bottom • Monolithic slab on grade may have 1-#5

  24. Permissible Diaphragm Aspect Ratios • Wood structural panels and particleboard, nailed all edges: • Horizontal Diaphragms: 4:1 • Particleboard Not Permitted as Horizontal Diaphragm • Vertical Diaphragms: • 3.5:1 in Zone 3 • 2:1 in Zone 4 Wood structural panels and particleboard, blocking omitted at intermediate joints: • Horizontal Diaphragms: 4:1 • Particleboard Not Permitted as Horizontal Diaphragm • *Vertical Diaphragms: Unblocked Not Permitted

  25. Diaphragm Aspect Ratios - Table 23-II-G 10’ 8’ 9’ 2.28’ (Zone 3) 4.00’ (Zone 4) 2.57’ (Zone 3) 4.50’ (Zone 4) 2.86’ (Zone 3) 5.00’ (Zone 4) • Footnote 1 of Tables 23-II-I-1 & 23-II-I-2 requires that all panel edges be backed with 2x or wider framing. • Section. 2315.5.3 requires that, “Framing members or blocking shall be provided at the edges of all sheets in shear walls.”

  26. A Failure Mechanism • Question: What if only one shear transfer mechanism were omitted? What is the result? Suppose only one shear wall has an incomplete load path. What could happen under design loading? • The share of its load never arrives at the disconnected shear wall. • Since that shear wall is not supporting its share of load, its load is distributed to the remaining shear walls. • The remaining shear walls are subjected to more load than they were designed for • The remaining shear walls are then overloaded so they in turn fail. • The Result: The entire lateral force resisting system fails because of incomplete load-path to only one resisting element.

  27. Complete Load Path Complete load path means Complete shear transfer details

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