Arch 435 Lecture 9: Lateral Force Resisting Systems Notes

Stability

  • Every building is a cantilevered beam.
  • Lateral loads on buildings are caused by:
    • Earthquakes
    • Wind

Vertical Lateral Force Resisting Systems (VLFRS)

  • Stability is achieved through:
    • Shear in Shear Walls
    • Tension & Compression in Braced Frames
    • Moment in Moment Frames

Shear Walls

  • Types:
    • Homogeneous Shear Walls
    • Stick Framed Shear Walls
    • Panelized Shear Walls
Homogeneous Shear Walls
  • Systems:
    • Site-Cast Concrete
    • Masonry
  • Shear is resisted by the cementitious material.
  • Moment resistance is similar to a concrete beam.
    • The cementitious material resists compression.
    • Steel reinforcement resists tension.
  • Ductility is achieved through steel reinforcement.
Stick-Framed Shear Walls
  • Systems include light framed wood and cold-formed metal framing.
  • Shear is resisted by sheathing.
  • Moment is decoupled:
    • Compression is resisted by studs, which act as columns.
    • Tension is resisted by hold-downs.
  • Ductility is achieved through hold-downs.
Panelized Shear Walls
  • Systems include precast concrete and CLT (Cross-Laminated Timber) panels.
  • Shear and moment are distributed similarly to homogeneous walls.
  • Steel connections create continuity between panels.
  • Ductility may be achieved through connections or through steel reinforcement.
  • Moment may be decoupled within each panel or over the entire wall.

Braced Frames

  • Systems include steel, and rarely wood or concrete if earthquake forces are negligible (SDC A).
  • Braces are horizontal trusses.
  • Story shear is converted through axial force.
  • Moment is decoupled and resisted through tension and compression in columns.
  • Ductility is achieved through yielding and buckling of braces.

Moment Frames

  • Systems include steel and concrete.
  • Shear is resisted by columns within the frame.
  • Moment-fixed connections between beams and columns provide stability.
  • Moment is decoupled and resisted by columns.
  • Ductility is achieved through yielding of beam flanges near the moment connections.

Hybrid Systems

  • Eccentric Braced Frames: Combining Braced and Moment Frames.
  • Coupled Shear Walls: Combining Shear Walls and Moment Frames.
  • Stacked Systems: Code allows for greater building heights of “soft” systems by stacking them on top of stiffer lateral systems.
  • Mixed Systems

Ductility of VLFRS

  • For wind loads, a design-level 3-second gust is only meant to occur once.
    • Ductility is important to control building failures but not to dissipate load.
  • For seismic loads, ductility not only protects against catastrophic structural failures but also decreases the design-level earthquake load by dissipating energy.
  • The ductility of each system is quantified in a lateral system’s Response Modification Factor (R).
  • In the equation used to calculate seismic loads, the R-value is inversely related to the magnitude of the acceleration factor and thus the seismic load.
  • RR values are provided by code for most systems.
  • Systems not included in this list may not be used except for buildings with SDC A.

Lateral Load Path

  • A building is just a cantilevered beam turned on its side.
  • A lateral system is just a gravity system turned on its side.
  • For steel gravity systems:
    • Load enters the system through a deck.
    • Deck load is supported by framing.
    • Supports (columns) transfer framing load to the ground.
  • The same is true for lateral systems. There is typically:
    • A mechanism for load to enter the system.
    • A system that uses flexure or truss behavior to transfer loads - a diaphragm.
    • Supports (VLFRS) that transfer the load to the ground.

Diaphragms

  • A building’s floor/roof system is also typically its diaphragm.
  • Like other beams, diaphragms must:
    • Resist Flexure
    • Resist Shear
    • Have Supports

Diaphragm Shear

  • In the analogy of a wide-flange beam, a floor’s deck or slab acts as the web.
  • When a deck or slab cannot provide diaphragm shear strength (there is no deck/slab, it is too weak, etc.), a horizontal truss may be used in place.

Diaphragm Chords

  • In the analogy of a wide-flange beam, diaphragm chords act as the flanges.
  • Moment is resisted by the force-couple provided by chords on opposite sides of the diaphragm.

Diaphragm Collectors

  • In the analogy of a wide-flange beam, the VLFRS and collectors act as supports.
  • Collectors “drag” load from the diaphragm into the VLFRS.

Diaphragm Stiffness

  • In reality, all diaphragms are somewhat continuous, and thus, indeterminate.
  • Designers choose from 1 of 3 options when distributing lateral load to VLFRS from a diaphragm:
    • Rationalize the diaphragm as infinitely flexible.
    • Rationalize the diaphragm as infinitely rigid.
    • Consider the VLFRS and the diaphragm’s stiffness and analyze the diaphragm-beam via calculation.
Flexible Diaphragms
  • Per code, diaphragms may be rationalized as “flexible” if they are significantly less stiff compared to a building’s VLFRS.
  • In general, “decks” (wood sheathing, steel deck) may be assumed to be flexible regardless of the VLRFS.
  • Flexible diaphragms assign load by tributary area.
Rigid Diaphragms
  • Per code, diaphragms may be rationalized as “rigid” if they are significantly more stiff compared to a building’s VLFRS.
  • In general, “slabs” (cast-in-place concrete) are typically assumed to be rigid regardless of the VLRFS.
  • Rigid diaphragms assign load to VLFRS based on their relative stiffness (in this case, we use the term “relative rigidity”).
Rigid Diaphragm Torsion
  • Because there is no “give” to a rigid diaphragm, the force application will be separate from the geometric center of the floor.
  • The distance between the center of force and the center of stiffness creates an eccentricity.
  • The moment due to this eccentricity is called “Inherent Torsion.”
Semi-Rigid Diaphragms
  • When neither flexible distribution nor rigid distribution are appropriate, engineers analyze the diaphragm as a continuous beam and the VLFRS as spring-supports.
  • This is the most accurate approach but also the most arduous.

Load Collection Into Diaphragms

Wind Loads
  • An Area Load is distributed across the facade of the building.
  • The structural members of a facade distribute the load vertically to diaphragms above and below.
    • Glazing: Primary mullions run up and down
    • Stick Framing: Studs span up and down
    • Homogeneous Walls: typically span 1-way up and down due to proportion
Seismic Loads
  • Most mass in the building is already on the floor plate.
  • In the case of an earthquake, the floor mass directly acts as a line load on the diaphragm.
  • Building facades are also dead load and thus carry momentum in the case of an earthquake.
  • The seismic force due to the mass of the facade distributes vertically into diaphragms - similar to wind load.

Lateral System Irregularities

  • The code has defined several “irregularities” that trigger special restrictions in seismic lateral resisting systems.
  • These irregularities have the same mechanical issues under wind loads and should be avoided by designers when possible.
Vertical Irregularities
  • Soft Story
  • Mass Irregularity
  • Vertical Geometric Irregularity
  • In Plane Discontinuity
  • Weak Story
Horizontal Irregularities
  • Horizontal Offset
  • Reentrant Corners
  • Torsional Irregularity
  • Diaphragm Discontinuity
  • Nonparallel Systems