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Chapter 1: Buildings

STEP 1-E: RISK ASSESSMENT - Added Risks and Benefits

 

Building-specific Load Path Analysis

Load Path - The lateral earthquake forces in a building are caused by the acceleration of the buildings weights when the ground moves. There must be a continuous path for these inertial forces to be cared from the ground to the building weight locations. A gap in this transmission path will result in failure of the structure at that location. The most direct load paths are lateral force resisting members along a continuous vertical path from the foundation to the roof.

Unique problems - many of which are identified in below - can best be identified by undertaking a load path analysis to determine that all seismically induced inertial forces from the mass of the building are conveyed from all parts of the superstructure to the foundation.

 

Specialized building-specific assets and shortcomings

("Modifiers-"Performance Modification Factors-PMF's)

1. ADDED RISKS

a. Building height: (NOTE: this could be an asset or a shortcoming based on whether height is sufficient to reduce seismic stresses below the design loads for wind, or either reduce or increase them because of unlikely or likely resonance with predicted ground motions.)

  • i. Possible soil/building resonance based on building's period (see Local Geology Screen above.)

b. Poor original construction (as it might effect seismic performance).

c. Poor condition - [LINK TO 178-pA-4 checklist]

d. Vertical Irregularities: "Defined in terms of discontinuities of strength, stiffness, geometry and mass.

Vertical Irregularity

 

Defined in FEMA 368 Section 5.2.3.3 and Table 5.2.3.3.  The classes are

1a soft story, [same as your description]

1b extreme soft story, [Not included in your list. Lateral stiffness is less than 60% of that in the story above, or less than 70% of the average stiffness of the three stories above]

2 weight irregularity, [same as your description]

3 vertical geometric irregularity [same as your description]

4 in-plane discontinuity in vertical lateral-force-resisting elements [different than your vertical discontinuity description]

5 weak story [expanded from your description]

 

FEMA 368 does not include short columns or non-structural elements as Vertical Irregularities.

 

  • 1)  Soft story (stiffness discontinuity)

Exists in a story where the lateral stiffness is less than 70% of that in the story above or less than 80% of the average stiffness of the three stories above.

  • 2)  Weak story (strength discontinuity)

Exists in a story where the strength is less than 80% of that in the story above.

  • 3)  Geometric Irregularities

Exists where the horizontal dimension of the lateral-force-resisting system is more than 130% of that in an adjacent story. (Penthouses excluded).

  • 4)  Mass Irregularity

Exists where the effective mass of any story is more than 150% of the affective mass of the adjacent story. (A lighter roof mass excepted).

  • 5)  Vertical Discontinuities

Exists where shearwalls or rigid infill walls or frame elements are not continuous to the foundations, thus threatening to impart large overturning forces onto columns.

  • 6)  Short columns (captured columns)

Exist where partial height walls prevent structural columns from bending across their floor to floor height, concentrating the story drift onto a reduced length portion of the column leading to shear failure.

e. Horizontal Irregularities

  • i. Torsion

Significant torsion exists where the distance between the story center of rigidity, and the story center of mass is greater than 20% of the width of the structure in either major plan dimension.

  • 2. re-entrant corners-need for tensile capacity at all re-entrant corners

  • Horizontal Irregularities

    Defined in FEMA 368 Section 5.2.3.2 Plan Irregularity and Table 5.2.3.2. The classes are
    1a - Torsional Irregularity for systems - diaphragms not flexible. "Torsional irregularity shall be considered to exist when the maximum story drift, computed including accidental torsion, at one end of the structure transverse to an axis is more than 1.2 times the average of the story drifts at the two ends of the structure."
    1b - Extreme Torsional Irregularity - diaphragms not flexible. "Extreme torsional irregularity shall be considered to exist when the maximum story drift, computed including accidental torsion, at one end of the structure transverse to an axis is more than 1.4 times the average of the story drifts at the two ends of the structure."
    2 - Re-entrant Corners - both projections of the structure beyond a re-entrant corner are greater than 15 percent of the plan dimension of the structure in the given direction.
    3 - Diaphragm Discontinuity - cutout or open areas greater than 50 percent of the gross enclosed diaphragm area or changes in effective diaphragm stiffness of more than 50 percent from one story to the next
    4 - Out-of-Plane Offsets - offset of vertical lateral-force resisting elements
    5 - Nonparallel Systems - not parallel or symmetric about the major orthogonal axes

    Adjacencies / pounding are not included in the FEMA 368 horizontal irregularities categories.

    Note: FEMA 369 provides a Commentary that describes the reasons for requiring increased force level designs for buildings with these irregularities.

g.  Non-structural elements: Large heavy cladding, un-braced parapets, and other falling hazards.

f. Adjacencies: Pounding risk

A structure is considered to be "immediately adjacent," and thus at risk for pounding if it is within 2 inches times the number of stories away from the building being evaluated.

1. ADDED BENEFITS

a. Redundancy: "Building will remain stable after failure of any one element."

b. Mid-block location (party walls)

c. Prior upgrade work undertaken already.

Return to RISK ASSESSMENT list

 

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