Chapter 1: Buildings

MITIGATION MEASURES

Text and graphics by FEMA Region X

I.  Typical Damages

When strong earthquake shaking occurs, a building is thrown mostly from side to side, and also up and down. That is, while the ground is violently moving from side to side, taking the building foundation with it, the building structure tends to stay at rest, similar to a passenger standing on a bus that accelerates quickly. Once the building starts moving, it tends to continue in the same direction, but the ground moves back in the opposite direction (as if the bus driver first accelerated quickly, then suddenly braked). Thus the building gets thrown back and forth by the motion of the ground, with some parts of the building lagging behind the foundation movement, and then moving in the opposite direction. 

The force F that an upper floor level or roof level of the building should successfully resist is related to its mass m and its acceleration a, according to Newton’s law, F = ma. The heavier the building the more the force is exerted. Therefore, a tall, heavy, reinforced concrete building will be subject to more force than a lightweight, one-story, wood-frame house, given the same acceleration.

Damage can be due either to structural members (beams and columns) being overloaded or differential movements between different parts of the structure. If the structure is sufficiently strong to resist these forces or differential movements, little damage will result. If the structure cannot resist these forces or differential movements, structural members will be damaged, and collapse may occur.

Building damage is related to the characteristics of the building, and the duration and severity of the ground shaking. Larger earthquakes tend to shake longer and harder and therefore cause more damage to structures. Earthquakes with Richter magnitudes less than 5 rarely cause significant damage to buildings, since acceleration levels (except when the site is on the fault) are relatively small and the durations of shaking for these earthquakes are relatively short. In addition to damage caused by ground shaking, damage can be caused by buildings pounding against one another, ground failure that causes the degradation of the building foundation, landslides, fires and tidal waves (tsunamis). Most of these “indirect” forms of damage are not addressed in this Handbook.  (from FEMA 154, p133)

II.  Examples of Mitigation Measures for protection of life and reduction in damage.

The following mitigation measures were described and illustrated by FEMA Region X for publication in the printed version of this Handbook.  The have here been reorganized under the specific building type to which they relate.  Some illustrations may relate to several types, so they may not be representative of the specific building type discussed.  This list of measures is not comprehensive, and may be expanded as more material is placed on this site.  Also, they are meant only to be representative of the kinds of measures that have been proven to be effective, rather than provide details on how the measure could be implemented.

General Structural Mitigation Measures:  

• SOFT/WEAK STORY (1st floor): 
  • Reinforce Building With Steel Moment Frames
  • Increase Lateral Support by Infilling Openings

• FLEXIBLE DIAPHRAGMS: 

  • Protect Walls by Stiffening Floors

• POUNDING: 

  • Create Separation Joints Between Structures  

• BASE ISOLATION & MECHANICAL DAMPING:
  • Base-Isolate Building With Vibration Isolation Bearings
  • Install Dampers in Superstructure

• CARBON-FIBER REINFORCEMENT:

  • Reinforce Walls With Fiber Materials

  • Confine Columns With Fiber Wraps

General Enclosure and Interior Element Mitigation Measures:

• HEAVY ARCHITECTURAL EXTERIOR ELEMENTS
  • Mitigate Danger of Falling Heavy Exterior Facade Elements
• WINDOWS
  • Protect windows that are vulnerable to breakage
• PARAPETS: 
  • Provide tie backs for parapets and heavy exterior architectural elements.
• CHIMNEYS:
  • Brace or Support Chimneys

• INTERIOR PARTITIONS

  • Ensure stability of interior partitions
• CEILINGS AND LIGHTS
  • Ensure collapse prevention of suspended ceilings and lights
• RAISED COMPUTER FLOORS
  • Ensure stability of raised false floors

Wood Frame Structures (W).  

Well-designed wood structures have generally performed well in earthquakes. Failures are often due to lack of foundation anchorage or unbraced crawlspace (cripple) walls.

(W1) WOOD - LIGHT FRAME

• Anchor Sill Plate to Foundation
• Reinforce Cripple Walls

(W2) WOOD - HEAVY TIMBER (Commercial and Industrial Buildings).

• [NEW] Strengthen Connections between beams and columns
• Reinforce Building With Cross Bracing
• Protect Walls by Stiffening Floors
• Install Tension Ties Between Elements

Steel Frame Structures (S).  

These structures generally perform better than other structure types. Steel moment frame structures may have damage to primary members, distress at connections, and broken or buckled braces and brace connections. Excessive Movement between floor levels (story drift) can cause nonstructural damage.

GENERAL: ALL STEEL TYPES

• Reinforce Building With Cross Bracing

• Reinforce Building with Shear Walls

(S1) STEEL: MOMENT RESISTING FRAME

(S2) STEEL: BRACED FRAME

(S3) STEEL: LIGHT FRAME

• Reinforce Building With Cross Bracing

• Reinforce Building Using Shear Walls

• Protect Walls by Stiffening Floors

• Provide for clearance between rigid wall panels

(S4) STEEL: FRAME WITH REINFORCED CONCRETE SHEAR WALLS

(S5) STEEL: FRAME WITH UNREINFORCED MASONRY INFILL

• Reinforce Walls With Shotcrete

• Reinforce Walls With Fiber Materials

Reinforced Concrete Structures (C). Concrete structures may be cast-in-place and/or precast.

Cast-in-place Concrete Structures.  Most typical of reinforced concrete buildings, cast-in-place concrete structures can be damaged to the point of collapse, if the design or detailing is inadequate.  The most vulnerable buildings are those constructed as frame structures without shearwalls, and with minimal ductility in the beam/column intersections, and inadequate ties in the columns.  Usually such vulnerable buildings were constructed after architectural styles favored open office or shop plans with exterior light-weight metal and glass curtain walls, and before building codes [DO LINK TO BENCHMARK YEARS] were altered to require ductile detailing.

GENERAL:  ALL REINFORCED CONCRETE (RC) TYPES

• Reinforce Building With Cross Bracing

• Reinforce Building Using Shear Walls

• Reinforce Walls With Fiber Materials

• Confine Columns

(C1) RC: MOMENT-RESISTING FRAME

(C2) RC: FRAME WITH RC SHEAR WALLS

(C3) RC: FRAME WITH INFILL MASONRY WALLS

• Reinforce Walls With Shotcrete

• Reinforce Walls With Fiber Materials

Precast Concrete Structures. Precast structures also may experience damage in joints and connections. Older "non-ductile" pre-cast concrete structures are vulnerable to collapse because earthquake damage was concentrated in the intersections between the pre-cast parts.  A subset of precast concrete structures, tilt-up construction has in the past proven to be particularly vulnerable to earthquake damage.  This construction method usually involves casting concrete walls at the site and "tilting" them into place. The most common failure is wall-roof separation resulting from inadequate ties. Other problems are weak connections between individual wall panels, failure of diaphragms and exterior elements, and failure in panels with large openings.

(PC1) PRE-CAST RC: TILT-UP BUILDINGS with large flexible diaphragms.

• [not yet published] Improve ties and connections

• [not yet published] Protect Walls by Stiffening Roof

• Reinforce Building With Cross Bracing

(PC2) PRE-CAST RC: PRE-CAST FRAME BUILDINGS 

• Reinforce Building With Cross Bracing

• Reinforce Building Using Shear Walls

• [not yet published] Improve connections

Masonry Structures (M). There are two kinds of masonry construction: unreinforced and reinforced. Unreinforced masonry structures, particularly bearing walls, are the form of construction most vulnerable to earthquake damages. Floors and walls of these structures are often not tied together or, when tied together, are only weakly connected. Some older structures have mortar that has deteriorated. Long, unreinforced masonry wall sections, unsupported by intersecting cross-walls, are particularly prone to severe cracking or failure due to the lack of bracing or reinforcing steel. Chimneys in older buildings are commonly damaged or destroyed, creating falling hazards.

GENERAL:  ALL MASONRY TYPES

• Reinforce Building With Cross Bracing
• Reinforce Building Using Shear Walls
• Protect Walls by Stiffening Floors
• Add Collectors to Strengthen Floors/Roofs
• Increase Lateral Support by Infilling Openings

• Reinforce Walls With Fiber Materials

(RM) REINFORCED MASONRY: WITH FLEXIBLE DIAPHRAGMS

(RM) REINFORCED MASONRY: WITH PRE-CAST RC DIAPHRAGMS

(URM) UNREINFORCED MASONRY

• Reinforce Walls With Shotcrete
• Reinforce Walls With Fiber Materials
• Brace Long Walls With Crosswalls
• Install Tension Ties Between Elements
• Install Shear Anchors
• [not yet published] Reinforce walls with center-core reinforcing.

 :

NOTE:  The "considerations" with the symbols that link to Appendix A: Regulations have not been included in this web version of this publication.  At this time, items such as cost, professional practice, and regulations are so often determined by the particular case, that the generalizations made by these symbols may not always be relevant.  It is recommended that Appendix A be reviewed for all mitigation measures.

Return to TOP