DIVISION 94

 VOLUNTARY – EARTHQUAKE HAZARD REDUCTION IN EXISTING HILLSIDE BUILDINGS

(Division 94 Added by Ord. No. 171,258, Eff. 8/30/96.)

 SEC. 91.9401. PURPOSE.

 (Amended by Ord. No. 172,592, Eff. 6/28/99, Oper. 7/1/99.)

 The purpose of this division is to promote public safety and welfare by reducing the risk of death or injury that may result from the effects of earthquakes on existing hillside buildings constructed on or into slopes in excess of one unit vertical in three units horizontal (33.3% slope). Such buildings have been recognized as life hazardous as a result of partial or complete collapse that occurred during the Northridge Earthquake.

The provisions of this division are minimum standards for structural systems established primarily to reduce the risk of loss of life or injury resulting from earthquakes and will not necessarily prevent loss of life or injury or prevent earthquake damage to an existing building which complies with these standards.

This division provides voluntary retrofit standards under which buildings shall be permitted to be structurally analyzed and retrofitted. When fully followed, these standards will strengthen the portion of the structure that is most vulnerable to earthquake damage.

SEC. 91.9402. SCOPE.

(Added by Ord. No. 171,258, Eff. 8/30/96.)

 The  provisions  of  this  division  may  be  applied  to  all   existing   hillside   buildings   designed under building codes effective prior to June 19, 1995. If only a portion of  the building is supported on or into the slope, these regulations may be applied  to the  entire building.

Seismic retrofit work as described in this division may be applied to the portion of the structure defined as the base-level-diaphragm  and  below.  Non-habitable  accessory buildings, decks not supporting the main building, and existing conditions above the base-level-diaphragm are exempt from these regulations.

Seismic strengthening constructed prior to the effective date of the ordinance may  be evaluated in accordance with the provisions of this division.

SEC. 91.9403. DEFINITIONS.

For the purpose of this division, in addition to the applicable definitions, symbols and notations in this Code, certain additional terms are defined as follows:

BASE (BASE LEVEL) is  the  level  at  which  the  earthquake  motions are considered to be imparted to the structure or the level at which the structure as a dynamic vibrator is supported.

BASE-LEVEL-DIAPHRAGM is the floor at, or closest to, the top of the highest level of the foundation.

DIAPHRAGM ANCHORS are assemblies that connect a diaphragm to the adjacent foundation at the uphill diaphragm edge.

DIAPHRAGM BACKSPAN is the horizontal cantilevered distance parallel to the direction of the lateral force, between the outermost vertical lateral force resisting elements and the edge  of the diaphragm.

DOWNHILL-DIRECTION is the descending direction of the slope approximately perpendicular to the slope contours.

FOUNDATION is the concrete or masonry which supports a building, including footings, stem walls, retaining walls, and grade beams.

FOUNDATION EXTENDING IN THE DOWNHILL-DIRECTION is a descending foundation and approximately perpendicular to the uphill foundation.

FOUNDATION, UPHILL is a foundation parallel and closest to the uphill diaphragm edge, as defined herein.

HILLSIDE BUILDING is any wood frame building or portion thereof constructed on or into a slope steeper than one unit vertical in three units horizontal (33.3% slope).

PRIMARY ANCHORS are diaphragm anchors designed for and providing direct connection as described in Sections 91.9406.2 and 91.9406.3 between the diaphragm and the uphill foundation.

RETROFIT is an improvement of the lateral force resisting system of the structure by an alteration of existing or addition of new structural elements.

SECONDARY ANCHORS are diaphragm anchors designed for and providing a redundant diaphragm to foundation connection, as described in Section 91.9406.4.

UPHILL DIAPHRAGM EDGE is the edge of the diaphragm adjacent to or closest to the highest ground level at the perimeter of the diaphragm.

SEC. 91.9404. GENERAL REQUIREMENTS.

(Added by Ord. No. 171,258, Eff. 8/30/96.)

 Except as modified herein, the analysis and design of the work within the scope of this division shall be in accordance with Division 16 of this Code.

SEC. 91.9405. PRE-DESIGN INVESTIGATION.

(Added by Ord. No. 171,258, Eff. 8/30/96.)

 The engineer or architect shall prepare a  pre-design field  investigation  report  in  accordance with Department  guidelines  and  shall  file  the  report  when  the  plans  are  filed for permit.  The  plans  shall  include  a  description  of  the  existing  lateral  force  resisting system at and below the base.

SEC. 91.9406. ANALYSIS AND DESIGN.

(Added by Ord. No. 171,258, Eff. 8/30/96.)

 91.9406.1. General. Every hillside building within the scope of  this  division  shall  be  analyzed, designed, and constructed in accordance with the following provisions.

91.9406.1.1.   Base for Seismic Design Defined.   The base for seismic design is defined   as follows:

  1. Downhill-Direction. For seismic forces  acting  in  the  downhill-  direction,  the base of the building shall be the floor at or closest to the top of the highest level of the
  2. Normal to the Downhill-Direction. For seismic forces acting normal to the downhill-direction, the distribution of  seismic  forces  over  the  height  of  the  building  using U.B.C. Section 1628.4, shall be determined  using  the  height  measured  from  the top  of  the  lowest  level   of  the  building      Retrofitting,   however,  shall  only be required at the base-level-diaphragm and below.

91.9406.1.2. Design Base Shear.  The  design  base  shear  shall  be that  required  at the time of the original building permit, but not less than V = 0.133 W.

91.9406.2. Base Shear Resistance in the Downhill-Direction.

 91.9406.2.1.  General.  The  base  shear  in  the  downhill-direction,  including  forces  from the  base-level-diaphragm, shall  be  resisted  through  primary  anchors  from  diaphragm  struts or collectors provided in the base level framing to the foundation.

91.9406.2.2. Location of Primary Anchors. A primary anchor and diaphragm strut or collector  shall  be  provided  in  line  with  each  foundation  extending  in   the downhill-direction. Primary anchors and diaphragm struts or collectors shall also be

provided where interior vertical lateral force resisting  elements  occur above and  in  contact with the base-level-diaphragm.

The base-level-diaphragm shall be provided with primary  anchors  designed  for  the tributary forces spaced at a maximum 30  feet  (9145  mm)  on  center.  Where  the  floor  below the base extends to the uphill foundation, and the foundation at  the  base  is  not  adequate to resist the forces  from  the  primary  anchors,  the  base  shear  may  be transferred to that lower level and the primary  anchorage  made  at  that  lower  level. The  connection shall be made directly to the foundation. The existing foundation shall  be  evaluated  as  specified in Section 91.9406.11.

91.9406.3. Seismic Forces on Floor Levels Below the Base in  the  Downhill-  Direction.

 91.9406.3.1.  General.  All   floor  diaphragm  connections   between  floor  diaphragms   below the base level  diaphragm  shall  be  designed  in  accordance  with  the  provisions  of  this section.

91.9406.3.2. Design. Each floor  level  below  the  base  defined  in  Section  91.9406.1.1 shall be designed for all tributary loads  at that level using a minimum seismic force factor not less than the base shear coefficient.

91.9406.3.3. Direct Connections. (Each floor level shall be directly connected to the foundation through a system of primary anchors at that level as required for the base as specified in Section 91.9406.1.1.

91.9406.4. Secondary Anchors  from  Diaphragm  to  Foundation  for  Seismic  Forces at and Below the Base in the Downhill-Direction.

 91.9406.4.1. General. In addition to the anchors required by Sections 91.9406.2 and 91.9406.3, the floor diaphragm for levels at and below the base shall  be anchored  to the uphill foundation at the level of the diaphragm, as specified in this section.

EXCEPTION:

Secondary anchors are not required where:

  1. the concrete or masonry foundations in the downhill-direction are spaced at not more than 30 feet (9145  mm) on center  and extend  up to and are directly  connected  to the base-level-diaphragm for at least 70 percent of the diaphragm depth; or
  2. the diaphragm is separated from the mudsill at  the  uphill  foundation  by  a  cripple wall which has anchor bolts and is braced in the plane  of  the  wall  and constructed with studs that are no less  than  12  inches  (305  mm)  in  height  and  primary anchors are spaced a maximum of 20 feet (6096 mm) on center; or
  3. the deflection of the plywood floor diaphragm  between  adjacent  primary anchors is calculated to be less than 1/4 of an inch (6.4 mm).

91.9406.4.2. Diaphragm Anchors. Secondary anchors required by this section shall be provided at each level at and below the base of the building.   Diaphragm  anchors shall be  fully developed into the diaphragm and be connected to  the  foundation  at  the  uphill diaphragm edge to develop the forces required by this section.

91.9406.4.3. Anchor Spacing. Secondary anchors required by this section shall be uniformly distributed along the uphill diaphragm edge and  shall  be  spaced  a  maximum  of  four feet (1219 mm) on center.

91.9406.4.4. Anchor Capacity for Floor Diaphragms at the Base and Below. Secondary anchors at the base and below shall be designed for a uniformly distributed  minimum force equal to the  total  primary  anchorage  design  force  at  that  level  divided  by the length of the uphill diaphragm edge, but  shall  not  be  less  than  300  pounds  per  lineal foot (4.37 kN/m). The existing foundation need not  be  checked  to  resist  the  additional  forces  induced  by  the  system  of  secondary  anchors;  however,  the  existing  foundation  shall be evaluated as specified in Section 91.9406.11.

91.9406.5. Design of Anchorage.

 91.9406.5.1. General. Primary and secondary anchors, and  diaphragm struts  and collectors, shall be designed in accordance with the provisions of this section.

91.9406.5.2.  Anchorage.  The  structure  shall  be anchored  to the  foundation  as specified in Sections 91.9406.2, 91.9406.3 and 91.9406.4.

91.9406.5.3. Fasteners. All bolted fasteners used to develop connections to wood members shall be provided with square plate  washers  at all bolt  heads  and nuts.  Washers shall be minimum 3/16 inch (4.8 mm) thick  and  two  inch  (51  mm)  square  for  1/2-inch (12.7 mm) diameter  bolts, and 1/4 inch (6.4 mm) thick and 2-1/2 inch (64 mm) square for     5/8 inch (15.9 mm) diameter or larger bolts. Nuts  shall  be  wrench  tightened  prior  to covering.

91.9406.5.4. Fastening. The  diaphragm  to  foundation  anchorage  shall  not  be accomplished by the use of  toe  nailing,  nails subject to  withdrawal,  or  wood  in  cross  grain bending or cross grain tension.

91.9406.5.5. Size of Wood Members. Wood  diaphragm  struts,  collectors,  and  other wood members connected to primary anchors shall not be less than three inch  (76  mm) nominal width members or doubled two inch  (51  mm)  nominal  width  members.  Doubled  two inch (51 mm) nominal width members shall be fastened together according to  the  provisions of Division 23 of this  Code.  Secondary  diaphragm  anchors  as  specified  in  Section 91.9406.4 may be developed through existing two inch (51 mm) nominal  width  framing members. The effects of eccentricity on wood members shall be  evaluated  as  required per Section 91.9406.5.9.

91.9406.5.6. Design. Primary and  secondary  anchorage,  including  diaphragm  struts,  splices, and collectors shall be designed for 125 percent of the tributary force.

91.9406.5.7. Allowable Stress Increase. (Amended by Ord. No. 172,592, Eff. 6/28/99, Oper. 7/1/99.) The one-third allowable stress increase permitted under C.B.C. Section 1612.3 or 2316.2 shall not  be permitted for   materials  using  allowable  stress  design methods.

91.9406.5.8. Seismic Load Factor. (Amended by Ord. No.  172,592,  Eff.  6/28/99,  Oper. 7/1/99.) Steel elements of the diaphragm anchorage systems and continuity ties shall be designed by the allowable stress design method using a load factor  of  1.7.  The strength design specified in C.B.C. Section 1923.2, using a load factor of 2.0 in lieu of

  • for earthquake loading shall be used for the design of embedment in

 

91.9406.5.9. Symmetry.  All  seismic  lateral  force  foundation  anchorage  and  diaphragm strut connections shall be symmetrical. Eccentric connections may be permitted when demonstrated by calculation or tests  that  all  components  of  force  have  been  provided  for in the structural analysis or tests.

 

91.9406.5.10. Load Path.

 

91.9406.5.10.1. Primary Anchors. The load path for primary anchors shall be fully developed into the  diaphragm  and  into  the  foundation.  The  foundation  must  be  shown  to be adequate to resist  the  concentrated  loads  from  the  primary  anchors and must be shown to comply with the following:

 

  1. Soil maximum  bearing  capacity  for  conventional  footings  shall  be   limited   to  3000 psf (143.7 kPa) unless an approved geotechnical report permits higher  bearing

 

  1. Conventional continuous footings shall be analyzed for uplift forces induced by primary

 

  1. Soil capacities need not be investigated for grade beam and caisson or pile

 

  1. Shear stress in grade beams and tie beams shall be investigated for vertical component of primary anchor forces. Unless otherwise known, a maximum 2000 psi (13.8 MPa) concrete strength shall be

 

91.9406.5.10.2. Secondary Anchors. The load path for secondary anchors need not be developed beyond the connection to the foundation.

 

91.9406.5.10.3. Above  Base  Uplift  Forces.  The load path for uplift forces generated  from above the base shall be analyzed and fully developed into the below base structural system.

 

 

91.9406.6. Base Shear Resistance Normal to the Downhill-Direction.

 

91.9406.6.1. General. Lateral force resisting elements acting in the normal to the downhill-direction shall be designed in accordance with  the  requirements  of  following Sections.

 

91.9406.6.2. Base Shear. In developing the base shear for seismic design, the structural system factor (Rw) shall not exceed six for bearing wall and building frame systems.

 

91.9406.6.3.  Vertical  Distribution  of  Seismic  Forces.  The  distribution   of  seismic forces acting normal to the downhill-direction shall be determined  using  U.B.C.  Section 1628.4 C.B.C. Section 1630.5. The height of the structure in Equation (28-8) of U.B.C. Section 1628.4 Equation (30-15) of C.B.C. Section  1630.5 shall be taken from the base  which shall be measured from the top of the lowest level of the building foundation.

 

91.9406.6.4.  Drift  Limitations.  The  interstory  drift  below  the  base-level-  diaphragm  shall not  exceed  0.005  times  the  story  height.  The  total  drift  from  the base-level-diaphragm to the top of the foundation shall  not  exceed 3/4  inch  (19  mm).  Where the story height or the height from the base-level-diaphragm to the top  of  the  foundation varies because of a stepped footing or  story offset, the height shall be measured from the average height of  the  top  of  the  foundation.  The  calculated story drift shall not be reduced by the effect of horizontal diaphragm stiffness.

 

91.9406.7. Lateral Force Resisting Systems at the Base and Below in the Downhill-Direction.

 

91.9406.7.1. General. As an alternative to providing primary anchor connections from diaphragms to foundation in the downhill-direction, the following systems may be used, provided their location and spacing is maintained as specified in Section 91.9406.2.2  for primary anchors.

 

91.9406.7.2. Wood Shear Walls. Wood structural panels or  existing  wood  diagonal sheathed shear walls may be used provided:

 

  1. The minimum length of shear wall shall be eight feet (2438 mm).

 

  1. The minimum level length between steps in the shear wall sill shall  be  eight  feet (2438 mm) and  the  maximum  step  height  between  adjacent  sills  shall  be  two  feet  eight inches (813 mm).

 

  1. Sill plates do not slope and they bear on a level

 

  1. The design  lateral  forces  shall  be  distributed  to  lateral  force  resisting  elements   of varying heights in accordance with the  stiffness  of  each  individual    The stiffness of a stepped wood structural panel shear wall may be determined by dividing

 

the wall into adjacent rectangular elements, subject to the same top of wall deflection. Deflections of shear walls  may be estimated  by U.B.C.  Standard  23-2 Section 23.223  or other equivalent  methods.  Sheathing  and  fastening  requirements  for  the  stiffest  section shall be used for the  entire  wall.  Each  section  of  wall  shall  be  anchored  for shear and uplift at each step as an independent shear wall.

 

  1. Actual configuration of steps shall be determined in the field at  the  time  of pre-design

 

  1. The drift limitations of Section 9406.6.4 are not exceeded.

 

91.9406.7.3.  Braced  Frames.  Structural steel  braced frames   with   concentric connections may be used as part  of  the  lateral  force  resisting  system.  All  members  in  braced frames shall be designed to resist tension  and  compression  forces.  Seismic  forces shall not induce flexural stresses  in  any  member  of  the  frame,  in  diaphragm  struts,  or  in  the collectors. Where existing anchor  bolts  are  used  for  anchorage,  existing  confinement shall be verified and additional confinement provided where  necessary.  When  the  braced frame is not rectangular,  distribution  of  forces  to  members  shall  account  for  the  variations in slope.

 

91.9406.7.4.  Rod-Braced  Frames  and  Diaphragms.  Existing tension only braces may be used provided they resist five times the design force, and  the  connections  have  the  capacity to resist the yield  strength  of  the  braces. Tension braces and  their  connections shall be exposed  for  evaluation.  Existing  anchor  bolts  shall  be tested  in shear  and  tension to five times the design force.

 

91.9406.7.5. Cement Plaster and Lath and  Gypsum Wallboard. The sheathing materials listed in Section 91.2513 of this Code are not permitted to  resist  seismic  lateral forces below the base-level-diaphragm.

 

91.9406.8. Lateral Force Resisting Systems at the Base and Below and Normal to  the Downhill-Direction. Lateral force resisting systems acting  normal  to  the downhill-direction may include steel moment frames and those systems  permitted  under  Section 91.9406.7, provided the drift limitations of Section 91.9406.6 are not exceeded.

 

91.9406.9. Diaphragms.

 

91.9406.9.1.  General.  Diaphragms  at the base   and  below  may  be of straight  one- inch by six-inch (25 mm by 152 mm) or two-inch by six-inch (51 mm by 152 mm) sheathing, provided vertical  lateral  force  resisting  elements  in  the  downhill-direction  or  primary anchors are spaced no more than 20 feet (6096 mm) apart and the diaphragm shear forces     do not exceed 100 plf (1.46 kN/m).

 

91.9406.9.2. Existing Diaphragms.  Existing plywood and diagonally sheathed diaphragms need not be investigated.

 

91.9406.9.3.  Existing  Cantilevered Diaphragms. Existing cantilevered wood diaphragms are acceptable provided they do not cantilever more than one-half the diaphragm backspan (anchor span).

 

91.9406.9.4. Wood Diaphragm Rotation. Diaphragm rotation is not permitted in resisting lateral forces.

 

91.9406.10. Steel Beam to Column Connections.

 

91.9406.10.1. General. All  steel beam  to  column connections  shall  be  braced at supports and locations of concentrated loads. The beam to column connection shall be  designed to prevent rotation of the beam.

91.9406.10.2.   Steel Beams.   Steel beams shall have stiffener plates installed on each side   of the beam web at the column  supports  and  points  of  concentrated  load. The  stiffener plates shall be welded  to  each  beam  flange  and  beam  web.  This  requirement  applies  at the base and below and only to those connections which  are  part  of  the  lateral  load resisting system or lateral load path.

 

91.9406.10.3. Column Bracing. All single length multi-level height columns shall be braced in each orthogonal direction at each diaphragm level.

 

91.9406.11. Foundations.

 

91.9406.11.1. Existing Foundations. Foundation soundness shall be  verified  by  the  engineer or architect.  Foundation types such as  unreinforced masonry, stone and ungrouted concrete block and unreinforced  concrete shall  be  retrofitted to  resist  lateral  loads applied through the diaphragm anchors.

 

91.9406.11.2. Damaged Foundations. Damaged foundations shall be evaluated by the engineer or architect. Cracks  in  excess  of  1/8  inch  (3.2  mm)  or  differential  displacement in excess of 1/4 inch (6.4 mm) shall be further investigated and repaired where necessary. Specifications for the restoration of the earth to wood separation shall be included  and be made a part of the plans.

 

91.9406.11.3. Stud Wall Attachment. Shot pinned anchors  shall  not  be used  to resist lateral forces.  Lateral  force  resisting  systems  which  utilize  shot  pins  shall  be  retrofitted  with approved drilled anchors.

 

91.9406.11.4. Existing Framing  Connections. Deteriorated framing and connections shall be repaired or replaced.

 

91.9406.11.5. Metal Connectors. Metal connectors  shall  not  be  in  contact  with,  or  below earth unless the connectors are hot  dipped  galvanized  and  further  protected  from  earth with four inches of concrete.

 

91.9406.12. Existing Materials.

 

 

91.9406.12.1. Allowable Stresses. Existing materials may be used as part of the lateral load-resisting system  provided that  the  stresses  in  these materials  do  not  exceed  the values shown in Table 94-A.

 

SEC. 91.9407. HISTORICAL BUILDINGS.

(Amended by Ord. No. 172,592, Eff. 6/28/99, Oper. 7/1/99.)

 

Qualified Historical Buildings shall be allowed to use alternate building regulations or deviations from this division in order to preserve their original  or  restored  architectural elements and features. See Section 91.8119 for these standards.

 

SEC. 91.9408. QUALITY CONTROL.

(Added by Ord. No. 171,258, Eff. 8/30/96.)

 

91.9408.1.  General.  All  hillside  building   construction   shall   comply   with   the requirements specified in this section.

 

91.9408.2. Department Called Inspections. (Amended by Ord. No. 172,592, Eff. 6/28/99, Oper. 7/1/99.) All anchors installed in accordance with Sections 91.9406.2,  91.9406.3 and 91.9406.4 shall be inspected by the Department prior to installation of any construction which might  restrict  access  to  the  anchors  or  prevent  a  visual  inspection  from the floor level above the anchors.

 

91.9408.3. Structural Observation by the Engineer or Architect of Record. The owner shall employ the engineer or architect of record, or other engineer or architect designated by the engineer or architect of record, to perform structural observations  as required by Section 91.1702.

 

91.9408.4. Anchor Installation. No installed anchor shall be  covered prior to all required Department framing inspections and structural observation by the  architect  or  engineer.

 

SEC. 91.9409. INFORMATION REQUIRED ON PLANS.

(Added by Ord. No. 171,258, Eff. 8/30/96.)

 

91.9409.1. General. The  licensed  engineer  or  architect  responsible  for  the  seismic  analysis of the building  shall  record  the  information  required  by  this  section  on  the approved plans. The  plans  shall  accurately  reflect  the  results  of  the  engineering  investigation and design and show all pertinent dimensions and sizes for plan review and construction.  The plans   shall   show   existing   framing   construction,   diaphragm  construction, proposed primary,  alternate  and  secondary  anchors,  proposed  shear  walls  and collectors for the base and below. All structural elements that are part of the design including existing nailing, anchors, ties, and  collectors,  shall  be  shown  on  the  plans.  The plans shall indicate  existing  construction  that  has  not  been  exposed  and  needs  verification  at the time of construction.

 

 

91.9409.2. Building  Elevations.  Elevations  showing  the  existing  conditions  shall  be  drawn to scale. Elevations shall show roof  and  floor  heights,  dimensions  of  openings,  location and extent of existing damage, and proposed repair and strengthening.

 

91.9409.3. Shear Walls. Plans shall include all information  pertinent  to  shear  walls, including typical wall panel thickness, length, and the location and size of all anchors.

 

91.9409.4.  Details.  Details  shall  include  the  existing  lateral  bracing  systems   to  be utilized including work required for the lateral  and  vertical  load  systems  and  new  anchors and the method of development of anchor forces into the diaphragm framing.

 

91.9409.5. Engineer’s or Architect’s Statement.  The responsible  engineer  or architect shall state on the approved plans the following:

 

“I am responsible for designing the strengthening  for  this  building’s  base  level  and below  in  compliance  with  the  minimum  regulations  of  Division  94  of   the   Los Angeles Building Code; and either I or someone under my responsible charge has performed the pre-design investigation.”

 

or when applicable:

 

  1. “The Registered Deputy Inspector, required as a condition of the use of structural design stresses requiring continuous inspection, will be responsible to me as required by Section 91.1701.1 of the Los Angeles Building Code.”

 

TABLE 94-A

 

VALUES FOR EXISTING MATERIALS

 

Existing Materials or Configuration of Materials 1 Allowable Values
1. Plain or reinforced concrete footings f c = 2000 psi (13.8 MPa) unless otherwise shown by tests.
2. Douglas fir wood Allowable stress same as No. 2 D.F.
3. Reinforcing steel fs = 0.4 Fy, maximum 16 ksi (110 MPa).
4. Structural steel fb = 0.6 Fy, maximum 22 ksi (152 MPa).
5. Anchor bolts Current code values.
6. Wood structural panels/diagonal sheathing Current code values.

1 Material must be sound and in good condition.