GUIDELINES FOR SEISMIC RETROFIT
OF EXISTING BUILDINGS
Chapter 1. Seismic strengthening Provisions
for Un-reinforced Masonry Bearing Wall
Presented by Doc X. Nghiem
June 15, 2002
Since the conception of LA Division 88 (ASD ELF
Method) in 1981, and LA RGA-87 (ASD version of
ABK for Zone 4), there have been many changes
to the code for new buildings:
n Seismic load has increased for soil effect and
proximity of faults.
n Strength design has been introduced with the
n The need for URM retrofit need has spread to other
zones other than zone 4.
n Experience was gained from recent earthquakes .
Ch.1 of 2001 GSREB has been updated to include the
above changes. It calls for the selection of 1 of 2
n The General Procedure (GP), based on a strength
design ELF method of IBC 2000, using 75% of
new buildings seismic load.
n The Special Procedure (SP), a limit state design
method based on the original ABK research on
dynamic behavior of URM buildings with flexible
Sec.106. Materials Requirements
n Table 1-D - Strength Values for
ExistingMaterials or configuration of Materials.
n Table 1-E – Strength Values of New Materials
used in Conjunction with Existing Construction.
n The materials values were obtained from
combined static and dynamic test data,
research, and engineering judgment.
Existing Un-reinforced Masonry
Lay-up of solid multi-wythe brick masonry:
n Bond with header bricks is required between
wythes of solid brick walls.
n Wythe not bonded with headers is veneer.
Veneer attachment shall be investigated.
n Veneer is not included in the thickness of
the wall for h/t calculations.
Lay up of other masonry:
n Running bond for hollow concrete or clay
Sec 106.3.3 Testing of Masonry
n Mortar tests: In-Place Shear Test.
n In-Place shear test of mortar bed joint of the
n Record Vtest at first movement of test brick.
n Record collar joint % coverage.
n Record test location for calculation of
Alternative tests of Masonry
n 1. Tensile splitting strength of 8” diam
n An intersection of mortar head and bed joints
shall be at the center of the core.
n Tensile splitting strength is determined per
ASTM C 476:
fsp = 2P/pa n
Alternative Tests of Masonry
n 2. Tensile splitting strength for hollow unit
masonry with through the wall units:
n Sawn 18” square sample tested with diagonal in
the vertical position
n Tensile splitting strength per ASTM E519:
fsp = 0.494P/a n
Location of Tests
n Representative of mortar condition
throughout the building. ( Class, Weather..)
n Determined by the engineer.
n Recorded by the testing agency.
Minimum quality of Mortar
n Mortar shear test value for each test:
vto = (V test/A b) – PD + L
n Mortar shear strength v t is the value exceeded
by 80% of v to . (Mean - s)
n URM walls with v t < 30 psi are not acceptable.
Repointing and retest, or replacement is
Minimum quality of Masonry
n Minimum average value of tensile splitting
strength f sp= 50 psi
n Repoint and retest, or replace if < 50 psi
Sec 108. Design Strengths
n Refer to Table 1-D and Table 1-E for values for
existing material and combinations of existing
and new materials.
n Strengths not specified in this chapter or the
Building Code may be as specified by NEHRP
n Capacity Reduction factors are not used.
n New materials not specified shall be
substantiated by research test data.
Masonry shear Strength
n Determined from in-place shear test:
vm = 0.56 v t + 0.75 P D/A <= 100 psi
n Determined from tensile splitting tests:
v m = 0.8 fsp + 0.5 P D/A
Other Design Strengths
n Masonry compression: 300 psi
n Masonry tension : 0
n Foundation strength without geotech
n New total DL may be increased over existing
DL by 25%.
n New DL & LL may be increased over existing
DL & LL by 50%.
Sec 109 Analysis And Design
n 1. Elements required to be analyzed:
Refer to Table 1-A.
n 2. Selection of Procedure:
n General Procedure (GP) is used to analyze buildings
with rigid diaphragms and flexible diaphragms.
n GP is merely the ELF method of the current code.
n One deviation: Rocking of masonry piers is allowed
Analysis and Design (cont.)
n Special Procedure (SP) is used to analyze
buildings with flexible diaphragms when
applicable. SP is based on dynamic testing of
diaphragms and URM walls responses.
n There is no redistribution of calculated base
shear similar to Building Code.
n Instead, SP uses concept of building element
response.The element response shear is an
upper bound or yield capacity, such as
diaphragm v u.
Analysis and Design (cont.)
n The summation of the response of the
elements will constitutes a base shear.
n When the diaphragm is yielding, large
deflections may result in instability of out-of-
plane walls. Cross-walls are used to reduce
the diaphragm deflection.
n In flexible diaphragms that stay near elastic,
yielding cross-walls also dampen the
diaphragm response and allow higher h/t
ratios for out-of-plane walls.
GP vs SP
Sec 110. General Procedure
n Sec. 110.1 Seismic loading is determined
at 75% of Building Code:
V = 0.75 SDS / R
S DS from IBC = 2.5 C a of UBC
S D1 from IBC = Cv of UBC
R = 1.5 from IBC for plain masonry
n Base shear is distributed according to the
n Sec.110.2. Elements of Structures:
n Diaphragms and their connection to shear walls
shall be analyzed and designed per the current
n Sec.110.3. Stability of out-of-plane walls:
URM walls with h/t ratios < set forth in Table
1-B need not be analyzed for out-of-plane
n Sec.110.5. Redundancy factor = 1, and vertical
component of earthquake force E v=0.
n Sec.110.4. In-plane analysis of URM walls is
according to Sec 112 - same as SP- using the
code distributed story shear.
Sec 111. Special Procedure
n Limits for the application of S.P.:
n Flexible diaphragms at all levels above base
n Vertical lateral force resisting system
consisting predominantly of masonry or
concrete shear walls.
n Minimum 2 lines of vertical lateral force
resistance parallel to each axis of the
n Lateral Forces on Elements of Structures:
n Same as GP, 110.2 to 110.5, except:
n Allowable h/t is determined by dynamic behavior
of diaphragm (zone 1,2 or 3 of Fig.1-1).
n Diaphragm deflection control provisions of 111.4
n Sec 111.3. Cross-walls Definition:
n Wood framed with materials of Table 1-D&E.
n Designed to yield.
n Couple diaphragms together and to ground.
n Dampen the diaphragm response, hence reduce
diaphragm amplification effects.
n Spaced no more than 40.' Minimum capacity
n Full story height and placed in each story of the
n 1. Cross-walls need not be provided at all levels if used
only to couple the roof to the diaphragm below.
n 2. Cross-walls need not be continuous to grade under
first floor crawl space if:
n Diaphragm is anchored to foundations for shear and
n There are minimum cross-walls coupling the
diaphragm to the foundations.
n DCR of the diaphragm sections within cross-walls do
not exceed 2.5.
n 111.3.2 Cross-wall Capacity:
n Sum of cross-walls shear capacities within any 40
ft of the diaphragm span > = 30% of the
stronger diaphragm shear capacity.
n Existing Cross-walls
n h/D of 1.5 to 1.
n Existing connections need not be checked if
cross-wall extends from floor to floor.
n New Cross-walls
n Connections to diaphragms to develop
cross- wall capacity.
n Design for OTM. OTM need not be cumulative
for more than 2 stories.
n Steel moment frame may be cross-wall.
111.4. Wood Diaphragms.
n Calculate DCR for each
diaphragm level, starting
from the roof level
n Diaphragms are acceptable
if the point DCR-Span is
located within the curve of
Diaphragm Displacement Control
n Demand Capacity Ratios calculations :
n 1. Any diaphragm w/o X-walls immediately
above and below:
DCR= 2.1 S D1 W d / 2vuD
n 2. Roof of one story with X-walls or roof coupled
with X-walls to diaphragm below:
DCR= 2.1 S D1 W d / (Sv uD + V cb )
n 3. Multistory building with x-walls at all
DCR= 2.1 SD1SW d / (SSvuD + V cb)
n 4. Roof and diaphragm immediately
below if coupled by x-walls:
DCR= 2.1 SD1SW d / SvuD
n Diaphragm chords need not be checked.
n Collectors need be designed.
n Diaphragm openings reduces the depth D if within
the end quarter of diaphragm span.
111.5. Diaphragm Edge Shear
Transfer to Shear Walls
n Shear connectors at each edge of a diaphragm is
designed for the lesser of:
V = 1.2 S D1 C p W d
C p from Table 1-C is dependent
on diaphragm make up.
V = vu D
111.6. Shear Wall in-Plane Loading
n 1. Wall Story Force at any diaphragm level:
F wx = 0.8 S D1 (W wx + W d/2)
Not to exceed:
F wx = 0.8 S D1 W wx + v uD
n 2. Wall Story Shear:
V wx = S F wx
n Shear wall analysis per Sec.112, same as for GP.
n Moment frames used in place of shear walls shall
have story drifts limited to 0.015 with further
restrictions per Sec.112.4.2.
111.7. Out-of-Plane Forces:
Stability of URM Walls
n h/t ratios for use in Table 1-B are determined
from Fig 1-1:
n Region 1: Diaphragm is an elastic amplifier,
hence cross-walls are effective.
Use h/t ratio for “buildings with cross-walls” if
qualifying cross-walls are present in all stories.
n Region 2: Diaphragm is heavily loaded and yielding,
and already damping the EQ force:
Use h/t ratio for “buildings with cross-walls” whether
or not qualifying cross-walls are present.
n Region 3: Diaphragm is strong and lightly loaded.
Cross-walls are not displaced enough to yield:
Use h/t ratio for “all other buildings” whether or not
qualifying cross-walls are present.
n Walls with diaphragms in different regions:
n When DCR of the diaphragm above and below the
level of consideration result in different regions of
Fig 1-1, use the lesser h/t.
Open Front Procedure
n Single story.
n Open on one side.
n Cross-walls parallel to open front.
n Use the method of mirror.
n Effective diaphragm span and DCR for use in Fig 1-1:
Li = 2[(W w/W d)L + L]
DCR = 2.12SD1(W d + W w)/[(vuD) + V cb]
n Note: If cross wall is not at the open front,
another DCR check is made for the cantilevered
Sec.112. Analysis and Design
- URM Pier Analysis
n Applicable to both GP and SP.
n In-plane shear force at any level of a shear wall is
distributed to each pier in proportion to the pier
stiffness, unless the pier rocks.
n Pier rigidity considers shear and neglects flexural
n Rocking of URM pier is allowed after tension
cracks form at the ends of the pier. The axial
load on the piers provides restoring resistance to
n The capacity of a pier is the lesser of the
n Shear capacity = Allowable shear x pier
V a = v mA/1.5
n Restoring rocking capacity:
For piers: V r = 0.9P DD/H
For solid walls:V r = 0.9(P D + 0.5P w)D/H
Shear vs Rocking
n Rocking controlled mode:
n When V r < V a for each pier, the story
shear V wx is distributed to each pier in
proportion to PDD/H.
n The wall will be rocking safely on its piers if:
0.7 V wx < V r
Shear Controlled Mode:
n When Va < Vr in at least one pier, the story
shear is distributed in proportion to D/H.
n Use flow chart of Figure 1-2 as a design aid.
n The calculated shear in the piers must not
exceed pier capacities as defined above or the
wall must be strengthened.
Analysis and design (cont.)
n Plywood sheathed shear walls allowed for
buildings with flexible diaphragm analyzed
with SP, Sec.111.
n Plywood shear walls may not share lateral
forces with other materials along same line
n Moment resisting frames may not be used in
line with URM wall unless the wall has piers
with adequate rocking capacity.
The story drift ratio shall be 0.0075.
Sec.113. Detailed System Design
n Wall anchorage at floor and roof levels.
n Minimum wall anchorage at 0.9SD1 x tributary
weight or 200 plf.
n Anchors at 6Ft max spacing, 2Ft from corner.
n Diaphragm shear transfer with shear bolts at
n Collectors are required.
n Ties and continuity per Building Code.
Detailed System (cont.)
n Wall Bracing
n Truss and beam independent