4. SITE SELECTION 
Because the motion of the ground induces 
earthquake lateral forces in buildings, it is 
desirable, but not always possible, to build 
on sites located on stable and solid 
geologic formations. Deep and unbroken 
rock formations, referred to as bedrock, 
generally will minimize earthquake damage. 
It is not always possible to restrict the 
location of buildings to such geologically 
stable sites. However, by adhering to good 
construction practices such as those 
illustrated in this Guide, earthquake resistive 
houses can be built on other more 
commonly encountered site conditions. 
Houses built on unstable, questionable 
terrain may by vulnerable to damage due 
to ground failure as well as earthquake 
shaking in any seismic risk area. Greater 
care is required in high and moderate 
seismic risk areas subject to frequent 
and/or strong earthquakes, especially where 
unusual ground conditions have previously 
caused damage to buildings during an 
earthquake. 
landsliding, 
subsidence, 
Lands1iding 
Landsliding 
The principal hazards -are 
liquefaction, settlement or 
and surface fault rupture. 
is a potential hazard on hillside 
sites. A landslide scarp is shown in Figure 
21 and damage to houses due to 
landsliding is shown in Figure 25 on page 
15. Landslide--prone hillside sites include 
those that are underlain by previous 
landslides and those that, while presently 
stable, are located in unstable geologic 
formations. Landsliding is a particular 
hazard where poorly compacted fill has 
been placed on a hillside slope. Landsliding 
may cause damage in the absence of 
earthquake shaking and should be of 
concern even in low seismic risk areas. 
Rockfalls or falling boulders are hazards 
that are similar to landsliding. If an 
unstable rack slope is present above a 
dwelling, boulders could be dislodged by 
earthquake shaking and impact the dwelling 
below. 
Stable hillside sites with slopes of three to 
one (3:I) or less are generally satisfactory 
providing there is no previous history of 
landslide or movement on similar slopes in 
the vicinity and the slope does not contain 
poorly compacted fill. Any hillside site for 
which a professional soil consultant has 
made a favorable evaluation of the slope 
or designed the slopes to be stable can be 
usually regarded as satisfactory. 
Figure 21 
Compacted fills placed on slopes with 
engineering design and control during 
construction may also be regarded as 
stable. See Figure 23. For steep sites, 
geotechnical and engineering assistance is 
recommended in all seismic risk areas. 
Liquefaction 
Liquefaction is a phenomenon where loose 
saturated sandy soil loses bearing capacity 
and becomes like quicksand during 
earthquake shaking. The soil can spread 
laterally or may lose the ability to provide 
support for building foundations. Young 
alluvial deposits near rivers, lakes, or bays, 
or reclaimed land formed by placing 
uncompacted fills in lakes or bays are 
most likely to exhibit liquefaction potential. 
Settlement 
Any site on which fill has been placed 
without professional control should be 
regarded as suspect and possibly subject 
to either long term or earthquake induced 
settlement or subsidence. Settlement may 
be most pronounced in high seismic risk 
areas. The potential for settlement should 
be carefully evaluated, however, even in low 
seismic risk areas. 
Surface Fault Rupture 
Dwellings should not be located directly on 
an active earthquake fault due to the 
possibility of surface fault rupture and 
abrupt shearing displacement occurring 
along the fault during an earthquake. To 
avoid construction directly on a fault, a 
zone of at least 100 feet (50 feet on 
either side of the fault) where no 
construction is allowed should be observed. 
In the event of a large or moderate 
earthquake a dwelling situated directly on 
an active fault or in a fault zone would be 
subject to severe damage even if it was 
well constructed. Figures 22 and 25 
illustrate the types of displacement that 
can occur along earthquake faults. Damage 
to two houses that were situated on faults 
during two different earthquakes are shown 
in Figure 26. 
Site Evaluation 
Before selecting a building site, the builder 
should investigate the existence of any 
known or recognized geotechnical or 
geologic hazards discussed above. Maps are 
available for some locales from the US 
Geological Survey (USGS), state geologic 
agencies or from county and city agencies 
that show locations of active earthquake 
faults, fault zones and areas of potential 
landsliding and liquefaction. Some local 
jurisdictions have used these maps to 
delineate special zones where residential 
construction could be vulnerable to damage 
in the event of an earthquake. 
Schematic Diagrams of Surface Fault 
Displacement A) Strike Slip. B) Normal 
Slip. C) Reverse Slip 
Figure 22 
In areas suspected of being exposed to geotechnical 
hazards, maps or information indicating such conditions 
should be sought. A professional consultant may be 
retained to make a site-specific assessment if a 
hazardous condition is known or suspected to exist at 
a site. Construction in areas of low seismic risk may 
not justify an extensive investigation. 
When site conditions are unstable or of questionable 
soundness, such sites should be evaluated by a 
professional consultant. If a hazardous condition exists, 
the site may still be suitable if special measures are 
taken for the site and/or for the construction of the 
dwelling. If the site conditions can be sufficiently 
improved, an earthquake resistive house can be built 
using the information and illustrations shown in the 
guide. 
Summary 
The following summary lists Qualified and Disqualified 
(hazardous) building sites. Qualified sites are stable sites 
for which the building details in this Guide are applicable. 
Disqualified sites require further evaluation as discussed 
above. Some of these types of sites are shown in 
Figures 23 and 24. 
Qualified Sites For Which Guide is Applicable 
1. Stable and solid geologic formations - bedrock 
2. Firm, stable soil deposits 
3. Stable hillside slopes 
4. Engineered land fill placed over stable soils 
5. Sites recommended by a professional soil consultant. 
Disqualified (Hazardous) Sites Requiring Further Evaluation 
1. Sites built on non-engineered fill 
2. Sites prone to landsliding or rockfalls 
3. Reclaimed waterlands, marshes, or alluvial soil sites 
having a potential for liquefaction. 
4. Sites directly over an active fault or within a 100 feet 
wide zone, i.e., 50 feet either side of the fault. 
NATURAL STABLE 
SLOPE 
MODERATE SLOPE 
ORIGINAL PROFILE 
A (STABLE) 
ENGINEERED 
BENCH CUT 
IN SLOPE 
STEEP SLOPE 
QUALIFIED 
Figure 23 
FOOTHILLS
SHALLOW ALLUVIAL 
DEPOSIT X 
ROCK OUTCROPPING 
AT BASE OF HILL 
UNCONSOLIDATED FILL 
COMPACTED 
Y SEAM 
ROCK X 
UNSTABLE SLOPE 
DISQUALIFIED 
Figure 24 
(14)
Figure 25 
Figure 26 
(1 6) 
5. ELEMENTS OF THE SEISMIC RESISTANCE 
SYSTEM 
This section identifies and describes the 
components of residential construction that 
resist earthquake forces. The text and 
illustrations show how a complete, 
uninterrupted and well defined load path is 
constructed. It is essential that all of 
the seismic resisting components be tied 
together to form an earthquake resisting 
system without weak links. Connections 
between the resisting elements provide 
the continuity necessary to assure an 
uninterrupted path. The three principal 
elements of the system are the floor and 
roof diaphragms, shear walls and the 
foundation. See Figure 27. 
ROOF DIAPHRAGM 
RESISTING MOTION 
INDUCED BY 
GROUND SHAKING 
FLANGE (CHORD) 
SHAPED BLO 
ROOF 
WEB 
CONNECT
WALLS
SIDE AN 
EACH E 
FLANGE 
WALL TOP 
GROUND MOTION 
Figure 27 
The function of the floor and roof diaphragms 
is to transfer earthquake forces to shear walls 
which are usually located at the exterior of 
the building. For wood framed houses, diaphragm 
framing usually consists of the floor or roof 
sheathing, joists or rafters, blocking at the ends 
of joists or rafters and wall top plates. Cold formed 
steel framed houses are framed in essentially 
the same manner. 
Wood Structural Panel is always acceptable 
for roof and floor sheathing. Diagonal 
sheathing, although not commonly used, but 
still permitted by Code, can be used instead 
although not as effectively. Straight 
sheathing may be used in areas of low 
seismic risk but should be avoided in high 
and moderate seismic risk areas. Spaced 
roof sheathing, often used for shingled roofs, 
is not recommended but may be used in low 
seismic risk areas where wind loads are also low. 
For best performance, sheathing should be applied 
to the framing with face grain across the joists 
or rafters and with nailing on all edges and at 
intermediate supports. See Figure 17 on 
page 7. Each sheet must be nailed 
along each short edge to a framing 
member. Abutting short edges 
of adjoining sheets must be nailed 
to the same (mutual) framing 
member. The long edge of the 
sheet may be nailed to blocking 
inserted between the joists or rafters 
with abutting edges nailed to 
the same member. Blocking of edges 
to resist lateral forces is not usually 
required for roof or floor diaphragms 
in residential construction, but can 
be used to increase the stiffness and 
strength of the diaphragm. Where tongue 
and groove sheets are used, blocking 
may be omitted along the long edges. 
A bead of construction adhesive should 
be used on the top edge of floor joists. 
This is usually done to prevent squeaking 
but tests have shown that it also 
increases stiffness and improves 
performance. Adhesive on roof framing 
members can be used to stiffen the roof 
diaphragm. 
Edge of sheet nailing of floor and roof 
sheathing is required at diaphragm 
boundaries, along exterior wall plate lines 
and along interior walls used as shear 
walls. Sheathing is usually edge nailed to 
blocking or rim joists or similar members 
attached to the wall top plates. See Figure 
17 on page 7. 
Nailing of floor and roof sheathing 
prescribed in building codes for 
conventional construction is ordinarily 
adequate for diaphragm strength. 
Exterior wall top plates function as flanges 
or chords for the diaphragm and resist the 
tension and compression resulting from 
beam action. The wall double top plates at 
the boundaries of the diaphragms must be 
spliced along the widths and lengths 
(perimeter) of the diaphragms in order for 
them to be "continuous". See Figure 28. 
Because of strength and stiffness 
limitations, diaphragms are restricted to 
certain diaphragm ratios of length (L) to 
width (W). See Table No. 3 on page 39. If 
the horizontal distance (span) between 
exterior walls is too large, the diaphragm 
becomes long and narrow and consequently 
too flexible. The diaphragm ratio (L/W) is 
important because excessive deflection of 
the diaphragm may cause wall damage and 
glass breakage. If the distance between 
exterior walls is too great, the proper 
diaphragm ratio may be achieved by using 
interior shear walls parallel to the exterior 
walls to reduce the diaphragm span. 
The primary vertical resisting elements are 
the shear walls that support the floor and 
roof diaphragms and resist earthquake 
forces generated in the diaphragms. See 
Figure 11 on page 6. Aspect ratios of 
shear walls (vertical diaphragms) are 
established to control drift. The 
recommended height (H) to width (W) 
ratios shown in Table 3 on page 39, 
should not be exceeded and spacing of 
shear walls should be as prescribed in the 
building code or as shown in this Guide. 
RAFTER TOE NAILED
TO PLATE 
SOLID BLOCKING (OR RIM JOIST) 
PLATE 
ON PAGE 45. 
SEE FIGURE 17 FOR 
NAILING OF BLOCKING 
OR RIM JOIST TO PLATE 
TOP PLATE SPLICE 
Figure 28 
Shear wall framing usually consists of wall 
top plates, studs, sheathing, floor sill plate, 
hold-downs and other connections. Top 
plates should be doubled, lapped and 
spliced for continuity. Studs should be 
doubled at the ends of shear walls and 
internailed stud to stud. Sheathing must be 
nailed to blocking or rim joists attached to 
the wall top plates, to the end or corner 
studs, and to the floor plate or sill. The 
floor sill plates must be bolted to the 
foundation or otherwise attached by special 
hardware designed to substitute for bolts. 
Several sheathing materials are available 
with Wood Structural Panel sheathing being 
the most effective. See Table No. 3 on 
page 39. It is important to use Wood 
Structural Panel sheathing in high seismic 
risk areas. 
Gypsum board wall sheathing applied to 
the studs can also be used to resist 
earthquakes. See Table No. 3 on page 39. 
As demonstrated in the Northridge and 
other earthquakes, when gypsum board is 
used for shear wall sheathing in high 
seismic risk areas severe damage can 
result from strong earthquake shaking. 
Gypsum board is usually adequate as 
sheathing on shear walls in moderate and 
low seismic risk areas. Gypsum board may 
be applied to the walls with long edges 
vertical or horizontal. But if long edges are 
horizontal they should be blocked. 
Stucco over wire lath applied directly to the 
studs can be used for seismic resistance 
in all seismic risk areas, although, it is not 
advised in high seismic risk areas. The lath 
must be securely nailed to the studs, wall 
top plates and sill. In high seismic risk 
areas, stucco can be vulnerable to severe 
cracking as demonstrated in the Northridge 
and other earthquakes. 
Let-in bracing provides only minimal 
seismic resistance and should not be relied 
on except in low seismic risk areas where 
wind loads are also low. 
For shear walls to be effective in resisting 
the diaphragm loads and controlling 
damage they should have height to width 
ratios in high and moderate seismic risk 
areas of 2:1 or less. See Figure 29. A 
larger ratio may be allowed by certain 
codes, but experience in earthquakes has 
demonstrated that limiting height to width 
ratios will improve performance. Shear 
walls, in two story construction need only 
satisfy this ratio in each story although 
better performance can be achieved by 
computing the ratio using the full height of 
the wall. A minimum width of 4'-0" for 
shear walls in high and moderate seismic 
risk areas is advised. 
As noted before, earthquake forces must be 
transferred from the roof diaphragm into 
the shear walls and through the walls to 
the ground at the foundations. At the 
interface of the horizontal diaphragms and 
shear walls, the roof and/or floor sheathing 
is nailed to the top of solid blocking or 
rim joists, and the wall sheathing is usually 
nailed to the vertical face of the same 
block or rim joists. Where the wall 
sheathing only extends to the top of the 
top plates, the blocking or rim joist must 
be securely nailed or connected to the top 
plates. See Figure 30 and Figure 32. 
DOUBLE 
END STUDS 
FOUNDATION 
FLOOR SILL PLATE NAILED 
TO RIM JOIST 
END SHEAR WALL 
Figure 30
Wall top plates act as flanges and 
collectors for the roof and floor 
diaphragms and can be loaded in tension 
or compression. Top plates must be spliced 
for continuity. Wall top plates are stabilized 
against buckling by nailing the joists or 
rafters to the plates or by means of a rim 
joist or end rafter where the framing runs 
parallel to the wall. Corner and/or end 
studs act as flanges for shear walls and 
carry tensile or compressive forces. Only 
full length studs may be used as flanges 
and the wall sheathing must be edge nailed 
to the end studs rather than to adjacent 
studs. See Figure 31. 
Because of the tendency of a tall, narrow 
shear wall to overturn, a wall with 
insufficient weight may uplift from the 
foundation at either end of the wall. To 
prevent this, the corner or end studs can 
be anchored to the footing with a 
hold-down device. See Figure 31 here 
and Figure 55 on page 29. Experience 
has shown that hold-down bolts must be 
inserted in correctly sized holes and must 
be tight. Bolts should be retightened before 
closing in the walls. Better performance 
can be obtained by using 1/4" diameter 
wood screws to connect the hold-down to 
the post. 
To prevent the building from sliding off of 
its foundation when exposed to earthquake 
shaking, the sills must be bolted to the 
footings. See Figure 33. The footings will 
then transfer the earthquake loads back to 
the ground. Bolts should be set accurately 
on the center line of the sills. A consistent 
spacing of the bolts should be maintained. 
Improperly installed sill anchor bolts have 
contributed to damage in past earthquakes. 
Care should be taken not to oversize the 
holes in the sills and the bolts should be 
tight with a washer under the nut for firm 
bearing. Use plate washers in high seismic 
risk areas. Anchor bolts should not be 
countersunk in the sill. Floor and wall 
sheathing must be nailed to the framing. 
See Figures 32 and 33. 
SHEATHING EDGE NAILED 
TO HOLD-DOWN STUDS 
OR END POST 
HOLD-DOWN FIXTURE 
FASTENED TO DOUBLE 
STUDS OR END POST 
HOLD-DOWN BOLT EMBEDDED 
IN FOUNDATION 
HOLD-DOWN 
Figure 31 
FLOOR SHEATHING - 
EDGE NAILED TO 
BLOCKING OR 
RIM JOIST 
SILL PLATE BOLTED 
TO FOUNDATION 
Figure 32 
Figure 33 
6. FOUNDATIONS AND FOUNDATION DETAILS 
Footing types are dictated by the underlying 
ground conditions and by their inherent 
ability to resist forces caused by 
earthquakes. Some consistent rules apply 
and should be followed regardless of the 
footing type selected. 
To avoid damage from unequal settlement 
and subsidence, foundations should be on 
uniform ground conditions, i.e., footings for 
the same building should not be part on 
rock and part on loose fill, soft soil or 
other significantly different ground 
conditions unless specially designed to 
accommodate the non-uniform conditions 
See Figure 34. 
Builders should investigate building sites for 
ground conditions that could have an 
unfavorable impact on the foundation. 
Information noted in Section 4, SITE 
SELECTION, in the Guide indicates the 
minimum satisfactory conditions for 
constructing foundations to resist 
earthquakes. Local building codes generally 
provide information regarding minimum 
standards for foundations. Supplemental 
information intended for earthquake 
resistance is contained in the Guide. 
Underground basement walls must support 
the weight of the structure above, floor 
and roof loads, earth pressures and 
earthquake loads perpendicular to the plane 
of the wall. Basement walls acting as shear 
walls may be thought of as a continuation 
of the shear wall above and resist the 
earthquake forces transmitted to these 
shear walls by the horizontal floor 
diaphragms, See Figure 35. 
Sill plates on basement walls, when properly 
anchored, keep the shear walls above from 
sliding or moving. The minimum bolting 
requirement for moderate and low seismic 
risk areas is 1/2" diameter by 10" long 
bolts spaced a maximum of 6'-0" on 
center with 7" of embedment. 
For high seismic risk areas bolts should be 
5/8" diameter by 10" long spaced at 
4'-O" maximum with 7" of embedment. 
Bolts should be centered on the sill plate. 
Sill plates are normally 2x material, but in 
areas of high seismic risk, 3x sill plates 
provide better protection from splitting. 
Avoid countersunk washers and nuts in sill 
plates. In areas of high risk, sill bolts 
should have plate washers under the nuts. 
Use 3" x 3" x 1/4" plate washers for 
1/2" and 5/8" bolts and 3" x 3" x 3/8" 
plate washers for 3/4" bolts. At ends of 
sill plates, bolts should be placed 9" from 
each end of each piece. 
Local building codes should be consulted 
for reinforcing requirements for basement 
walls. Reinforcement necessary to resist 
earth pressure and gravity load will usually 
be adequate for earthquake resistance. 
ROCK 
FILL 
NON-UNIFORM CONDITION 
Figure 34 
LATERAL QUAKE as 
LOAD GRADE
LATERAL EARTH 
LOAD 
SUPERSTRUCTURE 
FOUNDATION WALL 
SECTION 
BUILDING MUST RESIST 
EARTHQUAKE LOADS IN 
EACH DIRECTION 
EARTHQUAKE SHEAR WLSH 
LOADS DELIVERED REACTIONS 
TO SHEAR WALLS 
BY FLOOR DIAPHRAGM FLOOR SHEATHING ACTS 
AS DIAPHRAGM 
SHEATHING FLOOR JOISTS 
BASEMENT END WALLS 
BLOCKING ACTING AS SHEAR WALLS 
SHEAR WALL 
AS REACTIONS 
SECTION
Figure 35 
Basement or foundation walls supporting a framed floor 
where there is a difference in elevation from the top of 
the walls to the underside of the floor joists may have
cripple stud walls supporting the framing. See Figure 36. 
Cripple stud walls are vulnerable to earthquake shaking 
and must be braced or sheathed as appropriate for all 
seismic risk areas. Wall bracing or sheathing for cripple 
walls should conform to the same requirements as for 
full height shear walls and can be of any material 
proposed in this Guide. Plywood or oriented strand board 
is recommended for high seismic risk areas. Gypsum 
board sheathing and/or stucco can be used for moderate 
and low seismic risk areas. Let-in bracing is not 
recommended for cripple walls except that it may be C R 
used in low seismic risk areas. 
As observed in the Northridge Earthquake, cripple stud 
walls supported on stepped footings along steeply 
sloping sites performed poorly. They should not be used
in high seismic risk areas. Except for high walls, a 
concrete stem wall above the footing should continue to Figure 
the floor framing, thus eliminating the need for a cripple Figure 
wall. 
Continuous perimeter footings are suitable for ground TO FOOTING 
conditions consisting of firm natural sites, engineered STEM 
landfills and moderate stable slopes. Concrete footings are W PLA N 
usually placed in trenches without forms with the bottom REINF( 
of footing at a minimum depth required by code or at AT CC 
least 12" below adjacent grade. Wall sill plates can rest 
directly on the tops of footings if the footings are high 
enough relative to the adjacent grade. If a grade beam 
does not project above the ground, a concrete or 
masonry stem wall resting on it can be provided. Vent 
openings and crawl holes may be placed in the stem 
wall, but the grade beam should be continuous without 
interruption. 
In areas of high seismic risk, footings should have 
reinforcing. See Figures 37 and 38. Reinforcing is 
recommended as shown in moderate seismic risk areas, 
but reinforcing may be omitted in low seismic risk areas FOI 
if codes and local construction practices permit. 
To be effective, reinforcing must be lapped at splices and 
well tied together at the corners and intersections. Where 
placed separately, stem walls and grade beams should be 
doweled together. Sill plates must be bolted to the tops 
of grade beams or stem walls. See Figure 39. The 
spacing and size of bolts should be as described for 
Figure 37 
basement walls. Hold-down anchor bolts 
must be embedded in the grade beams as 
shown in hardware manufacturers' catalogs. 
Interior bearing walls or partitions used as 
shear walls should have continuous footings 
similar to perimeter foundations. See 
Figure 38. Reinforcing, where used in footings 
under interior walls, should be lapped with 
reinforcing in the perimeter footings at their - - 
intersections. 
REINFORCING 
LAPPED AND 
TIED AT 
CORNERS 
CONTACT LA SPLICE 
TOP & BOTTOM BARS
STAGGER TOP SPLICE
WITH BOTTOM MIN. 
FOOTING AT 
INTERIOR 
SHEAR WALL PERIMETER PLAN VIEW 
FOOTING 
GRADE BEAI 
Figure 39 
REINFORCING BARS 
REINFORCING BARS 
INTERSECTING FOOTINGS 
Figure 38 
Often, stud walls are prefabricated on the 
ground and lifted into place. In such cases, 
bolts holes in the sills should not be 
oversized to accommodate the preset bolts 
in the footings. Bolt holes should be marked 
and drilled in the sill plate using a template 
made from the known location of the bolts 
projecting from the footing. After locating 
and drilling the bolt holes in the sill, the 
wall can be lifted and set over the anchor 
bolts. 
Floor joists supported on interior bearing 
lines other than shear walls can be supported 
on beam and girder framing systems 
attached to posts bearing on individual 
spread footings or precast piers. See 
Figure 40. Sheet metal post caps should be 
used to connect beams/girders to 
the posts. If a beam/girder acts as a collector 
for a shear wall, it must be spliced 
for continuity with wood cleats or metal 
straps. Splices should occur only over posts. 
Girders should be in lengths as long as 
possible to minimize splices. 
and 43. Sills must be bolted to the slab 
with bolt sizes and spacing as previously 
described for continuous wall footings. 
EARTHQUAKE B 
LOAD
GRADE 
PERIMETER 
FOOTING OR SLAB-ON-GRADE (REINFORCED FOR 
THICKEN HIGH AND OPTIONAL FOR MODERATE 
EDGE SEISMIC RISK AREAS 
SECTION 
PERIMETER
FOOTING 
INTERIOR BEARING LINE 
Figure 40 
Slab-on-grade foundations have performed 
well in earthquakes. Slab-on-grade construction 
is a suitable foundation system for 
most sub-grade conditions. When highly expansive 
soil or clay soil is encountered, a 
geotechnical engineer should be consulted. 
Slabs-on-grade may be used on flat 
sites and are ideal for homes framed of 
wood or cold-formed steel with no basement 
or crawl space. Slabs, unless they are 12" 
or more in thickness, should have thickened 
edges at exterior bearing walls and integral 
thickened trenched footings for interior bearing 
walls and shear walls. See Figures 41, 42 
EARTHQUAKE LOADS 
IN SHEAR WALL 
RESISTED BY 
PERIMETER FOOTING AND SLAB-ON-GRADE 
CONCRETE 
SLAB-ON GRADE 
EARTHQUAKE LOADS 
RESISTED IN 
EACH DIRECTION 
PLAN 
Figure 41 
Figure 42 
VERIFY CLEARANCE WITH 
LOCAL BUILDING REGULATIONS 
BAR TOP AND 
MIN. AND 
THICKENED SLAB MODERATE SEISMIC 
AT INTERIOR RISK AREA. 
BEARING WALL REINFORCING 
OPTIONAL IN LOW 
THICKENED EDGE SEISMIC RISK AREA 
AT PERIMETER 
CONCRETE SLAB-ON-GRADE 
Figure 43 
The slab floor will act as a diaphragm and 
should have minimum reinforcing in high 
seismic risk areas so that cracks and 
differential displacements do not impair the 
diaphragm properties. Slab reinforcing is 
also effective in bridging across soft spots 
and depressions in the subgrade regardless 
of need for seismic resistance. Reinforcing 
is optional in low seismic risk areas. At a 
minimum, welded wire fabric is 
recommended to keep slabs integral and 
minimize cracking. 
Drilled-in-place concrete-filled piers with 
grade beams spanning between the piers 
are suitable for compressible soils, loose 
land fills, and known weak soil conditions. 
Sizes and depths of piers must be 
determined from local building regulations 
or from a geotechnical consultant. 
Reinforcing of the grade beams must be 
provided to support the weight of the 
building and contents. Reinforcing required 
for normal loading will be adequate for 
earthquake loads as well. Reinforcing should 
employ individual bar lengths as long as 
practical and be lapped and tied at splices 
and at corners and intersections. The piers 
should be doweled to the grade beams. 
See Figure 44. 
See Table No. 2 on page 26. Four 
conditions not covered in the Guide are 
listed below: 
1. Pole houses. 
2. Houses on pile supported platforms. 
3. Floating foundations on unengineered 
hydraulically placed landfill. 
4. Isolated footings not connected by 
continuous grade beams 
Information and details of construction shown 
in the Guide may not be universally suited to 
these foundation types. Construction should 
not be attempted without the assistance of a 
design professional. 
EARTHQUAKE GRADE BEAM 
LOAD 
GRADE 
SECTION DRILLED PIER 
REACTIONS 
EARTHQUAKE FLOOR DIAPHRAGM 
LOADS DELIVERED FLOOR JOISTS 
BY DIAPHRAGM FORJIT 
DRILLED PIER 
SHEAR WALL 
EATONS 
GREADE GRADE BEAM
BESAME 
PLAN 
FLOOR 
FRAMING 
DRIIIED GRADE BEAM
Some foundation types are not suitable for 
seismic resistance without special 
consideration related to either the site or 
special requirements of the building. 
DRILLED PIER DETAIL 
Figure 44 
Table No. 2 - Foundation Types Requiring Special Construction 
Pole House 
ISOLATED WOOD 
POLES 
Pile Supported Platform 
GRADE GIRDER 
POST 
INDIVIDUAL PIER 
FOOTING 
Individual Isolated Piers 
GIRDER 
PIER FOOTING 
GIRDER 
PLAN 
TIMBER 
POST 
Floating Foundation on 
an Unengineered Landfill