Reported by ACI Committee 349
american concrete institute
P.O. BOX 9094
FARMINGTON HILLS, MICHIGAN 48333
0.7845 [D - (0.9743/n)]2Ar = reduction in effective stress area to account for limited depth of concrete beyond the bearing surface of the embedment (see Fig. B.4-2), sq. in.
= strength reduction factor, dimensionless
B.1-Scope
B.1.1 This appendix provides minimum requirements
for design and anchorage of steel embedments used to transmit loads from
attachments into reinforced concrete structures by means of tension, bearing,
shear, friction, or any combination permitted by this appendix.
Typical embedment details and concepts as referenced
in this appendix are shown in Fig. B.1-1 and B.1-2. In addition to meeting
these requirements consideration shall be given to the effect of the forces
applied to the embedment on the behavior of the overall structure.
B.1.2 The requirements for the attachment
to the embedment shall be in accordance with applicable codes and are beyond
the scope of this appendix.
B.1.3 Design limits less conservative than
those specified in this appendix may be used by the Engineer if substantiated
by experimental or detailed analytical investigation.
B.2-Definitions
Anchor-Steel component used to transmit loads from the
attachment into the concrete. Anchors include, but are not limited to,
bolts, welded studs, threaded rods, bars, undercut anchors, and expansion
anchors.
Anchorage-The process of attaching a member or object
to the concrete structure by means of an embedment, taking into consideration
those factors (e.g., depth of embedment, edge distance, and spacing of
anchors) which determine the load capacity of the anchorage system.
Anchor head-A nut, flat washer, plate, stud, or bolt
head used to transmit loads from the tensile strength component to the
concrete by bearing.
Attachment-The attachment is that structure external
to the surfaces of the embedment which transmits loads to the embedment.
Embedment-The embedment is that steel component embedded
in the concrete used to transmit applied loads to the concrete structure.
The embedment may be fabricated of plates, shapes, bolts, reinforcing bars,
shear connectors, expansion anchors, inserts, or any combination thereof.
Expansion anchor-Component that is installed in a hole
drilled in hardened concrete and then is expanded in order to transfer
loads into the concrete by direct bearing and/or friction.
Expansion mechanism-A mechanism used to transmit load
from the tensile stress component to the concrete when used as part of
an expansion anchor.
Grouted Embedments-An embedment located in a formed or
drilled hole in hardened concrete utilizing a grout to provide load transfer
from the embedment to the concrete.
Inserts--Commercially available, predesigned, and prefabricated
embedments installed prior to concrete placement which are specifically
designed for attachment of bolted connections.
Strength, design-Nominal strength multiplied by a strength
reduction factor .
Strength, nominal-Strength of a connection calculated
in accordance with the provisions and assumptions of the strength design
method of this code before application of any strength reduction factors.
Tensile stress component-That part of the embedment attached
to the anchor head or expansion mechanism used to transmit tensile loads
to the concrete.
B.3-General requirements and loading combinations
B.3.1 The embedment and surrounding concrete
or grout shall be designed for transmitting to the concrete structure all
loads used in the design of the attachment. Assumptions used in distributing
loads within the embedment shall be consistent with those used in the design
of the attachment.
B.3.2 Reactions on the embedment due to individual
loads such as dead, live (including vibratory loads), thermal, seismic,
and accident loads shall be considered. The loading combinations for embedment
design shall be in accordance with 9.2 of this code.
B.3.3 Material and testing requirements for
embedment steel shall be specified by the Engineer to ensure that the embedment
design is compatible with the intended function of the attachment.
B.3.4 The design strength of embedment materials
may be increased in accordance with Appendix C for embedments subject to
impactive and impulsive loads.
B.3.5 The strength of embedments as affected
by the size and grade of steel, spacing, and depth of embedment and any
concrete dimensions which limit or restrict the transfer of loads from
steel to concrete shall be considered as defined in B.4, B.5, and B.6.
B.3.6 Plastic deformation of the embedment
is permitted for impactive and impulsive loading provided the strength
of the embedment is controlled by the strength of the embedment steel as
specified in B.5. For these conditions a maximum ductility ratio of 3 may
be considered. The definition of ductility ratio shall be as defined in
Appendix C.
B.3.7 A combination of bearing and shear
friction mechanisms shall not be used to develop the design shear strength
defined in accordance with 9.3 of this code. The available confining force
afforded by the tension anchors in combination with external loads can,
however, be utilized in determining the shear capacity of anchorages with
shear lugs.
B.4-Design requirements for concrete
B.4.1 The design provisions of this appendix
are based on the strength design method. The assumptions, principles, and
requirements of the code are applicable for all load combinations except
as modified herein.
B.4.2-Tension
The design pullout strength of concrete Pd
for any embedment shall be based on a uniform tensile stress of 
acting
on an effective stress area which is defined by the projected area of stress
cones radiating toward the attachment from the bearing edge of the anchor
heads. The effective area shall be limited by overlapping stress cones,
by the intersection of the cones with concrete surfaces, by the bearing
area of anchor heads, and by the overall thickness of the concrete (see
Fig. B.4-1 and B.4-2). The inclination angle for calculating the projected
area shall be 45 deg.
The strength reduction factor shall be as follows:
a) Embedments anchored beyond the member far face reinforcement . . . . . . . .0.85B.4.3-Shear
b) Embedments anchored in a compression zone of a member . . . . . . . . . . . . . . . .0.85
c) Embedments anchored in a tension zone of a member where the concrete tension stress (based on an uncracked section) at the concrete surface is less thanfor the load combinations and load factors defined in 9.2 . . . . . . . . . . .0.85
d) All other embedments . . . . . . . . . . . . . . 0.65
B.4.5-Bearing
B.4.5.1 The bearing requirements of 10.15
or 18.13 of this Code shall apply to the average bearing stress at an anchor
head except as permitted in B.4.5.2.
The design bearing strength used for concrete or
grout placed against shear lugs shall not exceed (1.3f 'cAb)
using a strength reduction factor of 0.70. For grouted installations,
the value f'c, shall be the compressive strength of the
grout or the concrete whichever is less.
B.4.5.2 For bolts meeting the requirements
of ASTM Specifications A 307, A 325, or A 490 or if the anchor head at
the base of the tensile stress component satisfies the following conditions:
(a) The bearing area of the anchor head including the area of the tensile
stress component is at least 2.5 times the area of the tensile stress component.
(b) The thickness of the anchor head is at least 1.0 times the greatest
dimension from the outer most bearing edge of the anchor head to the face
of the tensile stress component. (c) The bearing area of the anchor head
is approximately evenly distributed around the perimeter of the tensile
stress component.
B.5-Anchorage requirements
B.5.1 Anchorage design shall be controlled
by the strength of embedment steel unless otherwise specified in this appendix.
B.5.1.1-Tension
Steel strength controls
when the design pullout strength of the concrete Pd as
determined in B.4.2 exceeds the minimum specified tensile strength of the
tensile stress component (based on fut) of the embedment
steel, and full load transfer is accomplished from steel to concrete within
the depth of the anchorage by one of the following methods:
a) An anchor head at the base of the tensile stress
components which satisfies the requirements of Section B.4.5.2. To prevent
failure due to lateral bursting forces at anchor heads, the minimum side
cover distance m shall be determined such that the lateral concrete
design strength (based on a uniform tensile stress of
acting on an effective area, including overlapping stress cones, defined
by projecting a 45 deg cone from the anchor head to the free surface) exceeds
the lateral bursting force unless the requirements of B.4.4 are met. The
factor shall be taken as 0.85.
b) Reinforcing bars with development lengths in accordance
with the requirements of Chapter 12, for anchor steel composed of reinforcement.
B.5.1.2--Shear
B.5.1.2.1-Bolts, studs,
or bars
Bolts, studs,
or bars shall meet the requirements of B.5. 1. 1. The minimum edge distance
m
for shear loading toward a free edge shall be such that the concrete design
strength (based on a uniform tensile stress of acting
on an effective area defined by projecting a 45 deg half-cone to the free
surface from the centerline of the tensile stress component at the shearing
place) exceeds the ultimate shear strength of the bolts, studs, or bars
(based on fut).
B.5.1.2.2-Shear lugs
The shear strength of grouted
or cast-in-place anchorages with shear lugs shall include consideration
of the bearing strength of the concrete or grout placed against the shear
lugs, the shear strength of the concrete or grout placed between shear
lugs and the confinement afforded by the tension anchors in combination
with external loads. Shear loads toward free edges and displacement compatibility
between shear lugs shall be considered.
a) When multiple shear lugs are used to establish
the design shear strength in a given direction, the magnitude of the allotted
shear to each lug shall be in direct proportion to the total shear, the
number of lugs, and the shear stiffness of each lug.
b) For shear lugs bearing in the direction of a free
edge, the design shear strength for each lug shall be determined based
on a uniform tensile stress of acting on an effective stress area
defined by projecting a 45 deg plane from the bearing edges of the shear
lug to the free surface unless the requirements of B.4.4 are met. Bearing
area of the shear lug shall be excluded from the projected area. The
factor shall be taken as 0.85.
B.5.1.3 For combined tension and shear, the
depth of embedment shall be in accordance with B.5.1.1 and the minimum
edge distance in accordance with B.5.1.2.1.
B.5.1.4 Where reinforcement is provided in
accordance with B.4.4, the minimum edge distance shall not be less than
one-third that required by B.5.1.2. The reinforcement shall also satisfy
the concrete cover requirements in 7.7 of this code.
B.6-Design requirements for embedment steel
B.6.1 Design strength provided by the embedment
steel in terms of flexure, axial load, shear, and torsion, shall be taken
as the nominal strength calculated in accordance with the requirements
and assumptions of this section, multiplied by a strength reduction factor
.
B.6.2 strength reduction factor shall be
as follows:
B.6.2.1 Flexure and/or
axial load . . . . . 0.90
B.6.2.2 Shear and
torsion . . . . . . . . . . . . 0.85
B.6.3 Embedment materials other than reinforcing
bars shall have a minimum elongation of 14 percent in 2 inches when tested
in accordance with ASTM A 370.
Embedment materials without a distinct yield point
shall be permitted. For such materials the yield strength fy
shall be defined as the 0.2 percent strain offset method in ASTM A370.
B.6.4 Anchors that incorporate a reduced
section in the load path shall satisfy one of the following conditions:
a) The ultimate tensile strength
of the reduced section shall be greater than the yield strength of the
unreduced section.
b) For bolts, the length of thread
in the load path shall be at least two anchor diameters.
B.6.5-Anchors
Anchors shall be designed for tension and shear loads in accordance
with B.6.5.1, B.6.5.2 and B.6.5.3.
B.6.5.1-Tension
The nominal tensile strength of an anchor shall
be fyAe
B.6.5.2-Shear
The nominal shear strength
attributed to anchors shall be determined by B.6.5.2.1 or B.6.5.2.2, whichever
is applicable.
B.6.5.2.1 For connections
with the contact surface of the baseplate flush with the surface of the
concrete, the nominal shear strength of an anchor shall be . . .
. . 0.70fyAe
For built-up grout pads, the nominal shear strength
shall be multiplied by . . . . . 0.80
Friction between the baseplate and concrete may
be considered to contribute to the nominal shear strength of the connection.
The nominal shear strength resulting from friction between the baseplate
and concrete (i.e., without any contribution from anchors) may be taken
as 0.40C
B.6.5.2.2 For connections
with the contact surface of the baseplate below the surface of the concrete,
the shearfriction provisions of 11.7 of this code (as modified by this
section) shall be used. The shear-friction coefficient shall be as follows:
Baseplates without shear lugs . . . . . . . 0.9
Baseplates with shear lugs which are designed to remain elastic . .
. . . 1.4
B.6.5.3-Combined tension and shear
B.6.5.3.1 The interaction
of tension and shear for anchors designed in accordance with B.6.5.1 and
B.6.5.2.1 (shear transfer by anchor bearing) shall be assumed to be linear
(additive) or elliptical.
B.6.5.3.2 For anchors
designed in accordance with B.6.5.1 and B.6.5.2.2 (shear transfer by shear-friction),
the area required for tension due to applied load and the area required
for tension due to shear-friction shall be additive.
B.6.6-Structural shapes, fabricated shapes, and
shear lugs
The design strength of embedded
structural shapes, fabricated shapes, and shear lugs shall be determined
using a steel stress of fy, where shall be taken
as 0.9 for tension, compression and bending, and 0.55 for shear.
B.7-Expansion anchors
This section provides minimum requirements for the
design of typical expansion anchors used in concrete structures and does
not restrict the use of other expansion anchors provided the expansion
anchors are designed and tested in accordance with the requirements of
this section.
B.7.1-Design requirements
Expansion anchors shall be designed to assure that
the design strength of concrete for a given expansion anchor or group of
expansion anchors is greater than the strength of the anchor steel except
as permitted in B.7.2. The requirement shall be met by satisfying the requirements
of B.7.1.1 or B.7.1.2.
B.7.1.1-Design by analysis
a) Tension: The design pullout
strength of concrete Pd shall be as defined in B.4.2
except that the effective stress area shall be defined by the projected
area of the stress cone radiating toward the concrete surface from the
innermost expansion contact surface between the expansion anchor and the
drilled hole. Refer to Fig. B.7-1 for typical details. The design pullout
strength of concrete shall be equal to or greater than the minimum specified
tensile strength or average tensile strength if a minimum is not defined
for the expansion anchor. The minimum edge distance shall be in accordance
with the requirement of B.5.1.1(a).
b) Shear: Expansion anchors
subject to shear shall meet the requirements of B.5.1.2.1.
c) For combined tension
and shear, the depth of embedment shall be in accordance with B.7.1.1(a)
and the minimum edge distance in accordance with B.7.1.1(b).
d) The design requirements
for embedment steel shall be in accordance with B.6.0.
B.7.1.2-Design by testing
Tests shall be conducted
to verify that the concrete will develop the steel strength of the expansion
anchor. Design by test results shall be restricted to tests that are representative
of the anchor spacing and load application.
B.7.1.3-Strength reduction
factors
The requirements of B.6
shall apply except that the factors for expansion anchors shall be 0.9
times the values specified in B.6.2.
B.7.2-Alternative design requirements
For expansion anchors that do not meet the requirement
of B.7. 1, the design strength shall be 0.33 times the average tension
and shear test failure loads. The average test failure load shall be equal
to the average of the test loads carried by test anchors at failure (maximum
load) or at a magnitude of displacement of test anchors as specified by
the Engineer.
B.7.3 A single expansion anchor used to anchor
an attachment shall be designed for one-half of the design strength defined
herein.
B.7.4-Testing
B.7.4.1 Expansion
anchors designed in accordance with B.7.1.1 or B.7.1.2 shall be tested
to verify the ability of the expansion mechanism to develop the tensile
strength of the tensile stress component. Expansion anchors designed in
accordance with B.7.2 shall be tested to determine the average test failure
load. Tests shall be conducted by a testing agency other than the expansion
anchor manufacturer and shall be certified by a professional Engineer with
full description and details of the testing program, procedures, results,
and conclusions.
B.7.4.2 The expansion
mechanism of the expansion anchor shall be tested for the installed condition
by one of the following methods:
a) The mechanism shall be
actuated and tested during installation by preloading the expansion anchor
to a minimum value as specified by the Engineer.
b) A random selection of
the installed expansion anchors shall be load tested to a minimum of 100
percent of the design strength. The testing program shall be established
by the Engineer.
B.7.5-Expansion anchor selection
The Engineer shall review the expansion anchor design
features, failure modes, test results, and installation procedures prior
to selecting a specific expansion anchor for an application. Expansion
anchors shall not be used to resist vibratory loads unless tests are conducted
to verify the adequacy of the specific expansion anchor and application.
In the selection of expansion anchors, consideration shall be given to
expansion anchor performance in cracked concrete.
B.8-Inserts
Concrete inserts shall be specified in accordance
with B.6.1 and tested in accordance with B.7.4.1.
B.8.1-Design requirements
When inserts cannot be designed to meet the requirements
of B.4, B.5, and B.6, the design strength shall be based on actual test
data of tests performed on inserts embedded in concrete. The tests shall
cover the full range of possible loading conditions.
B.8.2-Strength reduction factor
When inserts cannot be designed to meet the requirements
of B.4, B.5, and B.6, a factor of 0.5 shall be applied to the average
test failure loads in determining design strength.
B.9-Grouted embedments
B.9.1 Grouted embedments shall meet the applicable
requirements of B.4, B.5 and B.6.
B.9.2 For general grouting purposes the material
requirements for cement grout shall be in accordance with Chapter 3 of
this code. Special grouts used to achieve certain properties such as high
strength, low shrinkage, or expansion shall be the responsibility of the
Engineer and specified in contract documents.
B.9.3 Grouted embedments shall be tested
to verify anchorage strength. Grouted embedments installed in tension zones
of concrete members shall be capable of sustaining design strength in cracked
concrete. Tests shall be conducted by an independent testing agency and
shall be certified by a professional Engineer with full description and
details of the testing programs, procedures, results, and conclusions.
B.9.4 Grouted embedments shall be tested
for the installed condition by testing randomly selected grouted embedments
to a minimum of 100 percent of the required strength. The testing program
shall be established by the Engineer.
B.9.5 The tests required by B.9.3 and B.9.4
may be waived by the Engineer if tests and installation data are available
to insure that the grouted embedment will function as designed or if the
load transfer through the grout is by direct bearing or compression.
B.10-Fabrication and installation
Welding of attachments to large embedments shall
be in accordance with good practice to avoid excessive expansion of the
embedment which could result in detrimental spalling or cracking of the
concrete or excessive stress in the embedment.