where: Masonry crushing where:
where:
where: c a1 = Distance from the center of an anchor shaft to the edge of masonry in one direction, in.
ψ ec, V, m = Breakout eccentricity factor( 17.7.2.3) =
1 1 + e ' V
1.5c a1
≤ 1.0
ψ ed, V, m = Breakout edge effect factor( 17.7.2.4) = 1.0 if c a2 ≥ 1.5c a1
= 0.7 + 0.3 c a2 1.5c a1 if c a2 < 1.5c a1
c a2 = distance from center of an anchor shaft to the edge of masonry in the direction perpendicular to c a1, in.
ψ m, V = Breakout cracking factor( 17.7.2.5)
= 1.0 where analysis indicates cracking at service load levels
= 1.4 where analysis indicates no cracking at service load levels
ψ h, V = Breakout thickness factor( 17.7.2.6) = 1.0 if h a ≥ 1.5c a1
h a = thickness of member in which an anchor is located, measured parallel to anchor axis, in.
V b, m = Basic single anchor breakout strength in shear, lb( 17.7.2.2)
V b, m1 =( 7) l e d a
0.2
= MIN [ V b, m1; V b, m2 ]
d a f ′ m c a1
1.5
V b, m2 = 9 f ′ m c a1
1.5
l e = MIN [ 8d a; h ef ] for anchors with a constant stiffness over the full length of embedded section
V mc = 1750 × 4 f ′ m A se, V
Masonry crushing failure is based on an equation that has been used by The Masonry Society( TMS) 402 anchor design provisions for cast-in anchors. Masonry is often a softer material compared to concrete. When exposed to high shear loading, the steel anchor may crush and sink into the block of the masonry. Thus, this failure mode is unique to anchor design in masonry.
Compared to anchor design in concrete, there are a couple notable differences unique to the design process for masonry. The effectiveness factor for breakout strength in masonry( k m) will be lower than what is typically used for concrete( k c) to account for the inhomogeneity of masonry materials in breakout. Breakout cones for CMU construction can be greatly influenced by the presence of hollow head joints, which is the vertical mortar joint between two closed-ended CMU blocks in the same course and wythe. In addition to the ends and edges of walls, the nearest hollow head joint on a horizontal projection from the anchor shall be treated as an edge for design purposes. The minimum distance from the nearest adjacent hollow head joint as measured from the centerline of the hollow head joint in CMU construction shall be 2 inches for adhesive anchors or the c min, HJ value for mechanical anchors, which is provided in the product installation information tables. Please see design considerations section for helpful illustrations on understanding edge distances to hollow head joints and projected breakout failure areas.
The design example provided later in the guide will be based on fully grouted CMU construction.
Ungrouted CMU construction
Based on Section 3.4 of ICC-ES AC58, the tension failure modes for adhesive anchors in ungrouted CMU construction are steel failure and pullout failure. The shear failure modes for adhesive anchors in ungrouted CMU construction are steel failure, anchorage failure, and masonry crushing failure. The corresponding equations are provided below:
Tension— Nominal strengths
Masonry pryout V mpg = k mp MIN [ N mbg; N mag( Adhesive Anchors Only)]
Nominal Strength Equation 1, 2 Steel Strength
N sa = A se, N ƒ uta
Pullout Strength
N k, ug 1 N sa discussed on page 4. 2 N k, ug shall be taken from 3rd party evaluation report such as ICC-ES ESR or IAPMO UES ER. where: k mp = 1.0 for h ef < 2.5 in. k mp = 2.0 for h ef ≥ 2.5 in. N mbg = Nominal masonry breakout strength in tension, lb N mag = Nominal bond strength in tension, lb
Shear— Nominal strengths
Nominal Strength |
Equation 1, 2 |
Steel Strength |
V sa = 0.6A se, V ƒ uta |
Anchorage Strength
V s, ug
Masonry Crushing V mc, ug = 1750 ×
4 f ′ m A se, V
1 V sa and V mc, ug discussed on pages 5-6. 2 V s, ug shall be taken from 3rd party evaluation report such as ICC-ES ESR or IAPMO UES ER.
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