Structural Components
The following parts of this subsection show by examples
and charts the methods for designing:
- axially loaded bearing walls
- walls subjected to lateral wind loads
- bond beams subjected to uplift
- shear walls
- lintels
all utilizing AERCON standard block, ValuBlock and U-block.
The design methods presented are for single story construction,
but are also applicable to multi-story construction.
- Block wall
- Tie-down
- Bond beam
- Shear wall
Allowable Vertical Loads
AERCON block walls are solid walls that provide excellent
load carrying capacity for axial loads. The solid block provides
a full bed surface area for AERCON mortar.
Formulas:
The superimposed eccentric axial load is applied at the top
of the wall. The dead weight of the total wall height is calculated and added to
the total superimposed load to determine the total axial design load. The
allowable axial compressive stress is calculated based on the slenderness ratio. The
allowable flexural compressive stress is then calculated. The actual axial
compressive stress (fa) and the actual flexural compressive stress (fb) are
derived in terms of the geometric characteristics of the AERCON wall. All of
these values are substituted into the unity equation and the allowable superimposed
axial load at the resultant eccentricity is solved for. The maximum axial load at
the resultant eccentricity is also calculated based upon the allowable flexural tensile
stress. The maximum axial load at the resultant eccentricity is then the smaller
of the values calculated using either the unity equation or the allowable flexural
tensile stress.
Wind Loads
AERCON block walls are solid walls that provide excellent
resistance to lateral wind loads. Walls built of solid AERCON block are easy to
design and construct. The solid block provides a full bed surface area for
AERCON mortar and, therefore, a full block sectional area for resisting lateral
wind loads which cause out of plane bending. All lateral wind loads are
resisted by the flexural capacity of the masonry with the tensile
stress governing the design. The tie-down reinforcing through the bond
beam, either a threaded rod in a narrow chase or standard reinforcing
bars grouted solid in pre-drilled cores, provides all of the resistance to uplift
that is required.
Formulas:
Design assumptions: (1) all uplift is transferred along the
bond beam to the vertical tiedown reinforcing; (2) all wind loads are distributed vertically
to the bond beam/roof diaphragm and to the floor slab; (3) AERCON units are
unreinforced and the allowable flexural tensile stress controls; (4) the
wall section is considered uncracked in order to use the flexural tensile method; (5) the
top of the wall is considered pinned; and (6) the bottom of the wall is considered
as providing some moment resistance.
The maximum moment at the base of the wall is calculated using
one-half of the AERCON allowable flexural tensile stress. This value is conservative
compared to the allowable stress specified in ACI 530-02. The actual maximum wind load
moment is then determined and the actual maximum bending stress is calculated based upon
the wall section properties. The actual axial compressive stress due to the dead
weight of the upper portion of the wall is determined and then added to the
allowable flexural tensile stress, which is increased by one-third for wind loads, to
give the allowable total flexural tensile stress. The actual flexural tensile stress is
then compared to the allowable total flexural tensile stress.
Bond Beams Utilizing AERCON U-Block
Bond beams can be constructed using AERCON U-block to
create a continuously reinforced structural element. Two continuous #5
reinforcing bars are held securely in place within the U-block, one above the
other, accurately positioning the reinforcing bars for resistance to uplift
loads.
Formulas:
Design assumptions: (1) the bond beam capacity is based
upon the size and strength of the concrete "core" within the Ublock;
(2) the bond beam is a multi-span, continuous beam;
(3) the capacity is based upon the cracked section of the reinforced
concrete "core" using Working Stress Design principles.
The bond beam is checked for shear capacity and moment
capacity (reinforcing in tension and concrete in compression). The allowable
deflection is L/600.
For wind uplift loads, all allowable stresses are increased by
one-third.
Shear Walls
AERCON shear walls are solid block walls that provide
excellent load carrying capacity for diaphragm loads that are
transmitted to them from floors or roofs. Shear walls built of
solid AERCON block are easy to design and construct. The solid
block provides a full bed surface area for AERCON mortar,
and therefore, a full block sectional area for shear loads.
The tie-down reinforcing, either a threaded rod in a narrow
chase, or a threaded rod in ungrouted pre-drilled cores, or standard reinforcing bars
grouted solid in pre-drilled cores, provides all of the tensile capacity required.
Formulas:
The total lateral force to each shear wall is determined using
a typical load apportioning analysis. The net overturning moment is determined and
then compared to the resisting moment capacity in compression of the AERCON block and
the resisting moment capacity in tension for the tie-down. Finally the shear capacity of the
AERCON block is checked.
AERCON Lintels
AERCON's product line includes two alternatives for
load bearing lintels: manufactured reinforced lintels and concrete filled
U-block. The minimum bearing length for either style of lintel is 8", across
the full thickness of the wall. A longer bearing length is acceptable in order to
utilize standard elements to accommodate openings of various widths. To ensure
a uniform bearing stress at each end of the lintel, the bearing surface
must be true and level. Usually the bottom of the rough opening does not match
the block coursing, so an adjustment for the bearing height of the lintel
must be incorporated. This is most easily accomplished by cutting pieces of
block to the height required to achieve the desired bearing elevation.
Adjustment pieces above the lintel may also be required to re-align the block
coursing. The minimum height of an adjustment piece is 3".
Manufactured Lintels
AERCON manufactures readyto-install reinforced lintels in a
variety of sizes and lengths. The accompanying table indicates the allowable
superimposed loads for these standard pieces. The minimum lintel thickness that
attains the tabulated values is 8". For opening widths not specifically listed
in the table, the value for the standard opening width that is larger than the
actual width can be conservatively used. An example is included to demonstrate
the use of this table. Since the reinforced lintels are uniquely manufactured
based on size and load capacity, they cannot be cut, penetrated, or modified
without authorization from an AERCON Representative. Each manufactured lintel
has a marking indicating the orientation for installation.
Concrete Filled U-Block Lintels
As an alternative, AERCON's U-block can also be used as the
"formwork" to create a cast in place concrete lintel. The benefits of using
AERCON Ublocks include: the exterior and interior surfaces of the wall
are AERCON material, no external formwork is required for the concrete pour,
design theory for conventional concrete is utilized to allow evaluation for
any opening size, desired elevations can be achieved by cutting the U-block height to
adjust for coursing differences, U-blocks can be arranged and poured on site
prior to installation in the finished wall if desired. Additionally,
U-blocks can be stacked in place to minimize the pouring sequence when the lintel
is located near the bond beam course. For this condition, the upper U-block
course must be modified to allow the concrete fill to reach the lower U-block course.
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