Summary Section of Stem Tab

The summary section indicates the results of the Stem design at-a-glance.

Interaction Ratio:

The interaction ratio indicates the efficiency of your design, not to exceed 1.0.

For masonry using ASD this is the computed ratio of fa/Fa + Mactual/Mallowable. For concrete and masonry using LRFD it is Mactual/Mallowable.

The weight of the stem will be included only if there is added axial load. For masonry stems, Fa is calculated by considering the wall as unsupported with "K" = 2.0. Since even a very small axial load will activate the unsupported height/slenderness calculation for masonry stems, we suggest you do not enter an axial load unless it is significant (e.g. greater than, say, 3000 plf.).

Actual Moment:

This is the maximum moment due to the lateral pressures and applied loads above the "Design Height" location entered. Note that when concrete is used, all soil pressures and loads are factored per default Load Factors for evaluation of moments and shears.

Allowable Moment:

This is the allowable moment capacity. It is Allowable Stress Design (ASD) for masonry, or based upon Strength Design for concrete and when LRFD is specified for masonry. For concrete strength design, the maximum reinforcing steel percentage is controlled by equilibrium at the prescribed strain limits.

Total Force:

This is the total lateral force from loads applied above the "Check Design at Height" location entered. This force is factored for concrete and masonry using the LRFD method. Forces applied to compute overturning, sliding, and soil pressure are not factored.

Actual Shear:

For masonry, the effective thickness is used to calculate the actual shear. The effective thickness is the actual "d" distance for the moment applied, considering partial or full grouting (equivalent solid thickness is not used). In other words, the unit shear is determined by dividing the total lateral force of the stem cross section by the product of "d" * 12" unit width strip. Shears are calculated at the "Design height" location entered, not at distance "d" above design height. Concrete stems use an area of "d" x 12" for the shear area, and masonry stems use "d" x 12" per ACI 530-11, Section 2.3.6.1.1.

Allowable Shear:

For masonry designed by ASD according to ACI 530-11, the allowable shear stress varies between 3*sqrt(f'm) and 2*sqrt(f'm) as a function of M/(Vd). No contribution of shear strength is assumed from reinforcing steel in a retaining wall.

For masonry designed by LRFD according to ACI 530-11, the nominal shear strength varies between 6*Anv*sqrt(f'm) and 4*Anv*sqrt(f'm) as a function of M/(Vd). Again, no contribution of shear strength is assumed from reinforcing steel in a retaining wall.

For concrete, the nominal shear strength is 2*λ*sqrt(f'c), per ACI 318-05 Section 11.3.1.1 or ACI 318-08 Section 11.2.1.1 or ACI 318-11 Section 11.2.1.1.

Rebar Lap & Embedment Lengths: Regardless of the stem material, there are two fundamental lengths to calculate: lap splice length and development length. These values are summarized in the "Rebar Lap & Embedment Lengths" table, which can be accessed from the button on the Stem tab. As of build 10.14.7.29, this table is only available in the Cantilevered Retaining Wall module.

The following presents the formulas used and the limits applied to generate the values in that table:

Straight Development Length of Rebar in Concrete: (Applies to all referenced codes)

ld calc = (3/40) * (fy / sqrt(f'c)) * (psis / 2.5) * (bar size / 8)

psis = 0.8 for bar sizes #6 and smaller, 1.0 for bar sizes #7 and larger

ld report = ld calc but not less than 12 inches

(This is Eq. (12-1) with appropriate assumptions for bar location, clear cover, spacing, transverse reinforcing, and epoxy coating.)

Lap Splice Length of Rebar in Concrete: (Applies to all referenced codes)

ls = 1.3 * ld calc but not less than 12 inches

Hooked Embedment of Rebar in Concrete: (Applies to all referenced codes)

ldh calc = 0.02 * (fy / sqrt(f'c)) * (bar size / 8) * 0.7 * (As required / As provided)

(As required / As provided) = the ratio of required to provided area of rebar (this is a user option checkbox)

ldh report = ldh calc but not less than the larger of 8 bar diameters or 6 inches

Development Length of Rebar in Masonry designed by ASD: (Applies to all referenced codes)

ld calc = (0.002) * (bar size / 8) * fs

fs = actual stress in rebar

ld report = ld calc but not less than 12 inches

(This is the IBC equation.)

Lap Splice Length of Rebar in Masonry designed by ASD: (Applies to all referenced codes)

ls = Factor * ld calc but not less than 12 inches or 40 bar diameters

Factor = 1.5 in regions where design tensile stresses in reinforcement are greater than 0.8 * fs, otherwise 1.0.

(As of build 10.14.7.29, the program conservatively assumes a value of 1.5 for the "Factor" referenced above.)

Development Length of Rebar in Masonry designed by LRFD: (Applies to all referenced codes)

ld calc = (0.13) * (bar size / 8)^2 * fy * gamma / (K * sqrt(f'm))

gamma = 1.0 for #3 through #5 bars, 1.3 for #6 through #7 bars, and 1.5 for #8 through #9 bars

K = 1.5 for #3 through #5 bars, 2.0 for #6 through #9 bars

ld report = ld calc but not less than 12 inches

(This is the ACI equation by direct reference from IBC. The value of K has conservatively been set to the required clear cover for the selected bar exposed to earth.)

Lap Splice Length of Rebar in Masonry designed by LRFD: (Applies to all referenced codes)

ls = 1.0 * ld calc but not less than 12 inches and need not be GREATER than 72 bar diameters

General Notes on Rebar Lap & Embedment Lengths:

For concrete stems, a Class B lap splice is assumed (see ACI 318-11, 12.15), therefore the lap length is the bar development length x 1.3. Concrete is assumed to be normal weight, and bars are assumed to be plain (not epoxy coated).

Concrete development lengths are computed per ACI 12.2.

For the bottom Design Height only (Ht. = 0.00), this displays the required hooked bar embedment into the footing. It assumes a bar with a 90° bend and at least a 12-diameter extension.

The minimum footing thickness required is based upon this embedment depth plus the clearance you have specified below the bar (usually 3 inches). If this totals more than the footing thickness you have chosen, a warning message will be displayed.

Note that if the bar extends straight down into a key, it must be embedded by a depth equal to the development length.

The program does not reduce embedment length by stress level unless the user selects the checkbox labeled Reduce Hook Embedment by Percent Rebar Stress.

The program never reduces lap splice lengths by the stress ratio. It is not permitted by the referenced codes.