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Summary Section of Stem Tab

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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 + fb/Fb.
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 stems, the shear stress is calculated as Total Shear Force / An. For concrete stems, the shear stress is calculated as Total Shear Force / (12" * 'd').

 

Allowable Shear: For masonry stems designed by ASD, the allowable shear stress is calculated per TMS 402-16 Eqn. 8-26 as follows:

               where need not exceed 1.0

 

Since no contribution of shear strength is assumed from reinforcing steel in a retaining wall, nor is the stem a partially grouted shear wall, TMS 402-16 Eqn. 8-22 reduces to Fv = Fvm

 

An upper bound limit Fv,max is imposed on Fv such that Fv ≤ Fv,max where:  

a)   where  

b)   where  

c)and Fv,max is linearly interpolated for values of between 0.25 and 1.0.
 

 

For masonry stems designed by LRFD, the nominal shear strength is calculated per TMS 402-16 Eqn. 9-20 as follows:

 

               where need not exceed 1.0

 

Since no contribution of shear strength is assumed from reinforcing steel in a retaining wall, nor is the stem a partially grouted shear wall, TMS 402-16 Eqn. 9-17 reduces to vn = vnm

 

An upper bound limit vn,max is imposed on vn such that vn ≤ vn,max where:  

a)   where  

b)   where  

c)and vn,max is linearly interpolated for values of between 0.25 and 1.0.

 

 

For concrete stems designed per ACI 318-14 and earlier, the nominal shear strength is 2*λ*sqrt(f'c).

 

For concrete stems designed per ACI 318-19, the nominal shear strength is per the one-way shear strength provisions of §22.5 and Table 22.5.5.1 (as referenced by §13.3.6.1 and §7.5.3.1). Concrete stems are assumed to be unreinforced for out-of-plane shear. Therefore, Av is less than Av,min, and equations (a) and (b) from Table 22.5.5.1 need not be considered.

 

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 20.22.10.9, 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. (25.4.2.3a) from ACI 318-14 and Eq. (25.4.2.4a) from ACI 318-19 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

 

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

 

 

Note:  IBC 2021 references ACI 318-19.  ACI 318-19 Section 25.4.10 prohibits the use of the (As required / As provided) term:

Excess reinf factor not allowed

 

 

In codes prior to IBC 2021, the following formula was used:

 

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

 

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

 

 

 

 

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 20.22.10.9, 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, 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 318.

 

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.

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