Concrete
The 28-day concrete strength (f’c), the density of the concrete, the modulus of the concrete (Ec), and the type of concrete may be specified. The modulus of elasticity of the concrete is automatically recomputed when changes are made to either the strength or the density. However, concrete modulus may be overridden. The program uses LRFD 5.4.2.4-1 for this calculation.
The concrete type may also be designated as normal, sand-lightweight, or all-lightweight. LRFD defines lightweight concrete as concrete containing lightweight aggregate and having an air-dry unit weight not exceeding 0.120 kcf. However, if a unit weight is specified as being less than 0.120 kcf and it is desired that the concrete be treated as lightweight for design purposes, the concrete type must be explicitly changed by the user. Note that under the LRFD 7th Edition, you can no longer select the concrete type, rather it is selected for you based on the unit weight, and will either be Normal Weight (>135 pcf) or Lightweight (< 135 pcf).
Mcr Factors
These factors are the gamma factors present in the new Mcr equation introduced in the 6th Edition (LRFD Eq. 5.7.3.3.2-1). Currently Eriksson Culvert only uses the first and third gamma factors. CHBDC uses neither of these factors.
Fr Factors
Here the user can change any of the multiplication factors used in calculating the modulus of rupture (Fr). The first factor is the generic normal weight concrete factor used in both the 5th and 6th Editions of LRFD, along with STND. The second factor is only used in the 5th Edition for the minimum steel cracking moment. The lightweight concrete factors are used in all editions of LRFD prior to the 7th Edition, along with STND and AREMA. The 7th Edition only uses the first factor, and uses this factor for both normal and lightweight concrete. CHBDC uses none of these factors.
Reinforcing Steel
Input values for the yield strength (fy), allowable stress, and modulus of elasticity (Es) of the reinforcing steel. The reinforcing diameter is used by the program to compute the depth to the reinforcement during the design phase. For the calculations of the critical section values, Eriksson Culvert uses the actual diameter of any the calculated or assigned flexural reinforcement. However, in Design Mode, at the time the program settles on a specific member thickness, the size of the reinforcing is not known, therefore the ‘reinforcing diameter’ is used to approximate the thickness of one layer of reinforcement. If you are using mesh, then the default value of 1 inch may be too large. You can also define the yield strength of the shear steel here as well. Note that if you switch from rebar to mesh, the program will automatically change the steel yield as well, defaulting to 60 ksi for rebar and either 65 or 70 ksi for mesh. However, the yield strength of the shear steel will not be changed.
Soil
Input a value for the density and Poisson’s ratio of the soil.
Exposure Factor
Select the exposure factor to be used for the Culvert. If using LRFD Specification the available values are Class 1 Exposure, Class 2 Exposure, and User-Defined. If User-Defined is selected, then you must input a value for Gamma-e. If using Standard or AREMA Specifications the program selects the appropriate value of Z for the user, based on the culvert type (cast-in-place or precast). The values selected by the program reflect the values in the specification. They may be overridden by the user. LRFD does not specify the exposure factor to be used. CHBDC uses a maximum crack width for service load checks, which functions similarly to an exposure factor.
Soil-Structure Interaction
Select either Compacted Fill, Uncompacted Fill or a User-Defined value. If User-Defined is selected, then you must input a value for Fe. Note that for compacted or uncompacted fills, the Fe value listed is used as the maximum possible value, assuming an embankment type of installation. The actual Fe value used by the program may be less than this. If you set the type of interaction to User-Defined, then the program uses whatever value you input for Fe. This is useful for trench installations, so you can calculate the actual Fe based on the soil conditions and input that value here. CHBDC uses both a vertical arching factor and a horizontal arching factor based on the installation type (B1 or B2).
Slope Factor
Select either Granular Soil, Other Soil Types, or User-Defined. If User-Defined is selected, then you must input a slope factor. The slope factor is used for fill depths greater than 2 feet to calculate the live load distribution factor. Note that if you change the specification, then this value may be changed by the program as well. CHBDC uses a fixed value of 1.75 for the slope factor.