Designing Single Layers of Reinforcement

Eriksson Culvert assumes two layers of reinforcement. In this document we will discuss the steps needed to use Eriksson Culvert for the design of culverts with one layer of reinforcement. For this discussion, we must also include some general discussion of minimum reinforcement (as found in LRFD 7^th Edition and newer). In addition, we will use the Eriksson Culvert reinforcement designations for reinforcement, which differ from ASTM designations.

Step 1: Coordinate the cover dimensions (shown in Figure 2), with the assumed reinforcing diameter (shown in Figure 3). Both the inside and outside cover dimensions should be set equal to ½ of the member thickness minus ½ of the reinforcing diameter. The input shown in Figures 2 and 3 assume member thicknesses of 10” and a reinforcing diameter of 1”. Both of these inputs are found in the Culvert Properties dialog box.

Step 2: In the Reinforcement Dialog Box, make sure that the Base Area is set appropriately (LRFD 12.11.4.3.2 recommends 0.03 in²/ft, some states have different requirements), and the Maximize check box has been checked, see Figure 4.

Step 3: Main flexural reinforcement. The main flexural reinforcement is the reinforcement that is placed in the tension zones. For typical 3- and 4-sided single cell culverts, the tension zones are the top and bottom outside corners, the bottom of the top slab, the top of the bottom slab, and the exterior of the walls. Therefore, the main flexural reinforcement is contained in the A1, A2, A100, A200 and B2 reinforcing zones.

Eriksson Culvert assumes a singly reinforced section, so for a proper analysis, set the interior and exterior reinforcement the same size and spacing in those zones listed above (note that there is no interior layer of reinforcement for the A1 and A2 zones), and check the design in Analysis mode. The reinforcement size and spacing used in these zones is the amount you need in your single layer cage. For example, the A100 and A300 zones should have the same size/spacing pairs, which will probably be controlled by the demand in the A100 zone. For the single layer cage, you need not add the reinforcement from the interior and exterior zones together. If your analysis requires #6 bars at 12” on center in both the A100 and A300 zones, the single layer cage need only contain #6 bars at 12” on center (NOT #6 bars at 6” on center).

To meet minimum reinforcement requirements, check that the amount of reinforcement in each of these zones is greater than the base area * 2, or for the typical case, 0.06 in²/ft.

Step 4: Transverse reinforcement for walls (C1 zones in Figure 1 above). The minimum transverse reinforcement required in the walls for a single layer is the base area * 2. Note that if you use mesh, this minimum may be met by the transverse wires in the mesh sheets.

Step 5: Transverse reinforcement for the bottom slab (C1 and C200 zones above). As with walls, the minimum transverse reinforcement required in the bottom slab for a single layer is the base area * 2. Again, note that for mesh users this requirement may be met by the transverse wires in the mesh sheets.

Step 6: Transverse reinforcement for the top slab (C1 and C100 zones above). For top slabs with fill depths less than 2’, the minimum transverse reinforcement for a single layer is the greater of the required distribution steel (LRFD 9.7.3.2) or 0.002 * b * h, plus the base area, or two times the base area (in mathematical terms: minimum area = max(max(distribution steel, 0.002*b*h) + base area), 2 * base area). You can run Eriksson Culvert in Design Mode and check the required area in the C100 zone to determine the greater of distribution steel and 0.002 * b * h, as this is the calculation the program performs for the C100 steel (LRFD 7^th Edition or newer). For top slabs with fill depths greater than or equal to 2’, use 2 * base area as a minimum. See note for mesh users above.

Step 7: Temperature and shrinkage steel. For box structures with fabricated lengths greater than 16’, you must also check shrinkage and temperature requirements.