FM 5-212 Chptr 5 MGB 13-22 Bays With LRS

CHAPTER 5

MGB 13-22 BAYS WITH LRS
(where water level or any obstructions are at least 3.7m below bank heights)

STEP 1. Measure the AR gap (see MGB Design, Step 1).

STEP 2. Select a bridge.

Using column (a) of Table 14, choose a bridge whose AR gap range brackets the AR gap measured. Always select the smallest range possible to avoid wasting assets. For example, if the AR gap measured 40.2m, choose the AR gap range of 37.8m to 40.5m even though the range of 39.8m to 42.4m also meets the criteria. Read the bay configuration column (c). Check the MLC of the bridge column (d) to ensure that it meets what is specified by the tasking authority.

STEP 3. Read the bridge length column (b).

STEP 4. Read the R distance column (f).

STEP 5. Read the nose construction column (e).

STEP 6. Identify key construction points.

These are constant for any DS bridge construction 13 through 22 bays with LRS (Figure 9).

F peg -- Designates the approximate location of the far bank end of bridge. It is initially placed the distance shown under far bank minimum column, Table 15, from the AR peg on the far bank.

CRB peg -- Designates the location of the CRB. It is placed 2.7m from the A' peg on the near bank as measured to the centerline of the CRB.

FRB peg -- Designates the location of the FRB. It is placed 9.1 m from the CRB peg to the centerline of the FRB.

RRB peg -- Designates the location of the RRB. It is placed 4.6m from the FRB peg (measured centerline to centerline).

O peg -- Marks the clear distance behind the CRB required to construct the bridge. It is positioned by measuring the R distance, Table 14, column (f), behind the CRB peg.

Bearing Check. The minimum/maximum bearings for any DS bridge 2E + 13 through 2E + 22 bays are shown in Table 15.

To calculate the actual locations of the F and F' pegs, the following procedure is used:

Near bank bearing = bridge length - (AR gap + far bank bearing) where --

Bridge length is obtained from column (b) of Table 14.

The AR gap was measured by you in the first step of this design procedure.

Far bank bearing is obtained from the appropriate column in Table 15.

If the near bank bearing is within acceptable limits, you do not have to adjust the position of the F peg. Its final position will be the initial value obtained from Table 15 as measured from the A peg. The F' peg will be located at a distance equal to the near bank bearing measured from the A' peg on the near bank.

If the near bank bearing is greater than the maximum allowable (2.3m), you must do one of the following:

Move the F peg further away from its present location to a point where the amount of bearing on near bank is less than or equal to 2.3m and greater than or equal to the value given in Table 15 for minimum bearing.

Crib up the near bank end of bridge to where the maximum allowable bearing is not exceeded.

Dig out the soil from the near bank until the maximum allowable bearing is not exceeded.

Physically locate the key construction points (Figure 10) on the ground and take elevations relative to the CRB peg. Locate the O, RRB, FRB, F', A', A, and F pegs on the ground along the bridge centerline. Estimate elevations of F, RRB, and O pegs relative to the CRB peg. A positive value indicates a point is above the CRB peg and a negative value indicates it is below the CRB peg. The CRB peg will always be 0.0 elevation. Place key construction point elevations on the baseline (Figure 10).

STEP 7. Slope check.

Ensure that the difference in elevation between the F' and F peg does not exceed one-twentieth of the total bridge length. If it does, you will have to crib up, undertake a major construction project, or choose another site. Note that the elevation of the F' peg cannot be lower than the elevation of the CRB peg or the bridge will not receive full bearing. In these cases, the normal procedure is to crib up or fill in until the elevation of the F' peg is at least as high as the CRB peg. Otherwise, you would have to remove soil next to the bank, level with the F' peg. This same rule applies to the F peg.

STEP 8.

Calculate the far bank height (H), and the near bank tall clearance (G) relative to the baseline.

STEP 9. RULE 1

Use the LNCG setting to give adequate nose clearance (N) and tail clearance (T). See Table 14.

Choose a setting from column (h), (i), or (j) which gives an N greater than H. If none of the three choices meet the criteria, choose the highest value available.

Check to see if the value from column (g) is greater than G.

If N is not greater than H or if T is not greater than G, proceed to RULE 2.

If N is greater than H and T is greater than G, the LNCG setting chosen has adequate nose clearance and the bridge selected has adequate tail clearance.

STEP 10. RULE 2

Lowering the RRB. If there is ample tail clearance, some increase in N can be obtained by keeping the CRB in its highest position and lowering the RRB to its lowest position. See Table 14, column (k), for values.

STEP 11. Loads required.

From Table 16, determine the truck and trailer loads required for the bridge.