Spread Footings On Rock
Spread Footings on Rock
When spread footings are utilized for bridge foundations in West Virginia, they must bear on bedrock at a minimum depth of 1 foot below the recommended rock scour depth. To ensure stability, footings are initially sized based on the relationship between breadth and eccentricity. Specifically, the breadth (and length, if applicable) should satisfy the following condition:
This requires that eccentricity values be established during the Log, Tip, and Type (LTT) meeting when site conditions indicate spread footings as the preferred foundation type.
Rock Mass Rating (RMR) System
Once the footing is initially sized, bedrock strength parameters and bearing resistance are calculated using the Rock Mass Rating (RMR) system. The WVDOH mandates the following criteria:
- Standard: Use the LRFD Bridge Design Specifications, 2012 edition. Do not use later editions for these specific calculations.
- RMR < 50: Utilize the general bearing resistance equation (LRFD Equation 10.6.3.1.2a-1).
- RMR ≥ 50: Utilize the lower bound equation (LRFD Equation 10.8.3.5.4c-2).
- Engineer Latitude: If RMR values vary within 3 points of the 50-point threshold, the geotechnical engineer may use professional judgment to select the most appropriate equation for the site.
Bearing Resistance and Equations
When utilizing the general equation, rock strength parameters are estimated using the Bieniawski (1989) correlations:
- c = 0.104 x RMR (ksf)
- φ = 5 + RMR/2
Correction Factors and Adjustments:
- Groundwater: Correction factors are taken as 1, as the RMR is already discounted for water. Use the buoyant weight of rock for footings subject to submergence.
- Joint Orientation: No discount to the RMR for joint orientation should be made from Table 10.4.6.4-2 of the LRFD.
- Inclined Bedding: If bedrock is dipping between 20° and 80°, the resistance factor must be reduced by 0.50 ($$\Phi = 0.225$$) to account for loss of bearing.
- Standard Resistance Factor: A factor of 0.45 is used for factored bearing resistance calculations under normal conditions.
Eccentricity and Maximum Bearing Pressure
Factored bearing resistance must be compared against the factored bearing pressure. LRFD standards require that the stress distribution on rock be modeled as triangular or trapezoidal for eccentrically loaded footings. The maximum bearing pressure at the toe is estimated using a 1-foot strip basis:
- σvmax = ΣV/B(1+6e/B) (LRFD Equation 11.6.3.2-2 on a 1-foot strip basis)
Sliding and Bedrock Settlement
Base Sliding: Because footings are recessed 1 foot into bedrock and poured directly against the rock, base sliding is typically not a primary concern. However, if bedrock dips more than 20° in an unfavorable orientation relative to the slope, a base sliding check must be performed in the strength limit state using the friction angle of the bedding plane.
Settlement and Tilt: While elastic compression of bedrock is usually negligible for standard bridge loads, it must be evaluated for high bridges or those supported on weak rock. In the case of tall piers, even minor differential elastic settlement can result in significant side-sway or tilt. The geotechnical engineer must specify in the report whether a bedrock settlement check is required or if it is considered negligible based on service limit state analysis.
