Shallow Foundations Flashcards
Definitions
What is ULS?
Ultimate Limit State corresponds to the maximum load-carrying capacity of the foundation. An ultimate limit state corresponds to collapse. AKA strength limit state.
What are the 5 failure modes of shallow foundations?
- Bearing capacity of soil exceeded
- Excessive loss of contact, eccentricity.
- Sliding at the base of footing.
- Loss of overall stability, global stability,
- Structural Capacity exceeded.
What is SLS?
Serviceability Limit State. Corresponds to loss of serviceability and occurs before collapse.
How may a serviceability limit state be reached?
- Excessive differential or total foundation settlements.
- Excessive lateral displacement
- Structural deterioration of the foundation.
What is the definition of a shallow foundation?
Wherein the depth, Df is small compared to the cross-sectional size (width, Bf, or length, Lf.
What are Isolated Spread Footings?
Footings with Lf/Bf ratio less than 20.
How are Isolated Spread Footings designed?
They are designed to distribute the concentrated loads delivered by a single column to prevent shear failure of the soil beneath the footing.
What is a Continuous or Strip Footing?
They are footings with an Lf/Bf ratio greater than or equal to 10.
What is a spread footings with a cantilevered stemwall?
An earth retaining system consisting of a spread footing supporting a cantilevered retaining wall is frequently used to resist lateral loads applied by backfill and other external loads that may be acting on top of the backfill.
What are the four functions of bridge abutments?
- Retain the earthen backfill behind the abutment
- Support the superstructure and distribute the loads to the bearing materials below the spread footings, assuming that a spread footing is the foundation system chosen for the abutment.
- Provide a transition from the approach embankment to the bridge deck.
- Depending on the structure type, accommodate shrinkage and temperature movements within the superstructure.
List the three steps to a foundation design.
- Determine the direction, type and magnitude of foundation loads to be supported, tolerable deformations and special constraints such as underclearance requirements that limit allowable total settlement, structure type and span length that limits allowable deformations and angular distortions, time constraints on construction, extreme event loading and construction load requirements.
- Evaluate the subsurface investigation and lab data with regard to reliability and completeness.
- Consider alternative foundation types where applicable
What is a mat foundation?
A mat foundation consists of a single heavily reinforced concrete slab that underlies the entire structure or a major portion of the structure. Mat foundations typically support a number of columns and/or walls in either direction or a uniformly distributed load such as that imposed by a storage tank.
What is an advantage of a mat foundation?
Is its ability to bridge over local soft spots and to reduce differential settlement. Column and wall loads are transferred to the foundation soils through the mat foundation.
Are mat foundations typically used for bridge support?
They have limited applicability for bridge support, except where large bridge piers, such as bascules or other movable bridge supports, bear at relatively shallow depth without deep foundation support.
What is the the two-part process of designing a spread footing?
- Allowable soil bearing capacity must be calculated to ensure the stability of the foundation and determine if the proposed structural loads can be supported on a reasonably sized foundation.
- The amount of settlement due to the actual structural loads must be predicted and the time of occurrence estimated.
