Structural Systems - Soils and Foundations

Rigid Diaphragm & Lateral Systems

• This week's content includes two problems focusing on rigid diaphragms and developing lateral systems for projects.

• A pre-recorded video example covering rigid diaphragms is available on Box, substituting the lab session.

• The second homework component involves developing a lateral system for projects, building on the previous week's work.

• The assignment includes four drawings: floor plan, roof framing plan (previously completed), and elevation views (or sections) of the lateral systems.

• Elevation views should call out lateral system components like HSS steel braces, wide flange steel braces, moment frame columns/beams, or shear walls.

• For brace frames, braces are called out; for moment frames, beams and columns are specified; for shear walls, material is identified.

• Plans are required to indicate where elevations are taken.

• Buildings must be stable, having at least two vertical lateral force resisting systems in each direction.

• Diaphragms with supports at major grid lines are statically determinate, acting as continuous beams.

• Minimum two supports are needed in each direction to prevent rotation.

• Diagonal members can be rationalized as supports in X and Y directions.

• The assignment aims to apply concepts from class to the project; designs don't need to be economical or architecturally conventional but must demonstrate a stable superstructure for both gravity and lateral loads.

• Individual questions can be addressed during office hours via scheduled email appointments.

Soils and Foundations

Introduction

• The lecture transitions to soils and foundations, completing the building's load path.

• The order of consideration is typically gravity system, lateral system, then foundations.

• Foundations resist forces in Y (vertical), X (horizontal), and moments to maintain building stability.

• The Earth resists building forces. Soils have structural qualities, causing deformation under load and limiting soil's structural capacity.

Bearing, Sliding, and Overturning

• Structural engineers check three main criteria:

  • Bearing: Soil's ability to resist vertical load.
  • Sliding: Soil's ability to resist horizontal load, primarily through friction ($Friction = Coefficient <br />\newline of <br />\newline Friction <br />\newline * Dead <br />\newline Load$).
  • Overturning: Building's resistance to toppling over, assessed using overturning calculations.

• Overturning resistance is analyzed by comparing the overturning moment ($Moment<em>{overturning} = Lateral \newline Force \newline * Eccentricity</em>{overturning}$) to the resisting moment ($Moment<em>{resisting} = Self \newline Weight \newline * Eccentricity</em>{resisting}$).

• Increasing weight by deepening the foundation or expanding its length can enhance overturning resistance.

Soil Properties and Geotechnical Engineers

• Geotechnical engineers determine soil properties; soils exhibit nonlinear behavior, unlike steel, concrete or wood.

• Soil behavior varies with depth and load, leading to unpredictable responses.

• Geotechnical engineers sample and test soils to provide reliable design numbers.

• Building codes offer guidance for simple, one-story homes without requiring geotechnical engineers; mid-rise, high-rise, and seismic areas require geotechnical input.

Unified Soil Classification System (USCS)

• The Unified Soil Classification System organizes soils by granular size using sieves.

• Particle size is correlated to soil strength (bearing, sliding), but saturation and plasticity also influence strength.

• USCS categorizes soils using group symbols (letter classifications) like well-graded gravels or sandy gravels.

• Classifications indicate particle size: boulders/cobbles (large), gravel/sand (smaller), silt/clay (molecular), peat/organics (biological).

• The International Building Code (IBC) provides baseline pressure values based on soil classification (Table 1806).

• Minimum assumed bearing pressure without a geotechnical engineer is 1,500 pounds per square foot (psf), associated with clay.

• For sliding resistance, friction (larger molecules) as well as cohesion (clays) are considered.

• Clays have cohesion due to molecular attraction, providing uniform pressure resistance (e.g., 130 psf).

• The lower value between friction and cohesion is typically used; geotechnical reports provide specific values.

Soil-Related Factors

• Factors affecting structural behavior include:

  • Settlement: Vertical deformation of soil under load; can be uniform or differential.

    • All structures settle to some extent.
    • Differential settlement causes unplanned structural failures; design avoids differential settlement.

  • Frost Heave: Water in soil expands upon freezing, causing upward pressure on foundations.

    • Frost line/depth: Elevation where water freezes; foundations should be built below the frost line.

  • Hydrostatic Pressure: Lateral pressure from water in soil against basement walls.

    • Water penetrates concrete micro-cracks, leading to interior water damage.
    • Drainage tile is used to remove water from walls.

  • Buoyancy: Upward pressure from water on submerged structures.

    • Water pressure increases linearly with depth.
    • Can cause slab uplift and water intrusion; drainage tiles are used under slabs.

  • Seismic Resistance: Soil's ability to maintain stability during lateral forces; influences site soil classification.

    • Liquefaction: Soil loses strength and behaves like liquid during earthquakes, causing buildings to sink.
    • Site soil classifications determine seismic design categories.

Types of Foundations

• Two general categories:

  • Shallow Foundations: Typically above the frost line, less than 10 feet deep.

  • Deep Foundations: Significantly deeper, used when near-surface soil is poor or for added capacity.

Shallow Foundations

• Common types include:

  • Strip Footings: Continuous concrete footings supporting walls.

  • Spread Footings: Square concrete footings supporting columns.

  • Mat Foundations: Solid concrete slabs supporting multiple elements.

Strip Footings

• Strip footings are analyzed on a per-foot basis due to uniform load distribution from the wall.

• Forces from the wall are transferred uniformly, and the footing wings act as cantilever beams.

• The linear load (w) on the footing is calculated as the wall force divided by footing width.

• Design considerations include bearing capacity, reinforcement for flexure (bending), and shear resistance.

• Eccentric loads create moments in the soil interface, resisting with a linearly changing bearing pressure.

Eccentric Footings and Kern

• Goal: keep the loads inside the kern.

• $Q = \frac{M*6}{L^2}$

• Kern: the area that is L/6 within the centroid of the foundation that results in still purely positive pressure.

Spread Footings

• Spread footings are similar to strip footings but designed as slabs, accounting for two-way bending and shear.

• Slab reinforcement is needed to handle cantilevered bending at each interface.

• Punching shear: Special consideration is given to punching shear, where the column tries to punch through the slab.

• Eccentric loads create compounding pressures, highest at one corner and lowest at the opposite.

Mat Foundations

• Mat foundations are also like spread footing, but over multiple elements.

• Mat foundations spread loads, connect foundations structurally, and increase the bearing area.

Other Shallow Foundation Systems

• Tie Beams: Tie foundations together; used to keep loads inside the edges of foundations.
• Grade Beams: Literally structural concrete beams that are supporting slab on grade at the foundation level; structural concrete beam spanning over weak areas of soil.

Load Combinations

• ASD Load combinations are to be used when checking soil pressures and soil qualities.
• Sliding check has a minimum of 1.5 factor of safety.

Deep Foundations

• Deep foundations are used when surface soils are poor, needing access to deeper, more robust strata (often bedrock).

• They offer added capacity through bedrock resistance and skin friction along the sides, resisting both uplift and downward forces.

• Deep foundations resist lateral force through direct interface or by socketing and bending.

• Two types of deep foundations:

  • Pile foundations: slender compared to length.

  • Pier foundations: Short and stout compared to length; wider compared to its length; smaller than the factor of 12 with the least dimension.

• Materials can be precast concrete, side cast concrete, hot rolled steel sections, heavy timber, or combinations of these.

• Piles can be displacement (compacting soil) or nondisplacement (removing soil).

• Caissons are tubing used as formwork and provide structural capacity; good for keeping water out, especially for infrastructure.

• Deep foundations can have belled bottoms for better bearing resistance.

• Micropiles are slender concrete piles, often used with pile caps for connectivity and lateral capacity, sometimes battered (angled) for truss action.