twenty three concrete construction: Bright Football Complex - PowerPoint PPT Presentation

A RCHITECTURAL S TRUCTURES : F ORM, B EHAVIOR, AND D ESIGN A RCH 331 D R. A NNE N ICHOLS S UMMER 2018 lecture twenty three concrete construction: Bright Football Complex www.tamu.edu foundation design Foundations 1 Architectural Structures Su2018abn Lecture 23 ARCH 331

Foundation • the engineered interface between the earth and the structure it supports that transmits the loads to the soil or rock Foundations 2 Architectural Structures Su2018abn Lecture 23 ARCH 331

Structural vs. Foundation Design • structural design – choice of materials – choice of framing system – uniform materials and quality assurance – design largely independent of geology, climate, etc. Foundations 3 Architectural Structures Su2018abn Lecture 23 ARCH 331

Structural vs. Foundation Design • foundation design – cannot specify site materials – site is usually predetermined – framing/structure predetermined – site geology influences foundation choice – no site the same – no design the same Foundations 4 Architectural Structures Su2018abn Lecture 23 ARCH 331

Soil Properties & Mechanics • unit weight of soil • allowable soil pressure • factored net soil pressure • shear resistance • backfill pressure • cohesion & friction of soil • effect of water • settlement • rock fracture behavior Foundations 5 Architectural Structures Su2018abn Lecture 23 ARCH 331

Soil Properties & Mechanics • compressibility – settlements • strength – stability • shallow foundations • deep foundations • slopes and walls – ultimate bearing capacity, q u q finehomebuilding.com a  – allowable bearing capacity, u q S.F. Foundations 6 Architectural Structures Su2018abn Lecture 23 ARCH 331

Soil Properties & Mechanics • strength, q a Foundations 7 Architectural Structures Su2018abn Lecture 23 ARCH 331

Bearing Failure • shear slip zone slip zone punched wedge punched wedge Foundations 8 Architectural Structures Su2018abn Lecture 23 ARCH 331

Lateral Earth Pressure • passive vs. active active (trying to move wall) passive (resists movement) Foundations 9 Architectural Structures Su2018abn Lecture 23 ARCH 331

Foundation Materials • concrete, plain or reinforced – shear – bearing capacity – bending – embedment length, development length • other materials (piles) – steel – wood – composite Foundations 10 Architectural Structures Su2018abn Lecture 23 ARCH 331

Basic Foundation Requirements • safe against instability or collapse • no excessive/damaging settlements • consider environment – frost action – shrinkage/swelling – adjacent structure, property lines – ground water – underground defects – earthquake • economics Foundations 11 Architectural Structures Su2018abn Lecture 23 ARCH 331

Generalized Design Steps • calculate loads • characterize soil • determine footing location and depth • evaluate soil bearing capacity • determine footing size (unfactored loads) • calculate contact pressure and check stability • estimate settlements • design footing structure* (factored loads) Foundations 12 Architectural Structures Su2018abn Lecture 23 ARCH 331

Types of Foundations • spread footings • wall footings • eccentric footings • combined footings • unsymmetrical footings • strap footings Foundations 13 Architectural Structures Su2018abn Lecture 23 ARCH 331

Types of Foundations • mat foundations • retaining walls • basement walls • pile foundations • drilled piers Foundations 14 Architectural Structures Su2018abn Lecture 23 ARCH 331

Shallow Footings • spread footing – a square or rectangular footing supporting a single column – reduces stress from load to size the ground can withstand Foundations 15 Architectural Structures Su2018abn Lecture 23 ARCH 331

Actual vs. Design Soil Pressure • stress distribution is a function of – footing rigidity – soil behavior RIGID RIGID sand clay • linear stress distribution assumed Foundations 16 Architectural Structures Su2018abn Lecture 23 ARCH 331

Proportioning Footings • net allowable soil pressure, q net      – q q h ( ) net allowable f c s – considers all extra weight (overburden) from replacing soil with concrete – can be more overburden • design requirement with total unfactored load: P  q net A Foundations 17 Architectural Structures Su2018abn Lecture 23 ARCH 331

Concrete Spread Footings • plain or reinforced • ACI specifications • P u = combination of factored D, L, W • ultimate strength     – V V 0.75 for shear : u c • plain concrete has shear strength     – M M : 0.9 for flexure u n Foundations 18 Architectural Structures Su2018abn Lecture 23 ARCH 331

Concrete Spread Footings • failure modes shear bending Foundations 19 Architectural Structures Su2018abn Lecture 23 ARCH 331

Concrete Spread Footings • shear failure one way shear two way shear Foundations 20 Architectural Structures Su2018abn Lecture 23 ARCH 331

Over and Under-reinforcement • reinforcement ratio for bending A s   – bd – use as a design estimate to find A s ,b,d – max  from  steel  0.004 – minimum for slabs & footings of uniform A s thickness  0 . 002 grade 40 / 50 bars bh  0 . 0018 grade 60 bars Foundations 21 Architectural Structures Su2018abn Lecture 23 ARCH 331

Reinforcement Length • need length, l d – bond – development of yield strength Foundations 22 Architectural Structures Su2018abn Lecture 23 ARCH 331

Column Connection • bearing of column on footing   f c      – P P 0 . 85 A u n 1   A 1 0.65 for bearing A 2 1 A – confined: increase x  2 2 2 A 1 • dowel reinforcement – if P u > P b , need compression reinforcement – min of 4 bars and 0.005A g Foundations 23 Architectural Structures Su2018abn Lecture 23 ARCH 331

Wall Footings – continuous strip for load bearing walls – plain or reinforced – behavior • wide beam shear • bending of projection – dimensions usually dictated by codes for residential walls – light loads Foundations 24 Architectural Structures Su2018abn Lecture 23 ARCH 331

Eccentrically Loaded Footings • footings subject to moments P P e M=Pe by statics: – soil pressure resultant force may not coincide with the centroid of the footing Foundations 25 Architectural Structures Su2018abn Lecture 23 ARCH 331

Differential Soil Pressure – to avoid large rotations, limit the differential soil P M pressure across footing – for rigid footing, simplification of soil pressure is a linear distribution based on constant ratio of pressure to settlement Foundations 26 Architectural Structures Su2018abn Lecture 23 ARCH 331

Kern Limit • boundary of e for no tensile stress • triangular stress block with p max wpx   volume N 2 2 N max  p wx Foundations 27 Architectural Structures Su2018abn Lecture 23 ARCH 331

Guidelines – want resultant of load from pressure inside the middle third of base (kern) • ensures stability with respect to overturning P  M R x M    resist 1 5 SF . M M overturnin g x R  – pressure under toe (maximum) q a – shortcut using uniform soil pressure for design moments gives similar steel areas Foundations 28 Architectural Structures Su2018abn Lecture 23 ARCH 331

Combined Footings – supports two columns – used when space is tight and spread footings would overlap or when at property line – soil pressure might not be uniform – proportion so pressure will uniform for sustained loads – behaves like beam lengthwise Foundations 29 Architectural Structures Su2018abn Lecture 23 ARCH 331

Combined Footing Types – rectangular – trapezoid – strap or cantilever • prevents overturning of exterior column – raft/mat • more than two columns over an extended area Foundations 30 Architectural Structures Su2018abn Lecture 23 ARCH 331

Proportioning – uniform settling is desired – area is proportioned with sustained column loads – want the resultant to coincide with centroid of footing area for uniformly distributed pressure P 2 P 1 assuming a rigid footing  y q q max a R = P 1 +P 2 Foundations 31 Architectural Structures Su2018abn Lecture 23 ARCH 331

Retaining Walls • purpose – retain soil or other material • basic parts – wall & base – additional parts • counterfort • buttress • key Foundations 32 Architectural Structures Su2018abn Lecture 23 ARCH 331

Retaining Walls • considerations – overturning – settlement – allowable bearing pressure – sliding – (adequate drainage) Foundations 33 Architectural Structures Su2018abn Lecture 23 ARCH 331

Retaining Walls • procedure – proportion and check stability with working loads for bearing, overturning and sliding – design structure with factored loads M    resist 1 5 2 SF . W M overturnin g F x F     o horizontal resist 1 25 2 SF . F resist R F sliding Foundations 34 Architectural Structures Su2018abn Lecture 23 ARCH 331