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E LEMENTS OF A RCHITECTURAL S TRUCTURES : Foundation F ORM, B EHAVIOR, AND D ESIGN ARCH 614 • the engineered interface between the D R. A NNE N ICHOLS earth and the structure it supports that S PRING 2018 transmits the loads to the soil or rock lecture twenty six concrete construction: Bright Football Complex www.tamu.edu foundation design Foundations 1 Elements of Architectural Structures S2009abn Lecture 26 ARCH 614 Foundations 2 Elements of Architectural Structures S2007abn Lecture 26 ARCH 614 Structural vs. Foundation Design Structural vs. Foundation Design • structural design • foundation design – choice of materials – cannot specify site materials – choice of framing system – site is usually predetermined – uniform materials and quality assurance – framing/structure predetermined – design largely – site geology influences foundation choice independent of – no site the same geology, climate, etc. – no design the same Foundations 3 Elements of Architectural Structures S2007abn Foundations 4 Elements of Architectural Structures S2007abn Lecture 26 ARCH 614 Lecture 26 ARCH 614 1

Soil Properties & Mechanics Soil Properties & Mechanics • compressibility • unit weight of soil • allowable soil pressure – settlements • factored net soil pressure • strength • shear resistance – stability • backfill pressure • shallow foundations • cohesion & friction of soil • deep foundations • slopes and walls • effect of water – ultimate bearing capacity, q u • settlement q finehomebuilding.com a  – allowable bearing capacity, u q • rock fracture behavior S.F. Foundations 6 Elements of Architectural Structures S2007abn Foundations 5 Elements of Architectural Structures S2007abn Lecture 26 ARCH 614 Lecture 26 ARCH 614 Soil Properties & Mechanics Bearing Failure • strength, q a • shear slip zone slip zone punched wedge punched wedge Foundations 7 Elements of Architectural Structures S2007abn Foundations 8 Elements of Architectural Structures S2007abn Lecture 26 ARCH 614 Lecture 26 ARCH 614 2

Lateral Earth Pressure Foundation Materials • passive vs. active • concrete, plain or reinforced – shear – bearing capacity – bending active – embedment length, development length (trying to • other materials (piles) move wall) – steel passive – wood (resists movement) – composite Foundations 9 Elements of Architectural Structures S2007abn Foundations 10 Elements of Architectural Structures S2007abn Lecture 26 ARCH 614 Lecture 26 ARCH 614 Basic Foundation Requirements Generalized Design Steps • safe against instability or collapse • calculate loads • no excessive/damaging settlements • characterize soil • determine footing location and depth • consider environment – frost action • evaluate soil bearing capacity – shrinkage/swelling • determine footing size (unfactored loads) – adjacent structure, property lines – ground water • calculate contact pressure and check – underground defects stability – earthquake • estimate settlements • economics • design footing structure* (factored loads) Foundations 11 Elements of Architectural Structures S2007abn Foundations 12 Elements of Architectural Structures S2007abn Lecture 26 ARCH 614 Lecture 26 ARCH 614 3

Types of Foundations Types of Foundations • spread footings • mat foundations • wall footings • retaining walls • eccentric footings • basement walls • combined footings • pile foundations • unsymmetrical footings • drilled piers • strap footings Foundations 13 Elements of Architectural Structures S2007abn Foundations 14 Elements of Architectural Structures S2007abn Lecture 26 ARCH 614 Lecture 26 ARCH 614 Shallow Footings Actual vs. Design Soil Pressure • spread footing • stress distribution is a function of – a square or rectangular footing supporting – footing rigidity a single column – soil behavior – reduces stress from load to size the ground RIGID RIGID can withstand sand clay • linear stress distribution assumed Foundations 15 Elements of Architectural Structures S2007abn Foundations 16 Elements of Architectural Structures S2007abn Lecture 26 ARCH 614 Lecture 26 ARCH 614 4

Proportioning Footings Concrete Spread Footings • net allowable soil pressure, q net • plain or reinforced      – q q h ( ) • ACI specifications net allowable f c s – considers all extra weight (overburden) • P u = combination of factored D, L, W from replacing soil with concrete • ultimate strength – can be more overburden     – V V : 0.75 for shear • design requirement u c • plain concrete has shear strength with total unfactored     – M M : 0.9 for flexure u n load: P  q net A Foundations 17 Elements of Architectural Structures S2007abn Foundations 18 Elements of Architectural Structures S2007abn Lecture 26 ARCH 614 Lecture 26 ARCH 614 Concrete Spread Footings Concrete Spread Footings • failure modes • shear failure shear one way shear two way shear bending Foundations 19 Elements of Architectural Structures S2007abn Foundations 20 Elements of Architectural Structures S2007abn Lecture 26 ARCH 614 Lecture 26 ARCH 614 5

Over and Under-reinforcement Reinforcement Length • reinforcement ratio for bending • need length, l d A s   – bond – bd – development of yield strength – use as a design estimate to find A s ,b,d – max  from  steel  0.004 – minimum for slabs & footings of uniform thickness A s  0 . 002 grade 40 / 50 bars bh  0 . 0018 grade 60 bars Foundations 21 Elements of Architectural Structures S2007abn Foundations 22 Elements of Architectural Structures S2007abn Lecture 26 ARCH 614 Lecture 26 ARCH 614 Column Connection Wall Footings • bearing of column on footing – continuous strip for load bearing walls   f c      – P P 0 . 85 A – plain or reinforced u n 1   A 1 0.65 for bearing – behavior A 2 1 A – confined: increase x  2 2 2 A • wide beam shear 1 • dowel reinforcement • bending of projection – if P u > P b , need – dimensions usually dictated compression by codes for residential walls reinforcement – min of 4 bars – light loads and 0.005A g Foundations 23 Elements of Architectural Structures S2009abn Foundations 24 Elements of Architectural Structures S2007abn Lecture 26 ARCH 614 Lecture 26 ARCH 614 6

Eccentrically Loaded Footings Differential Soil Pressure – to avoid large rotations, • footings subject to moments limit the differential soil P P P M pressure across footing e M=Pe by statics: – for rigid footing, simplification of soil pressure is a linear distribution based on – soil pressure resultant force may not constant ratio of pressure to settlement coincide with the centroid of the footing Foundations 25 Elements of Architectural Structures S2007abn Foundations 26 Elements of Architectural Structures S2007abn Lecture 26 ARCH 614 Lecture 26 ARCH 614 Kern Limit Guidelines • boundary of e for – want resultant of load from pressure inside the middle third of base (kern) no tensile stress • ensures stability with respect to overturning • triangular stress P block with p max  M R x M    resist 1 5 SF . M M wpx overturnin g   volume N x R 2  – pressure under toe (maximum) q a 2 N max  – shortcut using uniform soil pressure for p wx design moments gives similar steel areas Foundations 27 Elements of Architectural Structures S2007abn Foundations 28 Elements of Architectural Structures S2007abn Lecture 26 ARCH 614 Lecture 26 ARCH 614 7

Combined Footings Combined Footing Types – supports two columns – rectangular – used when space is tight and spread footings – trapezoid would overlap or when at property line – strap or cantilever • prevents overturning of exterior column – soil pressure might not be uniform – proportion so pressure will uniform for – raft/mat sustained loads – behaves like beam lengthwise • more than two columns over an extended area Foundations 29 Elements of Architectural Structures S2007abn Foundations 30 Elements of Architectural Structures S2007abn Lecture 26 ARCH 614 Lecture 26 ARCH 614 Proportioning Retaining Walls – uniform settling is desired • purpose – area is proportioned with sustained column – retain soil or other material loads • basic parts – want the resultant to coincide with centroid – wall & base of footing area for uniformly distributed – additional parts pressure P 2 P 1 • counterfort assuming a • buttress rigid footing • key  y q q max a R = P 1 +P 2 Foundations 31 Elements of Architectural Structures S2007abn Foundations 32 Elements of Architectural Structures S2007abn Lecture 26 ARCH 614 Lecture 26 ARCH 614 8