A RCHITECTURAL S TRUCTURES : Design F ORM, B EHAVIOR, AND D ESIGN • factors out of the designer’s control A RCH 331 D R. A NNE N ICHOLS – loads S PRING 2018 – occurrence lecture • factors within the designer’s control thirteen – choice of material – “cost” of failure (F.S., probability, location) design loads & methods, – economic design method – analysis method structural codes Methods & Codes 1 Architectural Structures Su2014abn Methods & Codes 2 Architectural Structures Su2014abn Lecture 13 ARCH 331 Lecture 13 ARCH 331 Design Methods Load Types • different approaches to meeting • D = dead load • L = live load strength/safety requirements – allowable stress design (elastic) • L r = live roof load – ultimate strength design • W = wind load • S = snow load – limit state design • E = earthquake load – plastic design – load and resistance factor design • R = rainwater load or ice water load http://mceer.buffalo.edu • assume a behavior at failure or other • T = effect of material & temperature • H = hydraulic loads from soil (F from fluids) threshold and include a margin of safety Methods & Codes 3 Architectural Structures Su2014abn Methods & Codes 4 Architectural Structures Su2014abn Lecture 13 ARCH 331 Lecture 13 ARCH 331 1
Dead Loads Weight of Materials • fixed elements • for a volume – structure itself – W V where is weight/volume – internal partitions – W tA for an extruded area with height of t – hung ceilings – all internal and external finishes A – HVAC ductwork and equipment – permanently mounted equipment t • F = mg (GRAVITY) Methods & Codes 5 Architectural Structures Su2014abn Methods & Codes 6 Architectural Structures Su2014abn Lecture 13 ARCH 331 Lecture 13 ARCH 331 Concentrated Loads Distributed Loads • for an area w A Methods & Codes 7 Architectural Structures Su2014abn Methods & Codes 8 Architectural Structures Su2014abn Lecture 13 ARCH 331 Lecture 13 ARCH 331 2
Dynamic Loads Load Locations • time, velocity, acceleration • centric • kinetics • eccentric – forces causing motion • bending or flexural load W = m·g • torsional load – work • combined loading – conservation of energy Methods & Codes 9 Architectural Structures Su2014abn Methods & Codes 10 Architectural Structures Su2014abn Lecture 13 ARCH 331 Lecture 13 ARCH 331 Load Paths Live Loads • tributary • occupancy areas • movable furniture • transfer and equipment • construction / roof traffic – L r • minimum values • reduction allowed as area increases Methods & Codes 11 Architectural Structures Su2014abn Methods & Codes 12 Architectural Structures Su2014abn Lecture 13 ARCH 331 Lecture 13 ARCH 331 3
Snow Load Wind Load • wind speed • latitude • gusting • solar exposure • terrain • wind speed • windward, leeward, • roof slope up and down! • drag • rocking • harmonic Moscow 2006 (BBC News) • torsion Methods & Codes 13 Architectural Structures Su2014abn Methods & Codes 14 Architectural Structures Su2014abn Lecture 13 ARCH 331 Lecture 13 ARCH 331 Seismic Load Dynamic Response • earthquake acceleration – F = ma – movement of ground (3D) – building mass responds – static models often used, V is static shear – building period, T ≈ 0.1N, ZICW determines C V R – building resistance – R W W – Z (zone), I (importance factor) Methods & Codes 15 Architectural Structures Su2014abn Methods & Codes 16 Architectural Structures Su2014abn Lecture 13 ARCH 331 Lecture 13 ARCH 331 4
Dynamic Response Frequency and Period • natural period of vibration • period of vibration or frequency – wave – avoid resonance – sway/time period – hard to predict seismic period • damping – affected by soil – reduction in sway – short period • resonance • high stiffness – long period – amplification of sway “To ring the bell, the sexton must pull • low stiffness on the downswing of the bell in time with the natural frequency of the bell.” Methods & Codes 17 Architectural Structures Su2014abn Methods & Codes 18 Architectural Structures Su2014abn Lecture 13 ARCH 331 Lecture 13 ARCH 331 Water Load Thermal Load • rainwater – clogged drains • stress due to strain • ponding • restrained expansion or contraction • ice formation • temperature gradients • composite construction mrfussycontracting.com Methods & Codes 19 Architectural Structures Su2014abn Methods & Codes 20 Architectural Structures Su2014abn Lecture 13 ARCH 331 Lecture 13 ARCH 331 5
Hydraulic Loads Building Codes • pressure by water in soil, H • documentation – laws that deal with planning, design, • fluid pressure, F construction, and use of buildings – normal to surface – regulate building construction for • flood • fire, structural and health safety – cover all aspect of building design – references standards • acceptable minimum criteria • material & structural codes Methods & Codes 21 Architectural Structures Su2014abn Methods & Codes 22 Architectural Structures Su2014abn Lecture 13 ARCH 331 Lecture 13 ARCH 331 Building Codes Prescribed Loads • occupancy • ASCE-7 • construction types – live load reductions allowed • structural chapters • International Building Code – loads, tests, foundations – occupancy • structural materials, assemblies – wind: pressure to static – roofs load – concrete – seismic: shear load – masonry function of mass and – steel response to acceleration – fire resistance Methods & Codes 24 Architectural Structures Su2014abn Methods & Codes 23 Architectural Structures Su2014abn Lecture 13 ARCH 331 Lecture 13 ARCH 331 6
Structural Codes Structural Codes • prescribe loads and combinations • Design Codes – Wood • prescribe design method • NDS • prescribe stress and deflection limits – Steel • backed by the profession • AISC • may require design to meet – Concrete performance standards • ACI • related to material or function • AASHTO – Masonry • MSJC Methods & Codes 25 Architectural Structures Su2014abn Methods & Codes 26 Architectural Structures Su2014abn Lecture 13 ARCH 331 Lecture 13 ARCH 331 Allowable Stress Design Design Methods • probability of loads and resistance • historical method • material variability • a.k.a. working stress, • overload, fracture, fatigue, failure strength design • allowable stress design • stresses stay in f P capacity f f ELASTIC range actual allowed A F S . . • limit state design – design loads & capacities Methods & Codes 27 Architectural Structures Su2014abn Methods & Codes 28 Architectural Structures Su2014abn Lecture 13 ARCH 331 Lecture 13 ARCH 331 7
ASCE-7 ASD Load Combinations Limit State Design (2010) • D • a.k.a. strength design • D + L • stresses go to limit (strain outside • D + 0.75(L r or S or R) elastic range) • loads may be factored • D + 0.75L + 0.75(L r or S or R) • resistance or capacity • D + (0.6W or 0.7E) reduced by a factor – D + 0.75L + 0.75(0.6W) + 0.75(L r or S or R) • based on material behavior – D + 0.75L + 0.75(0.7E) + 0.75S • 0.6D + 0.6W • “state of the art” • 0.6D + 0.7E Methods & Codes 29 Architectural Structures Su2014abn Methods & Codes 30 Architectural Structures Su2014abn Lecture 13 ARCH 331 Lecture 13 ARCH 331 ASCE-7 LRFD Load Combinations Limit State Design (2010) • load and resistance factor design (LRFD) • 1.4D – loads: • 1.2D + 1.6L + 0.5(L r or S or R) • not constant, • 1.2D + 1.6(L r or S or R) + (L or 0.5W) • possibly more influential on failure • 1.2D + 1.0W + L + 0.5(L r or S or R) • happen more or less often • 1.2D + 1.0E + L + 0.2S – UNCERTAINTY R R R • 0.9D + 1.0W D D L L n - Resistance factor • 0.9D + 1.0E - Load factor for (D)ead & (L)ive load • F has same factor as D in 1-5 and 7 • H adds with 1.6 and resists with 0.9 (permanent) Methods & Codes 31 Architectural Structures Su2014abn Methods & Codes 32 Architectural Structures Su2014abn Lecture 13 ARCH 331 Lecture 13 ARCH 331 8
Deflection Limits Load Conditions • based on service condition, severity • loads, patterns & combinations – usually uniformly distributed gravity loads Use LL only DL+LL – worst case for largest moments... Roof beams: Industrial L/180 L/120 – wind direction can increase moments Commercial plaster ceiling L/240 L/180 no plaster L/360 L/240 Floor beams: Ordinary Usage L/360 L/240 Roof or floor (damageable elements) L/480 Methods & Codes 33 Architectural Structures Su2014abn Methods & Codes 34 Architectural Structures Su2014abn Lecture 13 ARCH 331 Lecture 13 ARCH 331 9
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