THE BUILDING ENVELOPE: Lecture 4 Designing the Building Envelope Design Responsibility: Fee Structure • Architect: building function and aesthetics • Engineer: principal structural frame Design Responsibility: Building Type • High-rise and commercial - Architect • Industrial (metal) buildings- Engineer • Residential buildings - Builder/A/E Criteria for Design • Meeting Building Code Minimums • Risk Analysis Approach Criteria for Design: Code Minimums ASCE 7-02 outlines three methods for computing wind pressures • Method 1: Simplified Method • Method 2: Analytical Method • Method 3: Wind Tunnel Analysis ASCE 7-02 Method 1: Simplified Method Nine criteria must be met: (1) a simple diaphragm building (2) a low-rise building (3) an "enclosed" building (4) of regular shape (5) not flexible (6) have no special wind sensitivities (7) have no expansion joints (8) sited with no topographic effects, and (9) approximately symmetrical with roof slope < 45o ASCE 7-02 Method 2: Analytical Method p = q(GCp) – qi(GCpi) q = 0.00256 Kz Kzt Kd V2 I (ASCE 7-02, Equation 6-23) (ASCE 7-02, Equation 6-15) q is the "dynamic pressure" induced by the wind G addresses wind gusts and gust-structure interactions C (Cp and Cpi) is a factor reflecting building geometry Kz addresses terrain; varies with height above ground Kzt addresses topographic effects (hills, escarpments) Kd addresses directionality of the wind V is the design wind speed I is the importance factor for the building ASCE 7-02 Method 3: Wind Tunnel Method [ Note: Leighton Cochran will provide an illustration of wind loads on cladding determined from wind tunnel tests] Criteria for Design: Risk Analysis Approach • The Risk Model • Precedent and Cost • Appropriate Level of Risk The Risk Model: Hurricane Occurrence The Risk Model: V vs MRI 250 W in d S p e e d (m p h ) 200 150 M e a n H o urly 100 O ne M inute F a ste st M ile 50 P e a k G ust 0 10 100 1000 R e turn P e rio d (Ye a rs) 10000 The Risk Model: Precedent and Cost Risks associated with building function – Mass hurricane shelters - 500 yr MRI – Health care facilities - 300 yr MRI – Facilities with nuclear materials - 1M yr MRI Cost considerations – Costs are developed for several risk levels – Trade-off between cost and MRI