b'THE OFFICIAL NFBA MAGAZINEYOUR BUILDING IS UNDER SIEGE!!!!UNDERSTAND THE FORCES AT WORK TO IMPROVE BUILDING DESIGN AND PERFORMANCERESEARCH & TECHNOLOGYDavid R. Bohnhoff, P.E. Emeritus ProfessorUniversity of Wisconsin-MadisonT he forces which act upon buildings can be categorizeddestruction of a building, is almost entirely dependent bytheirmagnitude,duration,andfrequency. on how well siege forces have been addressed in initial Building codes and zoning ordinances have evolveddesign and construction, and during routine building over time to largely address high-magnitude or extreme forces.maintenance and repair.Realize that if damages due to These forces include fire (extreme heat), hurricanes andsiege forces are not addressed, the catastrophic loss of tornadoes (extreme wind), flooding (extreme water levels),the building to extreme forces becomes more probable.earthquakes (extreme mass acceleration/decelerations)Siege Forcesand extreme snowfalls.Although extreme forces tend to1.Photodegradationbe of short duration and are relatively infrequent, they are responsible for almost all catastrophic building losses. 2.Wood decay fungiOntheoppositeendofthespectrum(fromthe3.Animalsaforementionedextremeforces)areforcesoflower4.Corrosionmagnitude that are present over longer periods of time5.Water condensation and depositionand that occur with much greater frequency.This article is dedicated to a discussion of these forces, which for the6.Cold weatherpurpose of this article, I am referring to as siege forces.7.Temperature fluctuationsIn military parlance, a siege is a form of warfare characterized8.Windsby a constant, low-intensity assault on a party that holds a static, defensive position.It is a war of attrition whose9.Ventilation outcome is influenced by the initial size and strength of10.Equipment induced vibrationboth parties, and ultimately depends on how long each party can survive if not reinforced in some manner. PhotodegradationIn the present context, your building is the defendingPhotodegradation is the disruption of chemical bonds by party.Throughout its life, your building is under continualultraviolet (UV) radiation, which is present in sunlight. assault by siege forces.The extent to which your buildingPolymeric-typematerialsarehighlysusceptibleto succumbs to these siege forces depends on (1) how wellphotodegradation unless they contain polymer stabilizers it has been designed and constructed to defend againstor UV-absorbers. Once UV radiation disrupts chemical these forces, and (2) to what extent damage from thebonds, polymers are more likely to react with oxygen and/attacking forces are addressed during this prolonged siege.or water vapor causing additional changes to the material.Actual building life, which is defined as the elapsedPolymericmaterialsincludenumeroussynthetically time between the completed construction and the totalderived plastics such as polyethylene, polypropylene, 12 / FRAME BUILDER - JAN2023'