Symptoms of Complexity Prof. George Candea School of Computer & Communication Sciences Building Bridges A RTlClES A COMPUTER SCIENCE PERSPECTIVE OF BRIDGE DESIGN What kinds of lessonsdoes a classical engineering discipline like bridge design have for an emerging engineering discipline like computer systems Observation design?Case-study editors Alfred Spector and David Gifford consider the • insight and experienceof bridge designer Gerard Fox to find out how strong the parallels are. • bridges are normally on-time, on-budget, and don’t fall ALFRED SPECTORand DAVID GIFFORD • software projects rarely ship on-time, are often over- AS Gerry, let’s begin with an overview of THE DESIGN PROCESS bridges. AS What is the procedure for designing and con- GF In the United States, most highway bridges are budget, and rarely work exactly as specified structing a bridge? mandated by a government agency. The great major- GF It breaks down into three phases: the prelimi- ity are small bridges (with spans of less than 150 nay design phase, the main design phase, and the feet) and are part of the public highway system. construction phase. For larger bridges, several alter- There are fewer large bridges, having spans of 600 native designs are usually considered during the Blueprints for bridges must be approved... feet or more, that carry roads over bodies of water, preliminary design phase, whereas simple calcula- • gorges, or other large obstacles. There are also a tions or experience usually suffices in determining small number of superlarge bridges with spans ap- the appropriate design for small bridges. There are a proaching a mile, like the Verrazzano Narrows lot more factors to take into account with a large Bridge in New Yor:k. for structural integrity, earthquake and flood safety, etc. bridge: aesthetics, method of construction, cost of • materials, etc. The preliminary design report for a AS What are the requirements for a bridge? large bridge usually describes three or four alterna- GF There are several categories of requirements. tive bridge types, estimates their costs, and provides For instance, there are funcfionalify requirements: a rendering of what the bridge will look like. Usu- Foundations are inspected The lanes should be sufficiently wide, the bridge ally, the designer recommends one of the alterna- • should have safe barriers to deflect cars back onto tives to the client. There would also usually be hear- the roadway, and the lighting should be sufficient. ings to get the public’s reaction. There are serviceability requirements: We don’t want you don’t just build a pillar and test if it stands the bridge to vibrate excessively and scare people, DG Do you estimate both the initial cost and the • and we don’t want large cracks in concrete bridges. life-cycle cost for each of the alternatives? Of course, there is the ultimate strength requirement: GF Life-cycle costing is not in wide use for bridges, We don’t want the bridge to fail. Then there is an although I think it should be. For example, consider aesthetics requirement: The bridge should be pleas- the life-cycle cost of a bridge’s deck, the portion of ing to the eye. There’s also a long-term maintainability Fundamental difference: the bridge that comprises the riding surface. One requirement, which involves corrosion protection of • alternative is to design an orthotropic steel deck, various elements. For example, cables tend to be which can support traffic and also help to carry the very susceptible to stress corrosion, and therefore weight of the bridge itself. The alternative is a con- their protection is ‘very important. Finally, there is crete slab deck, which costs a lot less initially, but • laws of physics vs. laws of human intellect ... the cost-effectiveness requirement: The finished prod- does not last nearly as long as a steel deck. Since the uct should meet all of the above requirements at the initial cost is the primary thing that clients look at best possible cost. today, most new bridges in this country are being 01986 ACM OOOl-0762/66/0400-0268 7511 built with concrete decks. At the same time, many 268 Conmunications of fhe ACM April 1986 Volume 29 Number 4 George Candea Principles of Computer Systems 2 Four Symptoms of Complexity System • Large number of components • Large number of interconnections • Many irregularities and exceptions • High “Kolmogorov complexity” Interface Environment George Candea Principles of Computer Systems 3 Symptom #1: Many Components George Candea Principles of Computer Systems 4 Symptom #2: Many Interconnections srcpac abs devtools ypserv perl-io-stringyperl-mailtools openslp mdadm namcap memtest86+ v86d ppp grub gzip rsync perl-mime-tools snarf rp-pppoe grub2-efi-x86_64 ifenslave perl-convert-binhex fcpci xf86-video-nouveau screen capi4k-utils sed sshfs tiacx-firmware initscripts nilfs-utils python-iwscan isdn4k-utils lirc pptpclient mtools grep nano snd modemmanager dmraid nvidia-utils wireless_tools linux-firmware licenses bridge-utils cpio ntp perl-bit-vector perl-mime-lite nouveau-firmware procps perl-digest-nilsimsa 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