Architectural Structures

A A B B

6" MAX

12" MAX

6" MAX

6" MAX

E

E

H H

Critical wood-frame items

9-26 DESIGN METHODS Lateral Force Design

10

Conceptual Design

Introduction

Conceptual design usually starts with approximate sizing principle elements of a structure and possible alternatives, followed by thorough analysis during design development. Approximate methods are essential to quickly develop alternate designs. They are also useful to verify final designs and computer analysis. If based on good assumptions, approximate methods can provide results of remarkable accuracy, usually within ten percent of precise results. The following conceptual design examples introduce approximate methods, sometimes referred to as back-of-the-envelop design. They are not meant to replace accurate design but as precursor of accurate design and analysis.

System Selection

Structural design starts with the selection of a system and material; often informed by similar past projects, even if not appropriate. For example, light wood structures are common for residential building where hurricanes cause frequent destruction, though heavy concrete or masonry would resist wind load much better. A rational method is proposed with the objective to select more appropriate systems. However, since design criteria may be conflicting in some cases, selection is both art and science, yet the following criteria make the selection process more objective

•Capacity limit

•Code requirements

•Cost

•Load

•Location

•Resources

•Technology

•Synergy

Capacity limit is based on limits of systems and materials. For example, beams are economical for a given span range. To exceed that range would yield a bad ratio of dead load to live load. A beam’s cross section increases with span, resulting in heavier dead load. Eventually, the beam’s dead load exceeds its capacity and it would break. Approaching that limit, the beam gets increasingly uneconomical because its dead weight leaves little reserve capacity to carry live load. The span limit can be extended by effective cross section shape. For example, steel beam cross sections are optimized in response to bending and shear stress, to allow greater spans.

Trusses have longer span capacity than beams, due to reduced self weight. They replace the bulk of beams by top and bottom chords to resist global moments, and vertical and diagonal web bars to transfer shear between compression and tension chords. Compared to beams, the greater depth of trusses provides a greater lever arm between compression and tension bars to resist global moments. Similarly, suspension cables use the sag between support and mid-span as moment resisting lever arm. Since cables have higher breaking strength and resist tension only, without buckling, they are optimal for long spans; but the high cost of end fittings makes them expensive solutions for short spans. These examples show, most systems have upper and lower span limits.

Code requirements define structures by type of construction regarding materials and systems; ranging form type I to type V for least and most restrictive, respectively, of the Uniform Building Code (UBC) for example. Each type of construction has requirements for fire resistance, maximum allowable floor area, building height, and occupancy group. Codes also have detailed requirements regarding seismic design; notable structures are categorized by ductility to absorb seismic energy and related height limits. Some code requirements are related to other criteria described in the respective section.

Cost is often an overriding criteria in the selection of structures. in fact, cost is often defined by some of the other selection criteria. However, costs also depends on market conditions and seasonable changes. The availability of material and products, as well as economic conditions and labor strikes may greatly effect the cost of structures. For example, a labor strike in the steel industry may shift the advantage to a concrete structure, or the shortage of lumber, may give a cost advantage to light gauge steel instead of light wood framing. Sometimes, several systems are evaluated, or schematic designs are developed for them, in order to select the most cost effective alternative.

Load imposed on a structure is a major factor in selecting a system. For example, roofs in areas without snow must be designed only for a nominal load, yet roof load in mountain areas may be up to 20 times greater than the nominal load. Structures in earthquake prone areas should be lightweight and ductile, since seismic forces are basically governed by Newton’s law, force equals mass times acceleration (f=ma). In contrast, structures subject to wind load should be heavy and stiff to resist wind uplift and minimize drift. Structures in areas of daily temperature variations should be designed for thermal load as well, unless the structure is protected behind a thermal insulation skin and subjected to constant indoor temperature only.

Location may effects structure selection by the type of soil, topography, ground water level, natural hazards, such as fire, frost, or flood. Local soil conditions effect the foundation and possibly the entire structure. Soft soil may require pile foundations; a mat foundation may be chosen to balance the floating effect of high ground water. Locations with winter frost require deep foundations to prevent damage due to soil expansion in frost (usually a depth of about one meter). Hillside locations may require caisson foundations to prevent sliding, but foundations are more common on flat sites. Locations with fire hazards require non-combustible material. Raising the structure off the ground may be the answer to flooding.

Resources have a strong impact on the selection of structure materials. Availability of material was a deciding factor regarding the choice of material throughout history. The Viking build wood structures, a logical response to the vast forests of Scandinavia, yet stone temples of Egypt and Greece reflect the availability of stone and scarcity of wood. More recently, high-rise structures in the United States are usually steel structures, but the scarcity of steel in some other countries makes concrete structures more common.

Technology available at a area also effects the selection of structures. For example, light wood structures, known as platform framing, is most common for low rise residential structures in the United States, where it is widely available and very well known; but in Europe where this technology is less known, it is more expensive than more common masonry structures. Similarly, in some areas concrete technology is more familiar and available than steel technology. Concrete tends to be more common in areas of low labor cost, because concrete form-wok is labor intensive. On the other hand, prefab concrete technology is less dependent on low labor cost and more effected by market

conditions, namely continuity of demand to justify the high investments associated with prefab concrete technology.

Synergy, defined as a system that is greater than the sum of its parts is a powerful concept to enrich architecture, regarding both pragmatic as well as philosophic objectives. Pragmatic example are numerous: Wall system are appropriate for hotel and apartment projects which require spatial and sound separation; but moment frames provide better space planning flexibility as needed for office buildings. However, the core of office buildings, usually housing elevators, stairs, bathrooms, and mechanical ducts, without the need for planning flexibility, often consists of shear walls ore braced frames, effective to reduce drift under lateral loads. Long-span systems provide column-free space required for unobstructed views in auditoriums and other assembly halls; but lower cost short span systems are used for warehouses and similar facilities where columns are usually acceptable.

On a more detailed level, to incorporate mechanical systems within a long-span roof or floor structure, a Vierendeel girder may be selected instead of a truss, since the rectangular panels of a Vierendeel better facilitate ducts to pass through than triangular truss panels. A suspended cable roof may be selected for a sports arena if bleachers can be used to effectively resist the roof’s lateral thrust which is very substantial and may require costly foundations otherwise. Synergy is also a powerful concept regarding more philosophical objectives, as demonstrated throughout history, from early post and beam structures; Roman arches, domes and vaults; Gothic cathedrals; to contemporary suspension bridges or roofs. Columns can provide architectural expression as in post and beam systems, or define and organize circulation, as in a Gothic cathedral. The funicular surface of arches, domes and vaults can define a unique and spiritual space. The buttresses to resist their lateral thrust provides the unique vocabulary of Gothic cathedrals. Large retaining walls may use buttressing for rhythmic relieve, as in the great wall of Assisi, or lean backward to express increased stability as the wall of the Dalai Lama palace in Tibet.