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Structural Design: A Practical Guide for Architects

Structural Design: A Practical Guide for Architects

by James R. Underwood

ISBN: 9780471789048

Publisher Wiley

Published in Calendars/Architecture

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Sample Chapter


Chapter One

Introduction: Understanding Loads

1.1 LOADS

In statics, loads are forces acting on structural components and are represented as uniform or varying forces or points. In practice, they represent the weights of the building materials used to construct the building (Fig. 1.1), the weights of the people and equipment which will occupy the building, and the forces of nature that the building will be exposed to during its life.

Material weights are gravity loads which act down (surprise!). People and equipment are primarily gravity loads, but in some instances they may cause forces which act in some other direction. An example is a piece of horizontally moving equipment which suddenly comes to a stop, such as a gantry crane, causing a horizontal force to be induced in the structure. Highway bridges are constantly subjected to this loading condition.

Wind forces are primarily horizontal but can induce vertical forces when blowing over surfaces. Note that wind passing over an airplane wing causes the upward lift that keeps the plane in the air. Similar conditions can be induced in the roof structure of a building.

Earthquakes, by contrast, are wave-like forces which have both horizontal and vertical components; however, the horizontal force component is typically the more destructive of the two, since most structures are designed to be primarily vertical load-carrying systems. Both wind and earthquake loads are discussed in more detail later in this chapter.

The effect of these forces is to induce states of stress and deformation or deflection in the structure. Deflections are often the governing factors in the design of a structural system. Obviously, a structure fails when it collapses; however, excessive deflection which damages finishes or other building components without causing collapse is also defined as a structural failure.

Building codes categorize these loads into two classifications: dead loads and live loads. Dead loads are the permanent loads generated by the constructional system. Live loads are the nonpermanent loads applied to the structure after it is completed. Some loads may be in either category, depending upon their time of application. It is essential to understand the construction sequence of the building, and to design for deflection caused by live loads introduced after the construction is complete. For example, a typically permanent (dead) load such as an HVAC (heating, ventilating, air conditioning) unit should be considered a live load if installed after ceiling finishes are in place, since it would cause deflection of the ceiling/ floor components similar to that created by snow on a roof or human occupancy of a level above. This may occur even if a building component is assumed to be in place prior to finishes. A manufacturing delay, a labor dispute, a delivery problem, or even a design change may be responsible for an out-of-sequence installation which could have serious deflection implications.

An objective of the building codes is to limit the deflection of structural members to the extent that they do not damage the connected nonstructural components or affect the functionality of the building. In the 2003 International Building Code (IBC), as in previous codes, limitations are imposed on deflections due to both dead and live loads (Table 1.1).

This shouldn't suggest that dead loads don't cause deflection. The dead load deflection of the structure isn't considered in some cases since it is compensated for during the construction process. For example, the ceiling finish, which is (obviously) installed after the horizontal framing is enclosed, is installed "level." Any dead load deflection which exists in the framing will be hidden by adjusting the finish. The possible exception involves roof construction; consequently, care must be taken to ensure that "flat" roof systems have no water retention areas-ponding-mentioned earlier.

The load values to be used depend on the use or occupancy of the structure. Typically, loads are floor loads, roof loads, and wind loads acting on walls and roofs, and are given in lb/[ft.sup.2] (or psf) [kN/[m.sup.2]]. For example, floor loading for offices is 50 psf [2.39 kN/[m.sup.2]]; for school classrooms, it is 40 psf [1.92 kN/[m.sup.2]]. In both cases, the buildings will have corridors or circulation spaces on each level which will have a live loading of 80 to 100 psf [3.82 to 4.78 kN/[m.sup.2]]. As this suggests, structures are subjected to a variety of live loading conditions, and the design must work for the worst-case scenario: those loading conditions which cause the worst effect on the criteria being investigated. Why are we telling you this? No matter how well you understand the processes used in designing individual members, if you begin with a misunderstanding of what loads must be resisted, how they will be applied, when they will be applied (in what combinations), and how the total system transfers these loads, your calculations will be no more than mathematical exercises.

1.1.1 How Are Loads Determined?

Gravity Loads

Dead loads are calculated by looking at the construction system of the building and calculating the actual weights of the materials. This may be difficult since preliminary calculations must be made during the design of the project, when actual materials and systems have yet to be completely defined. Knowledge of a wide variety of alternative material systems is a great asset.

Probably the best technique for determining the dead load of a building system is to sketch a typical construction section of your projections (a roof or floor "sandwich") and determine the weights of the components from manufacturers' catalogs, from tables of standard weights, or from the AISC Manual of Steel Construction, Table 17.13.

Of course, you will have to add any nonuniform components to this system sketch in the final calculations, such as walls, movable point loads, and so on. Note that the IBC live load table (Table 1.2) has occupancy loading criteria which include both uniform loads and movable concentrated loads; also, in the case of offices, a partition load is mandatory.

Live loads are code specified, are a result of testing, and are somewhat subjective; consequently, they are usually conservative. For example, parking garages must support a code live load of 40 psf [1.92 kN/[m.sup.2]]. If a car weighs 2,500 lb and covers an area of 14 ft ? 6 ft = 84 [ft.sup.2], the corresponding distributed load on the floor is 30 psf [1.44 kN/[m.sup.2]]. If people and luggage are included in the car, the load increases. The code values are conservative since there is a wide range of exceptional conditions which could occur.

Snow loads, which were previously simply a psf value based on location, are now calculated based on a number of contributing factors: [C.sub.e], exposure factor, acknowledges the influence on the surrounding terrain of snow accumulation, [C.sub.t], the thermal factor, acknowledges the melting effect during accumulation due to heat loss from the supporting structure; [I.sub.s], the importance factor, acknowledges how critical the structure is to life safety, not simply of its occupants but of the community in general; and a rain on snow surcharge consideration which acknowledges the "snow cone" effect of water trapped in snow, thereby increasing its weight. The rain on snow load of an additional 5 psf [240 N/[m.sup.2]] applies to essentially flat roof systems-1/2 in./ft [4%] slope maximum-in areas that have a basic snow load of 20 psf [960 N/[m.sup.2]] or less (Table 1.3).

The basic snow load formula is [P.sub.s] = 0.7[C.sub.e][C.sub.t][I.sub.s][p.sub.g], where [P.sub.s] is the design snow load, 0.7 is a basic exposure factor for flat roofs, and [p.sub.g] is the geographically specified ground snow load. This load is indicated in Fig. 1.2.

The second factor of the basic snow load formula, [C.sub.t], acknowledges that the amount of heat that the structure loses will add to or subtract from the base quantity of snow retained.

Roof live loads account not only for snow, wind, or people having access for maintenance, but also for water which might accumulate if drains become clogged. The code for northern Indiana specifies 20 psf [0.96 kN/[m.sup.2]] (see Fig. 1.2), which is equivalent to 4 in. [102 mm] of water or as much as 40 in. [1.2 m] of snow. Water weighs 62.4 pcf [9.81 kN/[m.sup.3]], so every inch of water weighs 62.4 pcf/12 in./ft = 5.2 psf/in. [9.81 N/[m.sup.2]] per mm of depth; 40 in. [1.02 m] of snow is unusual. However, it is not uncommon to find drifts of this depth, especially on roofs where building forms of different heights are adjacent to one another.

1.2 TRIBUTARY AREAS

Loads are transferred from nonstructural parts to the load-bearing structure of the building; structural members transfer the loads to each other until the forces eventually reach the foundations (Fig. 1.3). Understanding how these transfers take place is essential to assign the correct loads to all members and to design the connections between members. The location and type of load, connections, and type of structural member affect the distribution of stress (axial, bending, shear, and torque) in the cross section of the member. The assumptions made by the designer regarding the type of structural system and loads must match the reality of construction as closely as possible.

If members support large areas of floor or roof surfaces, the likelihood of the entire area being fully loaded with live load decreases as the area increases, and code-specified live loads may be reduced based on the amount of area any individual structural member may be called upon to support. This reduction is the acknowledgment that the larger the tributary area is, the less likely that every square foot will be loaded to its maximum. For the purposes of this book, we will not incorporate this consideration and take the conservative position, applying the full live load to the entire system. This conservatism will come back to haunt you when you begin to acknowledge the cost of the system, especially on large-scale buildings. If you wish further information regarding specific load reduction criteria, see Section 1607.9 of the IBC.

1.3 LATERAL LOADS-WIND AND EARTHQUAKES

Lateral loads are forces acting horizontally or having a component which acts normal (perpendicular) to an inclined surface and are distinctly different from gravity loads. Lateral loads can be divided into two types: constant and variable. Lateral loads such as soil pressures on a retaining wall are relatively constant, while other loads such as wind and earthquakes have variable intensity. The code formulas use static loads in place of these dynamic actions. The magnitude of wind loads depends on the form of the structure or element, and the wind direction determines whether the load is positive (pushing) or negative (suction). Wind will exert a pressure on a roof (positive or negative) which is a function of the pitch and orientation of the roof. Earthquakes result from a rapid release of strain energy built up between tectonic plates which make up the crust of the earth. This movement is similar to that experienced in buildings when thermal stresses cause movement at expansion joints. These joints were originally "lubricated" with brass plates to reduce friction, and now these brass plates have been replaced by Teflon-coated steel or simply Teflon pads. Teflon pads are not yet available for fault lines, so we're forced to deal with the movement and vibration caused by this enormous release of energy.

The energy release (strength of the earthquake) is measured on two types of scales: the Modified Mercalli Intensity Scale, ranging from 1 to 12, which evaluates the earthquake's strength based on the degree of destruction observed, and the Richter Magnitude Scale, which measures the magnitude of movement observed at a distance of 62 mi [100 km] from the point of origin (epicenter) of the quake.

The Richter Scale is a logarithmic scale ranging from 1 to 10, with each step or whole number being 30 times stronger than the previous number. This means that a magnitude 8 quake is roughly 810,000 times as strong as a magnitude 4 quake: 30 x 30 x 30 x 30 = 810,000. To give you a sense of these abstract numbers, a Richter magnitude 2 quake cannot be felt without instrumentation, while a magnitude 9 quake is estimated to be the largest force which can be caused by tectonic plate movement. The moon striking the earth would probably be a magnitude 10.

In reinforced concrete structures, lateral loads can be resisted by a moment-resisting frame, with columns and beams connected rigidly together. In masonry structures, the walls can provide lateral resistance when designed as shear walls (Fig. 1.4).

In steel and wood frames, rigid connections are more expensive to build than in reinforced concrete; consequently, a variety of techniques ranging from rigid connections to shear walls to diagonal bracing are typically used. The structure must also be designed to resist overturning and uplift due to wind, and the same members must be designed for moment, shear, and axial load due to wind or earthquakes. These forces must be combined with the dead loads and a specified percentage of the live loads. Components such as windows and roof finishes must be designed and fastened to resist both wind pressure and wind suction. Lateral loads are assumed to act along the two principal axes of the building. If the structure is safe in these two directions, it is assumed to be adequate for loads acting in any direction. The same wind acting at an angle to a principal axis can be broken into its components acting parallel and perpendicular to the building. These components will produce only a proportion of the full force acting on the principal axis.

The problem in modern tall buildings built with a relatively light, flexible steel frame is not just strength, but also serviceability; in the case of horizontal loads, this means drift and vibrations caused by wind, earthquakes, or other dynamic loads such as machinery.

1.3.1 Structural Systems for Lateral Loads

Wind loads are transferred from the building envelope to the columns, bracing, or shear walls system anchored to the foundations. Earthquake loads similarly are assumed to act at each floor level, and the floor structure and vertical framing must be capable of transferring them to the foundations. Figure 1.5 illustrates how wind loads are resisted by vertical walls. Note that this diagram does not show all the loads on the building.

Two basic types of structural elements are used to transfer these loads through the building to the foundations (Fig. 1.6):

Horizontal Elements

Beams in a rigid frame

Trussed floor systems (horizontal cross-bracing)

Floor diaphragms

Vertical Elements

Columns in a rigid frame

Trussed walls (vertical cross-bracing)

Shear walls

Rigid cores

A trussed floor is designed for horizontal loads essentially in the same way that a truss is designed for gravity loads. As for bracing, when cross-bracing is used, the diagonal members are designed to work in tension only, the most efficient use of material utilizing the smallest possible sizes. Trussed floors are common in steel construction even when concrete-filled decks are specified.

Diaphragms work as deep horizontal beams or as plates spanning between the vertical elements. A diaphragm can be classifieed as rigid or flexible. Common types of deck construction that can be classified as rigid diaphragms are reinforced concrete slabs, steel decks with concrete toppings, and precast concrete tees connected to resist shear. Examples of flexible diaphragms are steel decks without concrete (common on roofs) and plywood. Rigid diaphragms can resist torsion, and some eccentricity in the design of vertical elements is possible. Flexible diaphragms, however, require vertical shear supports at both sides in each direction (Fig. 1.7).

(Continues...)

Excerpted from "Structural Design: A Practical Guide for Architects" by James R. Underwood. Copyright © 0 by James R. Underwood. Excerpted by permission. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher. Excerpts are provided solely for the personal use of visitors to this web site.
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