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Building Design and Construction Handbook - Merritt - Ventech!

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STRUCTURAL STEEL CONSTRUCTION 7.47<br />

provide section property values for the products they market. Table 7.9 lists several<br />

h<strong>and</strong>books widely used by design professionals, as well as other design guides<br />

which address specific design features.<br />

7.17 SERVICEABILITY CRITERIA<br />

Experienced designers are aware of certain practical limitations on the size of individual<br />

members. Flexural members which have marginal or too shallow a depth<br />

can cause deflections that can damage other building elements, as well as cause<br />

vibrations under moving loads that disturb a building’s occupants. Almost all building<br />

code leave stiffness design criteria to the designer. Experienced designers have<br />

found that to specify limits for all possible variations loads, occupancies, <strong>and</strong> types<br />

of construction is impracticable.<br />

This section outlines various criteria, originally based on experience but up-dated<br />

on the basis of testing, which the designer can incorporate to develop a serviceable<br />

design. The ASD specification (Table 7.1) restricts the maximum live-load deflection<br />

of beams <strong>and</strong> girders supporting plaster ceilings to 1 ⁄360 of the span. This<br />

requirement is not applicable to less rigid construction details. The AISC LRFD<br />

specification contains no numerical limits for serviceability criteria. Table 7.10 may<br />

be used to set limits on deflections of flexural members frequently encountered in<br />

building design.<br />

Minimum Depth-Span Ratios. Also, as a guide, Table 7.10 lists suggested minimum<br />

depth-span ratios for various loading conditions <strong>and</strong> yield strengths of steel<br />

up to F y � 50.0 ksi. These may be useful for estimating or making an initial design<br />

selection. Since maximum deflection is a straight-line function of maximum bend-<br />

TABLE 7.10 Guide to Selection of Beam Depths <strong>and</strong> Deflection Limits<br />

Yield stress F y, ksi<br />

36.0 42.0 45.0 50.0<br />

Specific beam condition Minimum depth-span ratio<br />

Heavy shock or vibration 1<br />

18<br />

Heavy pedestrian traffic 1<br />

20<br />

Normal loading 1<br />

22<br />

Beams for flat roofs* 1<br />

25<br />

Roof purlins, except for flat<br />

roofs*<br />

1<br />

28<br />

* Investigate for stability against ponding.<br />

1<br />

15.5<br />

1<br />

17<br />

1<br />

19<br />

1<br />

21.5<br />

1<br />

24<br />

1<br />

14.5<br />

1<br />

16<br />

1<br />

18<br />

1<br />

20<br />

1<br />

22<br />

1<br />

13<br />

1<br />

14.5<br />

1<br />

16<br />

1<br />

18<br />

1<br />

20<br />

Maximum stress, ksi<br />

0.60F y<br />

0.66F y<br />

Maximum ratio of<br />

deflection to span<br />

1<br />

357<br />

1<br />

320<br />

1<br />

290<br />

1<br />

258<br />

1<br />

232<br />

1<br />

324<br />

1<br />

291<br />

1<br />

264<br />

1<br />

232<br />

1<br />

210

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