Upload
others
View
38
Download
1
Embed Size (px)
Citation preview
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
Chapter 05
Structural Steel Design
According to the AISC Manual 13th Edition
Analysis and Design of Beams
By
Dr. Jawad Talib Al-Nasrawi
University of Karbala
College of Engineering
Department of Civil Engineering
71
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
Introduction Flexural Members/Beams: are defined as members acted upon primarily by
transverse loading, often gravity dead and live load effects. Thus, flexural members in a structure may also be referred to as:
Girders: usually the most important beams, which are frequently at wide spacing. Joists: usually less important beams which are closely spaced, frequently with
truss โ type webs. Purlins: roof beams spanning between trusses. Stringers: longitudinal bridge beams spanning between floor beams. Girts: Horizontal wall beams serving principally to resist bending due to wind on
the side of an industrial building, frequently supporting corrugated siding. Lintels: members supporting a wall over window or door openings.
Figure(5-1): Common Beam Members
72
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
Sections Used as Beams Among the steel shapes that are used as beam include: W โ Shapes, which normally prove to be the most economical beam sections, and
they have largely replaced channels and S โ Sections for beam usage. Channels are sometimes used for beams subjected to light loads, such as purlins,
and in places where clearances available require narrow flanges. Another common type of beam section is the open web joist or bar joist. This type of section, which commonly used to support floor and roof slabs, is
actually a light shop โ fabricated parallel chord truss. It is particularly economical for long spans and light loads.
Figure(5-2): W-Section as a Beam Bending Stresses The basic design checks for beams includes checking:- Bending, Shear, and Deflection The loading conditions and beam configuration will dictate which of the
preceding design parameters controls the size of the beam. When a beam is subjected to bending loads, the bending stress in the extreme
fiber is define as:- ๐๐ = ๐ด๐
๐ฐ= ๐ด
๐บ (5-1)
and the yield moment is defined as:- ๐ด๐ = ๐ญ๐๐บ (5-2) where ๐๐=Maximum bending stress, ๐๐ฆ=Yield moment, ๐น๐ฆ=Yield stress, ๐=Bending moment, c =Distance from the neutral axis to the extreme fiber, I =Moment of inertia, and
73
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
S =Section modulus. The above formulation is based on the elastic behavior of the beam.
Figure(5-3): Stress Distribution for Bending Members Plastic Moment
A plastic hinge occurs when the entire cross section of the beam is at its yield point, not just the extreme fiber. The moment at which a plastic hinge is developed in a beam is called the plastic moment and is defined as:- ๐ด๐ = ๐ญ๐๐ (5-3) where ๐๐=plastic moment, and Z =plastic section modulus.
Example (5-1):
For the built-up shape shown in Figure , determine: (a) the elastic section modulus S and the yield moment ๐ด๐. (b) the plastic section modulus Z and the plastic moment ๐ด๐. (c) The shape factor. Bending is about the x-axis, and the steel is A572 Grade 50.
74
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
Solution: (a) Because of the symmetry, the elastic Neutral axis ( x-axis) is locate at mid depth of
the cross section ( the location of the centroid). The moment of inertia of the cross section can be found by using the parallel axis theorem, and the results of the calculations are summarized in Table (5-1).
Table (5-1): Calculations of the Elastic Section Modulus Component I (in4) A (in2) d (in) ๐ฐ + ๐จ๐ ๐
Flange 0.6667 8 6.5 338.7
Flange 0.6667 8 6.5 338.7
Web 72 - - 72
Sum 749.4 The elastic section modulus is:
๐ =๐ผ๐
=749.4
7= 107 ๐๐3
and the yield moment is: ๐๐ฆ = ๐น๐ฆ๐ = 50(107) = 5350 ๐๐ โ ๐๐๐๐
= 446 ๐๐ก โ ๐๐๐๐ (b) Because this shape is symmetrical about the x-axis, this axis divides the cross section into equal areas and is therefore the plastic neutral axis. The centroid of the top half-area can be found by the principle of moments. Taking moments about the x-axis ( the neutral axis of the entire cross section) and tabulating the computations in Table (5-2), we get: Table(5-2)
Component A(in2) Y ( in) Ay (in3)
Flange 8 6.5 52
Web 3 3 9
Sum 11 61
๐ฆ =โ๐ด๐ฆโ๐ด
=6111
= 5.545 ๐๐
๐ = 2๐ฆ = 2(5.545) = 11.09 ๐๐ and that the plastic section modulus is:
๐ = ๐ด2 ๐ = 11(11.09) = 122 ๐๐3
The plastic moment is:
75
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
๐๐ = ๐น๐ฆ๐ = 50(122) = 6100 ๐๐ โ ๐๐๐๐
= 508 ๐๐ก โ ๐๐๐๐ (c) The shape factor of a member cross section can be define as the ratio of the plastic moment to the yield moment.
๐.๐น =๐๐
๐๐ฆ=
508446
= 1.14
The shape factor equals to 1.5 for rectangular cross sections and varies from about 1.1 to 1.2 for standard rolled beam sections. Example(5-2): Determine ๐ด๐, ๐ด๐, and Z for the steel tee beam shown below. Also, calculate the shape factor and the nominal load (๐ค๐)that can be placed on the beam for a 12-ft simple span. ๐น๐ฆ = 50 ๐๐ ๐.
Figure(5-6): Beam Details
Solution: Elastic Calculations:
๐ด = 8 112 + 6(2) = 24 ๐๐2
๐ฆ =12(0.75) + 12(4.5)
24= 2.625 ๐๐
๐ผ =8(1.5)3
12+ 8(1.5)(1.875)2 +
2(6)3
12+ 2(6)(1.875)2 = 122.6 ๐๐4
๐ =๐ผ๐
=122.64.875
= 25.1 ๐๐3
๐๐ฆ = ๐น๐ฆ๐ =50(25.1)
12= 104.6 ๐๐ก โ ๐๐๐๐
Plastic calculations (plastic neutral axis is at base of flange): ๐ = 12(0.75) + 12(3) = 45 ๐๐3
๐๐ = ๐๐ = ๐น๐ฆ๐ =50(45)
12= 187.5 ๐๐ก โ ๐๐๐๐
76
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
๐โ๐๐๐ ๐น๐๐๐ก๐๐ =๐๐
๐๐ ๐๐
๐๐
=45
25.1= 1.79
๐๐ =๐ค๐๐2
8 โ ๐ค๐ =
8(187.5)12
= 10.4 ๐/๐๐ก The values of the plastic section moduli for the standard steel beam sections are
tabulated in AISC Manual Table 3-2. Classification of Shapes AISC classified cross-sectional shapes as: Compact, Noncompact, and Slender
Depending on the values of the width-thickness ratios of the individual elements that form the shape. There are also two type of elements that are defined in the AISC Specification: Stiffened elements, and Unstiffened elements. For I-shape, the ratio for the projection flange ( an unstiffened element) is ๐๐
2๐ก๐ ,
and the ratio for the web ( a stiffened element) is โ๐ก๐ค
. The classification of shapes is found in Section B4 of the specification, โ Local
Bucklingโ, in Table B4-1. it can be summarized as follows: ๐=width-thickness ratio ๐๐=upper limit for compact category ๐๐=upper limit for noncompact category If ๐ โค ๐๐ and the flange is continuously connected to the web, the shape is
compact; If ๐๐ < ๐ โค ๐๐ , the shape is noncompact; and If ๐ > ๐๐ , the shape is slender.
77
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
Example (5-3): Determine the classification of a W18x35 and a W21x48 for ๐น๐ฆ =50 ๐๐ ๐. Check both the flange and the web. Solution: From Part 1 of the AISC Manual, get: W18x35 W21x48 ๐๐
2๐ก๐= 7.06 ๐๐
2๐ก๐= 9.47
โ๐ก๐ค
= 53.5 โ๐ก๐ค
= 53.6 Flange:
๐๐๐ = 0.38๐ธ๐น๐ฆ
= 0.3829000
50= 9.15 > 7.06
โด the W18x35 flange is compact
๐๐๐ = 1.0๐ธ๐น๐ฆ
= 1.029000
50= 24.0 > 9.47 > 9.15
โด the W21x48 flange is noncompact Web:
๐๐๐ค = 3.76๐ธ๐น๐ฆ
= 3.7629000
50= 90.5 > 53.6
โด the W16x35 and W21x48 webs are compact 78
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
Bending Strength of Compact Shapes The basic design strength equation for beams in bending is: ๐ด๐ โค โ ๐๐ด๐ ( For LRFD) ๐ด๐ โค
๐ด๐ฮฉ๐
( For ASD) where ๐๐ข= Ultimate Moment, โ ๐ = 0.9 ๐๐= Allowable Moment, ฮฉ๐ = 1.67 ๐๐= Nominal bending strength, โ ๐๐๐= Ultimate design bending strength, and ๐๐ฮฉ๐
= Allowable design bending strength. The nominal bending strength, ๐๐, is a function of the following:
1) Lateral โ torsional buckling ( LTB), 2) Flange local buckling (FLB), and 3) Web local buckling (WLB). Flange local buckling and web local buckling are localized failure modes and are
only of concern with shapes that have noncompact webs or flanges. Lateral โ torsional buckling occurs
when the distance between lateral brace points is large enough that the beam fails by lateral, outward movement in combination with a twisting action (ฮ and ฮธ ), as shown in Figure ( 5-7).
Figure (5-7) Figure ( 5-8) shows that beams have three distinct ranges, or zones, of behavior, depending on their lateral bracing situation.
79
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
Figure (5-8) The compact shapes defined as those whose webs are continuously connected to the flanges and that satisfy the following width-thickness ratio requirements for the flange and web:
๐๐2๐ก๐
โค 0.38๐ธ๐น๐ฆ
๐๐๐ โ๐ก๐ค
โค 3.76๐ธ๐น๐ฆ
If the beam is compact and has continuous lateral support, or if the unbraced length is very short (๐ณ๐ โค ๐ณ๐), the nominal moment strength, ๐ด๐, is the full plastic moment capacity of the shape, ๐ด๐.
๐ด๐ = ๐ด๐ = ๐ญ๐๐๐ ( AISC Equ. F2-1) Example (5-4): The beam shown in Figure(5-9) is a W16x31 of A992 steel. Its supports a reinforced concrete floor slab that provides continuous lateral support of the compression flange. The service dead load is 450 lb./ft. This load is superimposed on the beam; it does not include the weight of the beam itself. The service live load is 550 lb./ft. Does this beam have adequate moment strength?
Figure(5-9)
80
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
Solution: Check the compactness: ๐๐
2๐ก๐= 6.28 ( From Part 1 of the Manual)
0.38๐ธ๐น๐ฆ
= 0.3829000
50= 9.15 > 6.28
โด the flange is compact โ๐ก๐ค
< 3.76๐ธ๐น๐ฆ
( the web is compact for all shapes in the Manual for ๐น๐ฆ โค 65 ๐๐ ๐)
โด a W16x31 is compact The nominal flexural strength is: ๐๐ = ๐๐ = ๐น๐ฆ๐๐ฅ = 50(54) = 2700 ๐๐. ๐๐๐๐ = 225 ๐๐ก. ๐๐๐๐ Compute the max. bending moment:
The total service dead load, including the weight of the beam, is: ๐ค๐ท = 450 + 31 = 481 ๐๐/๐๐ก ๐๐๐๐ฅ = ๐ค๐2
8โ ๐๐ท = 0.481(30)2
8= 54.11 ๐๐ก. ๐๐๐๐
๐๐ฟ = 0.550(30)2
8= 61.88 ๐๐ก. ๐๐๐๐
๐๐ข = 1.2๐๐ท + 1.6๐๐ฟ = 1.2(54.11) + 1.6(61.88) = 164 ๐๐ก. ๐๐๐๐ Or ๐ค๐ข = 1.2(0.481) + 1.6(0.55) = 1.457 ๐๐๐๐ /๐๐ก
๐๐ข =๐ค๐2
8=
1.457(30)2
8= 164 ๐๐ก. ๐๐๐๐
The design strength is: โ ๐๐๐ = 0.9(225) = 203 ๐๐ก. ๐๐๐๐ > 164 ๐๐ก. ๐๐๐๐
โด the W16x31 is satisfactory The AISC Specification defines the unbraced length at which inelastic lateral-torsional buckling occurs as:
๐ฟ๐ = 1.76๐๐ฆ๐ธ๐น๐ฆ
( AISC Equ. F2-5)
๐ณ๐ is also the max. unbraced length at which the nominal bending strength equals the plastic moment capacity. The unbraced length at which elastic lateral-torsional buckling occurs is:
๐ฟ๐ = 1.95๐๐ก๐ ๐ธ
0.7๐น๐ฆ ๐ฝ๐๐๐ฅโ๐
1 + 1 + 6.76 0.7๐น๐ฆ๐ธ
๐๐ฅโ๐๐ฝ๐2 ( AISC Equ. F2-6)
81
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
where, ( AISC Equ. F2-7) ๐ = 1.0 ( for I-shapes) ( AISC Equ. F2-8a)
๐ = โ๐2
๐ผ๐ฆ๐ถ๐ค
( for Channel shapes) ( AISC Equ. F2-8b)
๐น๐ฆ = Yield strength ๐ธ = Modulus of elasticity ๐ฝ = Torsional constant ๐๐ฅ = Section modulus ( x-axis) ๐ผ๐ฆ = Moment of inertia ( y-axis) ๐ถ๐ค = Warping constant, and โ๐ = Distance between flange centroids= ๐ โ ๐ก๐. For compact I-shapes and C-shapes when ๐ณ๐ < ๐ณ๐ < ๐ณ๐, the nominal flexural
strength is:
๐๐ = ๐ถ๐ ๐๐ โ ๐๐ โ 0.7๐น๐ฆ๐๐ฅ ๐ฟ๐โ๐ฟ๐๐ฟ๐โ๐ฟ๐
โค ๐๐ (AISC Equ. F2-2)
For compact I-shapes and C-shapes, when ๐ณ๐ > ๐ณ๐, the nominal flexural strength is:
๐๐ = ๐น๐๐๐๐ฅ โค ๐๐ ( AISC Equ. F2-3) where
๐น๐๐ = ๐ถ๐๐2๐ธ
๐ฟ๐ ๐๐ก๐ 2 1 + 0.078 ๐ฝ๐
๐๐ฅโ๐๐ฟ๐๐๐ก๐ 2 ( AISC Equ. F2-4)
๐ถ๐ =Moment gradient factor, factor to account for non-uniform bending within the unbraced length ๐ฟ๐. ๐ถ๐ = 12.5๐๐๐๐ฅ
2.5๐๐๐๐ฅ+3๐๐ด+4๐๐ต+3๐๐ถ๐ ๐ โค 3.0 ( AISC Equ. F1-1)
where ๐๐๐๐ฅ =Absolute value of the maximum moment in the unbraced segment, ๐๐ด =Absolute value of the moment at the ยผ point of the unbraced segment, ๐๐ต =Absolute value of the moment at the centerline of the unbraced segment, ๐๐ถ =Absolute value of the moment at the 3/4 point of the unbraced segment, ๐ ๐ =Section symmetry factor ๐ ๐ = 1.0 , for doubly symmetric members ( I-shapes), ๐ ๐ = 1.0 , for singly symmetric sections in single-curvature bending,
๐ ๐ = 0.5 + 2 ๐ผ๐ฆ๐๐ผ๐ฆ2 , for single symmetric shapes subjected to reverse curvature
bending, and ๐ผ๐ฆ๐ =Moment of inertia of the compression flange about the y-axis. ๐ถ๐ = 1.0 , for cantilevers or overhangs where the free end is unbraced.
82
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
Some of ๐ถ๐ values are given in Table 3-1 of the AISC.
Example(5-5): Determine ๐ช๐ for the beam shown in Figure(5-10) parts (a) and (b). Assume the beam is a doubly symmetric member.
Figure(5-10) Solution:
83
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
๐ถ๐ =12.5๐๐๐๐ฅ
2.5๐๐๐๐ฅ + 3๐๐ด + 4๐๐ต + 3๐๐ถ๐ ๐ โค 3.0
๐ถ๐ =12.5 1
8
2.5 18 + 3 3
32 + 4 18 + 3 3
32= 1.14
๐ถ๐ =12.5 1
12
2.5 112 + 3 3
96 + 4 124 + 3 3
96= 2.38
Example (5-6): Determine the flexural strength of a W14x68 of A242 steel, Grade 50, subjected to:
a) Continuous lateral support. b) An unbraced length of 20 ft with ๐ถ๐ = 1.0 c) An unbraced length of 30 ft with ๐ถ๐ = 1.0
Solution: Determine whether this shape is compact, noncompact, or slender:
๐๐2๐ก๐
= 6.97 ( from Part 1 of the Manual)
0.38๐ธ๐น๐ฆ
= 0.3829000
50= 9.15 > 6.97
โด the flange is compact The web is compact for all shapes in the Manual for ๐น๐ฆ โค 65 ๐๐ ๐
a) Because the beam is compact and laterally supported (๐ฟ๐ = 0 < ๐ฟ๐), the nominal flexural strength is:
๐๐ = ๐๐ = ๐น๐ฆ๐๐ฅ = 50(115) = 5750 ๐๐. ๐๐๐๐ = 479.2 ๐๐ก. ๐๐๐๐
84
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
LRFD Solution: The design strength is:
โ ๐๐๐ = 0.9(479.2) = 431 ๐๐ก. ๐๐๐๐ ASD Solution: The allowable moment strength is:
๐๐
ฮฉ๐=
479.21.67
= 288 ๐๐ก.๐๐๐๐
b) ๐ฟ๐ = 20 ๐๐ก ๐๐๐ ๐ถ๐ = 1.0. First, determine ๐ฟ๐ and ๐ฟ๐:
๐ฟ๐ = 1.76๐๐ฆ๐ธ๐น๐ฆ
= 1.76(2.46)29000
50= 104.3 ๐๐ = 8.692 ๐๐ก
The following terms will be needed in the computation of ๐ฟ๐:
= ๐ผ๐ฆ๐ถ๐ค๐๐ฅ
= 121(5380)103
= 7.833 ๐๐2
๐๐ก๐ = โ7.833 = 2.799 ๐๐ (๐๐ก๐ can also be found in the dimensions and properties tables in Part 1 of the Manual).
โ๐ = ๐ โ ๐ก๐ = 14.0 โ 0.72 = 13.28 ๐๐ (โ๐ can also be found in the dimensions and properties tables in Part 1 of the Manual). For a doubly-symmetric I-shape, ๐ = 1.0 ( From AISC Equ. F2-6).
๐ฟ๐ = 1.95๐๐ก๐ ๐ธ
0.7๐น๐ฆ
๐ฝ๐๐๐ฅโ๐
1 + 1 + 6.76 0.7๐น๐ฆ๐ธ
๐๐ฅโ๐๐ฝ๐
2
๐ฟ๐ = 1.95(2.799)29000
0.7(50)
3.01(1.0)103(13.28)
1 + 1 + 6.760.7(50)29000
103(13.28)3.01(1.0)
2
๐ฟ๐ = 351.3 ๐๐ = 29.28 ๐๐ก Since ๐ฟ๐ < ๐ฟ๐ < ๐ฟ๐, then:
๐๐ = ๐ถ๐ ๐๐ โ ๐๐ โ 0.7๐น๐ฆ๐๐ฅ ๐ฟ๐ โ ๐ฟ๐๐ฟ๐ โ ๐ฟ๐
โค ๐๐
= 1.0 5750 โ (5750 โ 0.7 ร 50 ร 103) 20 โ 8.692
29.28 โ 8.692
๐๐ = 4572 ๐๐. ๐๐๐๐ = 381 ๐๐ก. ๐ < ๐๐ = 479.2 ๐๐ก. ๐ LRFD Solution: The design strength is:
โ ๐๐๐ = 0.9(381) = 343 ๐๐ก. ๐๐๐๐ ASD Solution: The allowable moment strength is:
๐๐
ฮฉ๐=
3811.67
= 229 ๐๐ก. ๐๐๐๐
85
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
c) ๐ณ๐ = ๐๐ ๐๐ ๐๐๐ ๐ช๐ = ๐.๐ Since ๐ฟ๐ > ๐ฟ๐ = 29.28, then: ๐๐ = ๐น๐๐๐๐ฅ โค ๐๐ ( AISC Equ. F2-3)
๐น๐๐ =๐ถ๐๐2๐ธ
๐ฟ๐ ๐๐ก๐ 2 1 + 0.078
๐ฝ๐๐๐ฅโ๐
๐ฟ๐๐๐ก๐ 2
๐น๐๐ =๐2(29000)
30 ร 122.799
2 1 + 0.0783.01(1)
103(13.28) 30 ร 12
2.7992
๐น๐๐ = 33.9 ๐๐ ๐ and
๐๐ = 33.9(103) = 3492 ๐๐. ๐ = 291 ๐๐ก.๐ < ๐๐ = 479.2 ๐๐ก. ๐ LRFD Solution: โ ๐๐๐ = 0.9(291) = 262 ๐๐ก. ๐๐๐๐ ASD Solution: ๐๐
ฮฉ๐= 291
1.67= 175 ๐๐ก. ๐๐๐๐
Non-Compact Shapes
The noncompact sections are those that have web-thickness ratios greater than ๐๐ , but not greater than ๐๐. For the noncompact range, the width-thickness ratios of the flanges of W or other I-shaped rolled sections must not exceed ๐๐ = 1.0๐ธ ๐น๐ฆโ , while those for the webs must not exceed ๐๐ = 5.7๐ธ ๐น๐ฆโ . Other values are provided in AISC Table B4.1b for ๐๐ and ๐๐ for other shapes From AISC F3, Flange local buckling, if ๐๐ < ๐ โค ๐๐, the flange is noncompact, buckling will be inelastic, and
๐๐ = ๐๐ โ ๐๐ โ 0.7๐น๐ฆ๐๐ฅ ๐โ๐๐๐๐โ๐๐
( AISC Equ. F3-1)
where ๐ = ๐๐2๐ก๐
, ๐๐ = 0.38๐ธ๐น๐ฆ
, ๐๐๐ ๐๐ = 1.0๐ธ๐น๐ฆ
Example(5-7): A simply supported beam with a span length of 45 ft is laterally supported at its ends and is subjected to the following service loads: Dead load=400 lb./ft ( including beam weight). Live load=1000 lb./ft. If ๐น๐ฆ = 50 ๐๐ ๐, is a W14x90 adequate?
86
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
Solution: Determine whether the shape is compact, noncompact, or slender:
๐ =๐๐2๐ก๐
= 10.2 , ๐๐ = 0.38๐ธ๐น๐ฆ
= 9.15 , ๐๐๐ ๐๐ = 1.0๐ธ๐น๐ฆ
= 24.1
Since ๐๐ < ๐ โค ๐๐ , this shape is noncompact. Check the capacity based on the limit state of flange local buckling:
๐๐ = ๐น๐ฆ๐๐ฅ = 50(157) = 7850 ๐๐.๐๐๐๐
๐๐ = ๐๐ โ ๐๐ โ 0.7๐น๐ฆ๐๐ฅ ๐ โ ๐๐๐๐ โ ๐๐
๐๐ = 7850 โ (7850 โ 0.7 ร 50 ร 143) 10.2 โ 9.1524.1 โ 9.15
= 7650 ๐๐. ๐๐๐๐ = 637.5 ๐๐ก. ๐๐๐๐
Check the capacity based on the limit state of lateral-torsional buckling: From the ๐๐ฅ table: ๐ฟ๐ = 15.2 ๐๐ก ๐๐๐ ๐ฟ๐ = 42.6 ๐๐ก ๐ฟ๐ = 45 ๐๐ก > ๐ฟ๐ โด failure is by elastic LTB From Part 1 of the Manual:
๐ผ๐ฆ = 362 ๐๐4, ๐๐ก๐ = 4.11 ๐๐ , โ๐ = 13.3 ๐๐ , ๐ฝ = 4.06 ๐๐4, ๐ถ๐ค = 16000 ๐๐6
For a uniformly loaded, simply supported beam with lateral support at the ends: ๐ถ๐ = 1.14 ( AISC Table 3-1) For a doubly-symmetric I-shape, c=1.0 ( AISC Equ.F2-4)
๐น๐๐ =๐ถ๐๐2๐ธ
๐ฟ๐ ๐๐ก๐ 2 1 + 0.078
๐ฝ๐๐๐ฅโ๐
๐ฟ๐๐๐ก๐ 2
๐น๐๐ =1.14๐2(29000)
45 ร 124.11
21 + 0.078
4.06(1)143(13.3)
45 ร 124.11
2
๐น๐๐ = 37.2 ๐๐ ๐ From AISC Equ. F2-3:
๐๐ = ๐น๐๐๐๐ฅ = 37.2(143) = 5320 ๐๐. ๐ < ๐๐ = 7850 ๐๐. ๐ This is smaller than the nominal strength based on flange local buckling, so lateral-torsional buckling controls. LRFD Solution: The design strength is:
โ ๐๐๐ = 0.9(5320) = 4788 ๐๐. ๐๐๐๐ = 399 ๐๐ก. ๐๐๐๐ The factored load and moment are:
๐ค๐ข = 1.2๐ค๐ท + 1.6๐ค๐ฟ = 1.2(0.4) + 1.6(1) = 2.08 ๐/๐๐ก 87
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
๐๐ข =๐ค๐ข๐ฟ2
8=
2.08(45)2
8= 527 ๐๐ก. ๐ > 399๐๐ก. ๐ ๐.๐บ
Summary of Moment Strength This summary is for compact and noncompact shapes (noncompact flanges) only ( no slender shapes):
1) Determine whether the shape is compact. 2) If the shape is compact, check for lateral-torsional buckling as follows:
Using ๐ฟ๐ = 1.76๐๐ฆ๐ธ๐น๐ฆ
,
If ๐ฟ๐ โค ๐ฟ๐, there is no LTB, and ๐๐ = ๐๐ If ๐ฟ๐ < ๐ฟ๐ โค ๐ฟ๐, there is inelastic LTB, and
๐๐ = ๐ถ๐ ๐๐ โ ๐๐ โ 0.7๐น๐ฆ๐๐ฅ ๐ฟ๐ โ ๐ฟ๐๐ฟ๐ โ ๐ฟ๐
โค ๐๐
If ๐ฟ๐ > ๐ฟ๐, there is inelastic LTB, and ๐๐ = ๐น๐๐๐๐ฅ โค ๐๐
Where ๐น๐๐ = ๐ถ๐๐2๐ธ
๐ฟ๐ ๐๐ก๐ 2 1 + 0.078 ๐ฝ๐
๐๐ฅโ๐๐ฟ๐๐๐ก๐ 2
3) If the shape is noncompact because of the flange, the nominal strength will be the smaller of the strength corresponding to flange local buckling and lateral-torsional buckling.
a) Flange local buckling: If ๐ โค ๐๐, there is no FTB, If ๐๐ < ๐ โค ๐๐, the flange is noncompact, and
๐๐ = ๐๐ โ ๐๐ โ 0.7๐น๐ฆ๐๐ฅ ๐ โ ๐๐๐๐ โ ๐๐
b) Lateral-torsional buckling:
Using ๐ฟ๐ = 1.76๐๐ฆ๐ธ๐น๐ฆ
,
If ๐ฟ๐ โค ๐ฟ๐, there is no LTB, If ๐ฟ๐ < ๐ฟ๐ โค ๐ฟ๐, there is inelastic LTB, and
๐๐ = ๐ถ๐ ๐๐ โ ๐๐ โ 0.7๐น๐ฆ๐๐ฅ ๐ฟ๐ โ ๐ฟ๐๐ฟ๐ โ ๐ฟ๐
โค ๐๐
If ๐ฟ๐ > ๐ฟ๐, there is elastic LTB, and ๐๐ = ๐น๐๐๐๐ฅ โค ๐๐
88
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
Where ๐น๐๐ = ๐ถ๐๐2๐ธ
๐ฟ๐ ๐๐ก๐ 2 1 + 0.078 ๐ฝ๐
๐๐ฅโ๐๐ฟ๐๐๐ก๐ 2
Design for Shear Generally, shear is not a problem in steel beams, because the webs of rolled shapes
are capable of resisting rather large shearing forces. Perhaps it is well, however, to list the most common situation where shear might be excessive:
1) Should large concentrated loads be placed near beam supports. 2) Probably the most common shear problem occurs where two members ( as a beam
and column) are rigidly connected together so that their webs lie in a common plane.
3) Where beams are notched or coped, as shown in Figure(5-11).
Figure(5-11) 4) Theoretically, very heavily loaded short beams can have excessive shears, but
practically, this does not occur too often unless it is like case 1. 5) Shear may very well be a problem even for ordinary loading when very thin webs
are used, as in plate girders or in light-gage cold-formed steel members. From mechanics of materials, the general formula for shear stress in a beam is:
๐๐ฃ =๐๐๐ผ๐
where ๐๐ฃ = Shear stress at the point under consideration, ๐ = Vertical shear at a point along the beam under consideration, ๐ผ = Moment of inertia about the neutral axis, and ๐ = Thickness of the section at the point under consideration The AISC specification allows the design for shear to be based on an approximate or average shear stress distribution as shown in Figure(5-12b), where the shear stress is concentrated only in the vertical section of the beam. In the AISC specification, the shear yield stress is taken as 60% of the yield stress, ๐น๐ฆ. The nominal shear strength of unstiffened or stiffened webs is specified as: ๐ฝ๐ = ๐.๐๐ญ๐๐จ๐๐ช๐ ( AISC Equ. G2-1)
89
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
Figure(5-12): Shear in a beam where ๐ด๐ค = Area of the webโ ๐๐ก๐ค ๐ = Overall depth of the beam ๐ถ๐ฃ =Web shear coefficient The value of ๐ถ๐ฃ depends on whether the limit state is web yielding, web inelastic buckling, or web elastic buckling. For the special case of hot-rolled I-shapes with:
โ๐ก๐ค
โค 2.24๐ธ๐น๐ฆ
The limit state is shear yielding, and ๐ถ๐ฃ = 1.0, โ ๐ฃ = 1.0, ฮฉ๐ฃ = 1.5
Most W shapes with (๐ญ๐ โค ๐๐ ๐๐๐) fall into this category. Except for the following shapes for (๐ญ๐ = ๐๐ ๐๐๐):
W12x14, W16x26, W24x55, W30x90, W33x118, W36x135, W40x149, and W44x230 For all other doubly and singly symmetric shapes, except for round HSS
โ ๐ฃ = 0.90, ฮฉ๐ฃ = 1.67 and ๐ถ๐ฃ is determine as follows:
For โ๐ก๐คโค 1.1
๐๐ฃ๐ธ๐น๐ฆ
, there is no web instability, and
๐ถ๐ฃ = 1.0 ( AISC Equ. G2-3)
For 1.1๐๐ฃ๐ธ๐น๐ฆ
< โ๐ก๐คโค 1.37
๐๐ฃ๐ธ๐น๐ฆ
, inelastic web buckling can occur, and
๐ถ๐ฃ =1.1
๐๐ฃ๐ธ๐น๐ฆ
โ๐ก๐ค
( AISC Equ. G2-4)
For โ๐ก๐คโฅ 1.37
๐๐ฃ๐ธ๐น๐ฆ
, there limit state is elastic web buckling, and
90
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
๐ถ๐ฃ = 1.51๐ธ๐๐ฃ
โ ๐ก๐ค 2๐น๐ฆ
( AISC Equ. G2-5)
where ๐๐ the web plate shear buckling coefficient, is specified in the AISC Specification G2.1b. For webs without transverse stiffeners and with โ
๐ก๐ค< 260:
๐๐ฃ = 5 Except that ๐๐ = ๐.๐ for the stem of T-shapes. For all steel shapes, ๐ช๐ = ๐.๐ , except for the following for ๐น๐ฆ = 50 ๐๐ ๐ :
M10x7.5, M10x8, M12,10, M12x10.8, M12x11.8, M12.5x11.6, and M12.5x12.4 Example (5-8): Check the beam in Example(5-7) for shear. Solution : From the dimensions and properties tables in Part 1 of the Manual, the web width-thickness ratio of a W14x90 is:
โ๐ก๐ค
= 25.9
and the web area is: ๐ด๐ค = ๐๐ก๐ค = 14(0.44) = 6.16 ๐๐2
2.24๐ธ๐น๐ฆ
= 2.2429000
50= 54
Since โ๐ก๐ค
< 2.24๐ธ๐น๐ฆ
the strength is governed by shear yielding of the web and ๐ถ๐ฃ =
1.0. ( as pointed out in the Specification User Note, this will be the case for most W shapes with ๐น๐ฆ โค 50 ๐๐ ๐ ). The nominal shear strength is:
๐๐ = 0.6๐น๐ฆ๐ด๐ค๐ถ๐ฃ = 0.6(50)(6.16)(1.0) = 184.8 ๐๐๐๐ LRFD Solution: Determine the resistance factor โ ๐ฃ
Since โ๐ก๐ค
< 2.24๐ธ๐น๐ฆ
โ โ ๐ฃ = 1.0
and the design shear strength is: โ ๐ฃ๐๐ = 1.0(184.8) = 185 From Example(5-7), ๐ค๐ข = 2.08 ๐๐๐๐ /๐๐ก and L=45 ft. For a simply supported, uniformly loaded beam, the max. shear occurs at the support and is equal to the reaction:
๐๐ข =๐ค๐ข๐ฟ
2=
2.08(45)2
= 46.8 ๐๐๐๐ < 185 ๐๐๐๐ ๐.๐พ Answer: The required shear strength is less than the available shear strength, so the beam is satisfactory.
91
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
The values of โ ๐ฃ๐๐๐ฅ ๐๐๐ ๐๐๐ฅ ฮฉ๐ฃ with ๐น๐ฆ = 50 ๐๐ ๐ are given for W shapes in the
Manual Table 3-2. A very useful Table(3-6) is provided in Part 3 of the AISC Manual for
determining the max. uniform load each W shape can support for various spans. Beam Design Tables The design bending strength of W-shapes and C-shapes with respect to the
unbraced length is given in AISC, Tables 3-10 and 3-11, respectively. These tables assume a moment gradient factor of Cb=1.0, which is conservative for all cases, and yield strengths of Fy=50 ksi for W-shapes and Fy=36 ksi for C-shapes.
For beams with Cb greater than 1.0, multiply the moment capacity calculated using these tables by the Cb value to obtain the actual design moment capacity of the beam for design moments that correspond to unbraced lengths greater than Lp. Note that ๐ถ๐โ ๐๐must always be less than โ ๐๐ .
AISC, Tables 3-2 through 3-5 can be used to select the most economical beam based on section properties. AISC, Table 3-2 lists the plastic section modulus, Zx, for a given series of shapes, with the most economical in one series at the top of the list in bold font.
AISC, Table 3-6 provides a useful summary of the beam design parameters for W-shapes. The lower part of the table provides values for รMp, รMr, รVn, Lp, and Lr for any given shape. The upper portion of the table provides the maximum possible load that a beam may support based on either shear or bending strength.
AISC, Table 3-6 can also be used to determine the design bending strength for a given beam if the unbraced length is between Lp and Lr. When the unbraced length is within this range, the design bending strength is:
โ ๐๐๐ = โ ๐๐๐ โ ๐ต๐น๐ฟ๐ โ ๐ฟ๐ where BF is a constant found from AISC Table 3-6. Note that this equation is simpler version of (AISC Equ. F2-2) Deflection The deflections of steel beams are usually limited to certain maximum values. Among the several excellent reasons for deflection limitations are the following:
1) Excessive deflections may damage other materials attached to or supported by the beam in equation.
2) The appearance of structures is often damaged by excessive deflections. 3) Extreme deflections do not inspire confidence in the persons using a structure. 4) It may be necessary for several different beams supporting the same loads to
deflect equal amounts.
92
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
The student should note that deflection limitations fall in the serviceability area.
Therefore, deflections are determined for service loads, and thus the calculations are identical for both LRFD and ASD designs.
For the common case of a simply supported, uniformly loaded beam, the max. vertical deflection is:
โ=5
384๐ค๐ฟ4
๐ธ๐ผ
Deflection formulas for a variety of beams and loading conditions can be found in Part 3 of the Manual.
Beam Design Procedure The design process can be outlined as follows:-
1) Determine the service and factored loads on the beam. Service loads are used for deflection calculations and factored loads are used for strength design. The weight of the beam would be unknown at this stage, but the self-weight can be initially estimated and is usually comparatively small enough not to affect the design.
2) Determine the factored shear and moments on the beam. 3) Select a shape that satisfies strength and deflection criteria. One of the following
methods can be used: a) For shapes listed in the AISC beam design tables, select the most economical
beam to support the factored moment. Then check deflection and shear for the selected shape.
b) Determine the required moment of inertia. Select the most economical shape based on the moment of inertia calculated, and check this shape for bending and shear.
c) For shapes not listed in the AISC beam design tables, an initial size must be assumed. An estimate of the available bending strength can be made for an initial beam selection; then check shear and deflection. A more accurate method might be to follow the procedure in step b above.
4) Check the shear strength. 5) Check the deflection.
Example(5-9): For the floor plan shown in Figure (5-13), design members B1 and G1 for bending, shear, and deflection. Compare deflections with L/240 for total loads and L/360 for live loads. The steel is ASTM A992, grade 50; assume that ๐ถ๐ = 1.0 for bending. The dead load ( including the beam weight) is assumed to be (85 psf) and the live load is (150 psf). Assume that the floor deck provides full lateral stability to the top flange of B1 Ignore live load reduction. Use the design tables in the AISC where appropriate.
93
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
Figure (5-13): Floor Plan for Example (5-9)
Solution: Since the dead load is more than half of the live load, the total load deflection of L/240 will control. Summary of loads ( see Figure(5-14).
Figure(5-14): Loading for B1 and G1 ๐๐ = 85 + 150 = 235 ๐๐ ๐ ( total service load) ๐๐ข = 1.2(85) + 1.6(150) = 342 ๐๐ f ( total service load) Design of Beam B1: Tributary width= 6โฒ โ 8โฒโฒ = 6.67โฒ
๐ค๐ = 6.67(0.235) = 1.57 ๐๐๐๐ /๐๐ก ๐ค๐ข = 6.67(0.342) = 2.28 ๐๐๐๐ /๐๐ก
๐๐ข =๐ค๐ข๐ฟ
2=
2.28(30)2
= 34.2 ๐๐๐๐
๐๐ข =๐ค๐ข๐ฟ2
8=
2.28(30)2
8= 257 ๐๐ก.๐๐๐๐
From AISC Table 3-10, for: ๐๐ข = 257 ๐๐ก. ๐๐๐๐ ๐๐๐ ๐ฟ๐ = 0 , try with W18x40, the most economical size for bending (โ ๐๐๐ =294 ๐๐ก. ๐๐๐๐ , ๐๐๐ ๐ผ = 612 ๐๐4)
94
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
๐ =๐ค๐ฟ2
8โ 8๐ = ๐ค๐ฟ2
โ=5๐ค๐ฟ4
384๐ธ๐ผ=
5(8๐)๐ฟ2
384๐ธ๐ผ=
40๐๐ฟ2(1728)384(29000)๐ผ
=๐๐ฟ2
161.1๐ผ
๐ฟ(12)240
=๐๐ฟ2
161.1๐ผ
๐ผ๐๐๐. =๐๐ฟ2
8.056=
๐ค๐ฟ2
64.44=
1.57(30)2
64.44= 658 ๐๐4
The required moment of inertia is greater than the moment of inertia of the W18x40, which is 612 ๐๐4; therefore, a new size needs to be selected. From AISC Table 1-1, get:
๐16 ร 50, ๐ผ = 659 ๐๐4 ๐18 ร 46, ๐ผ = 712 ๐๐4
๐21 ร 44, ๐ผ = 843 ๐๐4 โ ๐๐๐๐๐๐ก ๐24 ร 55, ๐ผ = 1350 ๐๐4
Try W21x44 is the lightest: ๐ผ = 843 ๐๐4 > ๐ผ๐๐๐. = 658๐๐4 ๐.๐พ
From AISC Table 3-10, find, โ ๐๐๐ = 358 ๐๐ก. ๐๐๐๐ > ๐๐ข = 257 ๐๐ก. ๐๐๐๐ ๐.๐พ
Checking shear, note that a W21x44 does not have a slender web; therefore, the design shear strength is determined from equation G2-1, with ๐ถ๐ฃ = 1.0 and โ ๐ฃ = 1.0:
โ ๐ฃ๐๐ = โ ๐ฃ0.6๐น๐ฆ๐ด๐ค๐ถ๐ฃ = 1.0(0.6)(50)(0.35)(20.7)(1.0) = 217 ๐๐๐๐ > ๐๐ข = 34.2 ๐๐๐๐ O.K Alternatively, the shear strength can be found from AISC Table 3-6: โ ๐ฃ๐๐ = 217 ๐๐๐๐ same as above a W21x44 is selected for member B1 Design of Beam G1: Tributary width= (6โฒ โ 8โฒโฒ)(30) = 200๐๐ก2
๐๐ = 200(0.235) = 47 ๐๐๐๐ ๐๐ข = 200(0.342) = 68. 4 ๐๐๐๐
๐๐ข = ๐๐ข = 68.4 ๐๐๐๐
๐๐ข =๐๐ข๐ฟ
3=
68.4(20)3
= 456 ๐๐ก โ ๐๐๐๐
For ๐๐ข = 456 ๐๐ก โ ๐๐๐๐ ๐๐๐ ๐ฟ๐ = 6.67 ๐๐ก, and from AISC Table 3-10, select a W24x55 the W24x55 is the most economical size for bending, with โ ๐๐๐ = 460 ๐๐ก โ ๐๐๐๐ Checking deflection:
โ=๐๐ฟ3
28๐ธ๐ผ=
47[20(12)]3
28(29000)(1350) = 0.593๐๐ <๐ฟ
240= 1๐๐
95
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
Check Shear: The design shear strength is determine from Equation G2-1, with ๐ถ๐ฃ = 1.0 ๐๐๐ โ ๐ฃ =0.9:
โ ๐ฃ๐๐ = โ ๐ฃ0.6๐น๐ฆ๐ด๐ค๐ถ๐ฃ = 0.9(0.6)(50)(0.395)(23.6)(1.0) = 251 ๐๐๐๐ > ๐๐ข = 68.4 ๐๐๐๐ O.K Alternatively, the shear strength can be found from AISC Table 3-6: โ ๐ฃ๐๐ = 251 ๐๐๐๐ same as above a W24x55 is selected for member G1 Webs and Flanges with Concentrated Loads If flange and web strengths do not satisfy the requirements of AISC Specification Section J.10, it will be necessary to use transverse stiffeners at the concentrated loads. These situations are discussed as follow:
1) Local Flange Bending The flange must be sufficiently rigid so that it will not deform and cause a zone of high stress concentrated in the weld in line with the web as shown in Figure (5-15). Figure (5-15) The nominal tensile load that may be applied through a plate welded to the flange of a W section is to be determined by the expression to follow, in which ๐น๐๐ is the specified minimum yield stress of the flange (ksi) and ๐๐ is the flange thickness (in): ( AISC Equ. J10-1)
ร=0.9 ( LRFD) and ฮฉ=1.67 ( ASD) It is not necessary to check this formula if the length of loading across the beam
flange is less than 0.15 times the width ๐๐ or if a pair of half-depth or deeper web stiffeners are provided.
2) Local Web Yielding Local web yielding is illustrated in Figure (5-16). The nominal strength of the web of a beam at the web toe of the fillet when a concentrated load or reaction is applied is to be determined by one of the following two expression, in which k is the distance from the outer edge of the flange to the web toe of the fillet, ๐ is the length of bearing (in) of the force parallel to the plane of the web, ๐ญ๐๐
96
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
is the specified minimum yield stress (ksi) of the web, and ๐๐ is the thickness of the web:
Figure (5-16) If the force is a concentrated load or reaction that causes tension or compression
and is applied at a distance greater than the member depth, d, from the end of the member, then:
๐น๐ = (๐๐ + ๐)๐ญ๐๐๐๐ ( AISC Equ. J10-2) ร=1.00 ( LRFD) and ฮฉ=1.50 ( ASD)
If the force is a concentrated load or reaction applied at distance d or less from the member end, then:
๐น๐ = (๐.๐๐ + ๐)๐ญ๐๐๐๐ ( AISC Equ. J10-3) 3) Web Crippling
When web crippling occurs, it is located in the part of the web adjacent to the loaded flange. Thus, it is thought that stiffening the web in this area for half its depth will prevent the problem. Web crippling is illustrated in Figure (5-17).
Figure (5-17) The nominal web crippling strength of the web is to be determined by the appropriate equation of the two that follow ( in which d is the overall depth of the member). If the concentrated load is applied at a distance greater than or equal to d/2 from
the end of the member, then: 97
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
( AISC Equ. J10-4)
ร=0.75 ( LRFD) and ฮฉ=2.00 ( ASD) If the concentrated load is applied at a distance less than d/2 from the end of the
member, then: For ๐
๐ โค ๐.๐,
( AISC Equ. J10-5a)
ร=0.75 ( LRFD) and ฮฉ=2.00 ( ASD) For ๐
๐ > ๐.๐,
( AISC Equ. J10-5b)
ร=0.75 ( LRFD) and ฮฉ=2.00 ( ASD) If one or two web stiffeners or one or two doubly plates are provided and extend
for at least half of the web depth, web crippling will not have to be checked.
4) Sidesway Web Buckling Should compressive loads be applied to laterally braced compression flanges, the web will be put in compression and the tension flange may buckle, as shown in Figure (5-18). Figure (5-18) Should members not be restrained against relative movement by stiffeners or lateral bracing and be subject to concentrated compressive loads, their strength may be determined as follows:
98
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
When the loaded flange is braced against rotation and ๐๐๐
๐ณ๐๐๐โค ๐.๐,
( AISC Equ. J10-6)
ร=0.85 ( LRFD) and ฮฉ=1.76 ( ASD)
When the loaded flange is braced against rotation and ๐๐๐
๐ณ๐๐๐
> ๐.๐, the limit state
of web sidesway buckling does not apply.
When the loaded flange is not braced against rotation and ๐๐๐
๐ณ๐๐๐โค ๐.๐,
( AISC Equ. J10-7)
ร=0.85 ( LRFD) and ฮฉ=1.76 ( ASD)
When the loaded flange is not braced against rotation and ๐๐๐
๐ณ๐๐๐
> ๐.๐, the limit
state of web sidesway buckling does not apply. It is not necessary to check Equations ( J10-6 and J10-7) if the webs are
subjected to distributed load. Furthermore, these equations were developed for bearing connections and do not apply to moment connection
In these expressions: ๐ช๐ = ๐๐๐๐๐๐ ๐๐๐ ๐๐๐๐:
๐ด๐ < ๐ด๐ (๐๐๐ ๐) ๐จ๐ซ ๐.๐๐ด๐ < ๐ด๐ ( ๐จ๐บ๐ซ) at the location of the force ๐ช๐ = ๐๐๐๐๐๐ ๐๐๐ ๐๐๐๐:
๐ด๐ โฅ ๐ด๐ (๐๐๐ ๐) ๐จ๐ซ 1.๐๐ด๐ โฅ ๐ด๐ ( ๐จ๐บ๐ซ) at the location of the force
5) Compression Buckling of the Web The equation to follow is applicable to moment connections, but not to bearing ones.
( AISC Equ. J10-8)
ร=0.90 ( LRFD) and ฮฉ=1.67 ( ASD) If the concentrated forces to be resisted are applied at a distance from the member
end that is less than d/2, then the value of ๐น๐ shall be reduced by 50 percent.
99
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
Example (5-10): A W21x44 has been selected for moment in the beam shown in Figure (5-19). Lateral bracing is provided for both flanges at beam ends and at concentrated loads. If the end bearing length is 3.5 inch and the concentrated load bearing lengths are 3 inch, check the beam for web yielding, web crippling, and sidesway web buckling. Solution: Using a W21x44( ๐ = ๐๐.๐ ๐๐, ๐๐ = ๐.๐ ๐๐, ๐๐ = ๐.๐๐ ๐๐ ๐๐ = ๐.๐๐ ๐๐, ๐ = ๐.๐๐ ๐๐ )
Figure (5-19)
LRFD ASD
๐ ๐ข = 1.2(1.044 ๐/๐๐ก) 152 + 1.6(35)
= 65.4 ๐ Concentrated load
๐๐ข = 1.6(35) = 56 ๐
๐ ๐ = (1.044 ๐/๐๐ก) 152 + 35
= 42.83 ๐ Concentrated load
๐๐ = 35 ๐ Local Web Yielding:
๐ =bearing length of reactions=3.5 in. ๐ =3.0 in. for concentrated loads At end reactions ( AISC Equ. J10-3):
๐น๐ = (๐.๐๐ + ๐)๐ญ๐๐๐๐ = (๐.๐ ร ๐.๐๐ + ๐.๐)(๐๐)(๐.๐๐) = ๐๐๐.๐ ๐๐๐๐
LRFD โ = ๐.๐๐ ASD ฮฉ = ๐.๐
โ ๐ ๐ = 1.0(102.8) = 102.8 ๐ > 65.4 ๐ O.K
๐ ๐ฮฉ
= 102.81.5
= 68.5 ๐ > 42.83 ๐ O.K
At concentrated loads ( AISC Equ. J10-2): ๐น๐ = (๐๐ + ๐)๐ญ๐๐๐๐
100
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
= (๐ ร ๐.๐๐ + ๐.๐)(๐๐)(๐.๐๐) = ๐๐๐.๐ ๐๐๐๐
LRFD โ = ๐.๐๐ ASD ฮฉ = ๐.๐
โ ๐น๐ = ๐.๐(๐๐๐.๐) = ๐๐๐.๐ ๐ > ๐๐ ๐ O.K
๐น๐ฮฉ
= ๐๐๐.๐๐.๐
= ๐๐.๐๐ > ๐๐ ๐ O.K
Web Crippling: At end reactions ( AISC Equ. J10-5a) since ๐
๐ โค ๐.๐:
๐๐
=๐.๐๐๐.๐
= ๐.๐๐๐ < ๐.๐
๐น๐ = ๐.๐(๐.๐๐)๐ ๐ + ๐ ๐.๐๐๐.๐
๐.๐๐๐.๐๐
๐.๐
๐๐๐๐๐(๐๐)(๐.๐๐)๐.๐๐
= ๐๐.๐ ๐๐๐๐
LRFD โ = ๐.๐๐ ASD ฮฉ = ๐.๐
โ ๐น๐ = ๐.๐๐(๐๐.๐) = ๐๐.๐ ๐ > ๐๐.๐ ๐ O.K
๐น๐ฮฉ
= ๐๐.๐๐.๐
= ๐๐.๐๐ > ๐๐.๐๐ ๐ O.K
At concentrated loads ( AISC Equ. J10-4):
๐น๐ = ๐.๐(๐.๐๐)๐ ๐ + ๐ ๐.๐๐๐.๐
๐.๐๐๐.๐๐
๐.๐
๐๐๐๐๐(๐๐)(๐.๐๐)๐.๐๐
= ๐๐๐.๐ ๐๐๐๐
LRFD โ = ๐.๐๐ ASD ฮฉ = ๐.๐
โ ๐น๐ = ๐.๐๐(๐๐๐.๐) = ๐๐๐. ๐๐ > ๐๐ ๐ O.K
๐น๐ฮฉ
= ๐๐๐.๐๐.๐
= ๐๐.๐๐ > ๐๐ ๐ O.K
Sidesway Web Buckling:
The compression flange is restrained against rotation.
101
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
๐๐๐
๐๐๐๐
=(๐๐.๐ โ ๐ ร ๐.๐๐) ๐.๐๐โ
๐(๐๐) ๐.๐โ = ๐.๐๐ > ๐.๐
โด Sidesway web buckling does not have to be checked The preceding calculations can be appreciably shortened if use is made of the
Manual tables numbered (9-4) and entitled โBeam Bearing Constantsโ. In these tables, values are shown for โ ๐น๐,โ ๐น๐,โ ๐น๐, ๐น๐ ฮฉโ ,๐น๐ ฮฉ,๐น๐ ฮฉโโ , and so on. The values given represent parts of the equations used for checking web yielding and web crippling and are defined on page 9-19 in the Manual.
Instructions for use of the tables are provided on pages 9-19 and 9-20 of the Manual. Then, those expressions and the table values are used to check the previous calculations.
LRFD ASD
โ ๐น๐ = โ ๐น๐ + โ ๐๐๐น๐ = ๐๐.๐ + ๐.๐(๐๐.๐)
= ๐๐๐.๐ ๐๐๐๐
๐น๐ฮฉ
=๐น๐ฮฉ
+ ๐๐ ๐น๐ฮฉ
= ๐๐.๐ + ๐.๐(๐๐.๐) = ๐๐.๐ ๐๐๐๐
Beam Bearing Plates The material used for a beam support can be concrete, brick, or some other material, but it usually will be concrete. This material must resist the bearing load applied by the steel plate. The basic design checks for beam bearing are web yielding and web crippling in the beam, plate bearing and plate bending in the plate, and bearing stress in the concrete or masonry. The nominal bearing strength specified in AISC J8 is the same as that given in the ACI Code 2005.
102
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
If the plate covers the full area of the support, the nominal strength is:
๐ท๐ = ๐.๐๐๐โฒ๐๐จ๐ ( AISC Equ. J8-1) If the plate does not covers the full area of the support, the nominal strength is:
๐ท๐ = ๐.๐๐๐โฒ๐๐จ๐๐จ๐๐จ๐โค ๐.๐๐โฒ๐๐จ๐ ( AISC Equ. J8-2)
๐โฒ๐= 28-day compressive strength of the concrete ๐จ๐=bearing area
๐จ๐=full area of the support, ๐จ๐๐จ๐โค ๐.๐
For the design of such a plate, its required area ๐จ๐is:-
LRFD โ ๐ = ๐.๐๐ ASD ฮฉ๐ = ๐.๐๐
๐ด1 =๐ ๐ข
โ ๐0.85๐โฒ๐ ๐ด1 =
ฮฉ๐๐ ๐0.85๐โฒ๐
Note that the strength reduction factor given in the AISC specification for bearing on concrete is 0.60. However, ACI 318 recommends a value of 0.65, which will be used here.
Plate Thickness:-
The required thickness of a 1- inch wide strip of plate can be determined as follows:- Z of a 1-inch wide piece of plate of (t) thickness=(๐) ๐
๐ ๐
๐ (๐) = ๐๐
๐
The moments ๐ด๐ and ๐ด๐ are computed at a distance k from the web center line and equated, respectively, to โ ๐๐ญ๐๐ and ๐ญ๐๐/ฮฉ๐; the resulting equations are then solved for the required plate thickness.
LRFD โ ๐ = ๐.๐ ASD ฮฉ๐ = ๐.๐๐
103
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
โ ๐๐๐ โฅ ๐๐ข =๐ ๐ข๐ด1
๐ ๐2
0.9๐น๐ฆ๐ก2
4โฅ๐ ๐ข๐2
2๐ต๐
๐ก โฅ 2๐ ๐ข๐2
0.9๐ต๐๐น๐ฆ
๐๐ ๐ก โฅ 2.22๐ ๐ข๐2
๐ต๐๐น๐ฆ
๐๐
ฮฉ๐โฅ ๐๐ =
๐ ๐๐ด1
๐ ๐2
๐น๐ฆ๐ก2 4โ1.67
โฅ๐ ๐๐2
2๐ต๐
๐ก โฅ 3.34๐ ๐๐2
๐ต๐๐น๐ฆ
The design procedure for bearing plates can be summarized as follows:
1. Determine the location of the load relative to the beam depth . 2. Assume a value for the bearing plate length, N. 3. Check the beam for web yielding and web crippling for the assumed value of N; adjust the value of N as required. 4. Determine the bearing plate width, B, such that the bearing plate area, A1=BN, is sufficient to prevent crushing of the concrete or masonry support. 5. Determine the thickness,๐๐, of the beam bearing plate so that the plate has adequate strength in bending. Example (5-11): Design a bearing plate to distribute the reaction of a W21x68 with a span length of ( 15 ft 10 inches) center to center of supports. The total service load, including the beam weight, is ( 9 kips/ft), with equal parts dead and live load. The beam is to be supported on reinforced concrete walls with ๐โฒ๐ = ๐๐ ksi for the plate. Solution by LRFD:
๐๐ = ๐.๐๐๐ซ + ๐.๐๐๐ณ = ๐.๐(๐.๐) + ๐.๐(๐.๐) = ๐๐.๐ ๐๐๐๐/๐๐ And the reaction is:-
๐น๐ =๐๐๐ณ๐
=๐๐.๐(๐๐.๐๐)
๐= ๐๐.๐๐ ๐๐๐๐
Determine the length of bearing N required to prevent web yielding:- From AISC Equ. J10-3, the nominal strength for this limit state is:-
๐น๐ = (๐.๐๐ + ๐ต)๐ญ๐๐๐ For โ ๐น๐ โฅ ๐น๐
๐.๐[๐.๐(๐.๐๐) + ๐ต](๐๐)(๐.๐๐) โฅ ๐๐.๐๐ Resulting in the requirement: ๐ต โฅ ๐.๐๐ ๐๐
104
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
Use AISC Equ. J10-5 to determine the value of N required to prevent web crippling. Assuming ๐ต/๐ > ๐.๐ and try the second form of the equation, J10-5b. For โ ๐น๐ โฅ ๐น๐
๐.๐๐(๐.๐)(0.๐๐)๐ ๐ + ๐๐ต๐๐.๐
โ ๐.๐ ๐.๐๐๐.๐๐๐
๐.๐
๐๐๐๐๐(๐๐)(๐.๐๐๐)๐.๐๐
โฅ ๐๐.๐๐
This results in the requirement: ๐ต โฅ ๐.๐ ๐๐ Check the assumption:
๐ต๐
=๐
๐๐.๐= ๐.๐๐ < ๐.๐ ๐ต.๐ฎ
For ๐ต/๐ โค ๐.๐, use AISC Equ. J10-5a: For โ ๐น๐ โฅ ๐น๐
๐.๐๐(๐.๐)(๐.๐๐)๐ ๐ + ๐
๐ต๐๐.๐
๐.๐๐๐.๐๐๐
๐.๐
๐๐๐๐๐(๐๐)(๐.๐๐๐)๐.4๐
โฅ ๐๐.๐๐
Resulting in the requirement: ๐ต โฅ ๐.๐๐ ๐๐, and ๐ต๐
=๐.๐๐๐๐.๐
= ๐.๐๐ < ๐.๐ ๐ถ.๐ฒ Try N=6 in. determine dimension B from a consideration of bearing strength. If we conservatively, assume that the full area of the support is used, the required plate area ๐จ๐ can be found as follows:-
โ ๐๐ท๐ โฅ ๐น๐ From AISC Equ. J8-1, ๐ท๐ = ๐.๐๐๐โฒ๐๐จ๐. then,
โ ๐(๐.๐๐๐โฒ๐๐จ๐) โฅ ๐น๐ ๐.๐๐(๐.๐๐)(๐.๐)๐จ๐ โฅ ๐๐.๐๐
๐จ๐ โฅ ๐๐.๐๐ ๐๐๐ The min. value of dimension B is:-
๐ฉ =๐จ๐๐ต
=๐๐.๐๐๐
= ๐.๐ ๐๐ > ๐๐ = ๐.๐๐ (๐ฉ โฅ ๐๐) Try B=10 in., then compute the required plate thickness:-
105
Karbala University College of Engineering Department of Civil Eng. Lecturer: Dr. Jawad T. Abodi
๐ =๐ฉ โ ๐๐
๐=๐๐ โ ๐(๐.๐๐)
๐= ๐.๐๐ ๐๐
๐ก = 2.22๐ ๐ข๐2
๐ต๐๐น๐ฆ=
2.22(99.73)(3.81)2
10(6)(36) = 1.22 ๐๐
Use a PL ๐๐ ๐ ร ๐ ร ๐๐
If we were to check to see if the flange thickness alone is sufficient, we would
have ๐ = ๐๐2 โ ๐ = 8.27
2 โ 1.19 = 2.95 ๐๐
๐ก = 2.22๐ ๐ข๐2
๐ต๐๐น๐ฆ= 2.22(99.73)(2.95)2
8.27(6)(36) = 1.04 ๐๐ > ๐ก๐ ๐ต.๐ฎ
106