CHAPTER 5 BEAMS Prepared By Ahmed F. Hassan Professor, Steel Structures and Bridges Structural Engineering Department Structural Engineering Department Cairo University
CHAPTER 55BEAMS
Prepared Byp yAhmed F. HassanProfessor, Steel Structures and BridgesStructural Engineering DepartmentStructural Engineering DepartmentCairo University
I. INTRODUCTIONBeams are structural members subjected to:B di (Si l Bi A i l)Bending moment (Simple or Bi‐Axial)Shear Force (Usually associated with the ( ybending moment)T iTorsion
I. INTRODUCTIONI. INTRODUCTIONMoment and shear result from l d li d l loads applied normal (perpendicular) to the longitudinal (perpendicular) to the longitudinal axis of the beam.
I INTRODUCTIONI. INTRODUCTION
I. INTRODUCTIONIf this transverse If this transverse load does not pass through the h t f shear center of the cross section, t e c oss sect o ,Torsion occurs.
I INTRODUCTIONI. INTRODUCTIONBeams are designed to meet the Beams are designed to meet the following requirements:STRENGTH Cross section must safely resist bending moment and shear force resist bending moment and shear force
STABILITY Cross section must be safe i t l l b kli f t d l t l against local buckling of component and lateral
torsional buckling
SERVICEABILITY deformation should not exceed certain limits
II APPLICATIONSII. APPLICATIONSPurlins supporting roof covering materialpp g gGirts supporting side covering materialFloor Beams to carry RC slabs in buildingsFloor Beams to carry RC slabs in buildingsFrame Girders (ignore normal force)Crane Track GirdersCrane Track GirdersStringers and X‐girders for bridge floors
Beams can be simply supported, continuous, or t f fpart of a frame
System can be different is both directions X, Y (U f Ti d )(Use of Tie rods)
II APPLICATIONSII. APPLICATIONS
II APPLICATIONSII. APPLICATIONS
II APPLICATIONSII. APPLICATIONS
III CROSS SECTIONSIII. CROSS SECTIONSInertia is an important factor (in Inertia is an important factor (in axially loaded members, area is the important f t )factor)
UPN IPE/IPNGood for
HEA/HEB/BFIGood for Bi‐
Cold Formed SectionsGood for moment about
moment about major axis Mx
axial Moment Mx & My
major axis Mx for light elements (Purlins and Girts)
III CROSS SECTIONSIII. CROSS SECTIONS
Built‐Up SectionsGood for Bi‐axial Moment Mx & My
IV. SERVICEABILITYDeflection Limitations
d fl l dExcessive deflections can cause non‐structural damage to finishing materialsB d Bad appearanceFear / non‐confidence of occupantsM lf i i i iMalfunction in equipment operationEgyptian Code limits the Live Load deflection to the following (clause 9 1 3 ):following (clause 9.1.3.):Beams carrying Plaster or Other Brittle Finish SPAN
All Other Beams
300
200SPAN
Crane Track Girders200
800SPAN
IV. SERVICEABILITYStiffness Limitations
d h ff f bTo reduce the effect of vibrationsThe following limitations shall Preferably be applied to th b h i ht (E ti C d Cl )the beam height (Egyptian Code Clause 9.2.1.):
Floor Beams
24SPAN
Floor Beams subject to shocks and vibrations (Crane track girders)
Roof Purlins20
SPAN
SPANRoof Purlins40
SPAN
V. STRENGTHThree failure modes:YieldingLateral Torsional Buckling (LTB)Lateral Torsional Buckling (LTB)Local Buckling
The allowable stress in bending is the SMALLER of the Three failure modesSMALLER of the Three failure modes
V. STRENGTH ‐ YieldinggThe stresses should not exceed the yield stress of the material by a reasonable factor of safetyof the material by a reasonable factor of safety.
Material F (t/cm2)Material FY (t/cm )
t ≤ 40‐mm 40‐mm < t < 100‐mm
Mild Steel (St. 37) 2.4 2.15
High Grade Steel (St. 52) 3.6 3.35
The factor of safety is not constant for all cases (Depends on whether the section is cases (Depends on whether the section is Compact or Non‐Compact)
V. STRENGTH ‐ LTBLateral Torsional Buckling is a global buckling phenomena for beams loaded buckling phenomena for beams loaded about their major axis.LTB involves:Vertical deflectionVertical deflectionLateral deflectionfTorsional rotation
V. STRENGTH ‐ LTBFor a simply supported beam with uniform bending the lateral torsional buckling bending, the lateral torsional buckling critical moment is:
242WYTY IIEKGIEM ππ
+= 42uu
cr LLM +=
E, G = Young’s and Shear Modulus of beam materialIY = Moment of Inertia of the section about Y axisKT = St. Venant Torsional ConstantIW = Warping ConstantL U t d l th f C i flLu = Unsupported length of Compression flange
V. STRENGTH ‐ LTBFor beams with different loading and support conditions, the lateral torsional buckling critical conditions, the lateral torsional buckling critical moment is:
242 IIEKGIE4
24
2
2
u
WY
u
TYbcr L
IIEL
KGIECM ππ+=
Uniform Torsion (St. Venant)
Warping Torsion
Cb= Factpr for loading and support conditions
22
21, fff LTBallowable +=
b p f g pp(Table 2.2 ECP – Table 5.2 Prof. Bahaa Mashaly Text Book)
For unequal end moments without transverse loads
3.23.005.175.12
2
1
2
1 ≤⎟⎟⎠
⎞⎜⎜⎝
⎛+⎟⎟
⎠
⎞⎜⎜⎝
⎛+=
MM
MMC b
V. STRENGTH ‐ LTBAf
f CA
f ×800
bu
f CdL
f ×=1
V. STRENGTH ‐ LTB A
For shallow thick flanged section:
Af
bu
f CdLA
f ×=800
1
For deep flanged Thin Section
L 2
⎟⎞
⎜⎛ ⎞⎛
Y
b
T
u
Y
bY
b
YT
u
FC
rL
FCF
C
FrL
f 1888410176.1
64.0 52 ≤≤
⎟⎟⎟⎟⎞
⎜⎜⎜⎜⎛
×
⎟⎠⎞⎜
⎝⎛
−=
bub F
CLCf 1881200022 >×⎞⎛
=
⎟⎠
⎜⎝
YTb
T
uFr
rL
f 22
⎟⎠⎞⎜
⎝⎛
di f ti b t i i f ti rT = radius of gyration about minor axis for a section comprising the compression flange plus 1/6 of the web area
V. STRENGTH – Local BucklinggThe various steel sections are classified as one of the f ll ifollowing:
Compact Can reach the plastic moment without local buckling
N CNon‐Compact Can reach the yield moment without local buckling
Slender Local buckling occurs prior to reaching yield moment
Photos
V. STRENGTH – Local BucklinggσY
432
1 1
εY
1
2
4F = σYF < σY
34321
4Point 1 = (stress < yield, strain < yield)Point 2 Yield Moment (Non Compact section) 4Point 2 = Yield Moment (Non‐Compact section)Point 3 = (stress = yield, strain > yield)Point 4 = Plastic moment (Compact section)
V. STRENGTH – Local BucklinggCompact b 58
≤
Table 2.1 ECP or Table 5.1 Prof. Mashaly text book
p
Non‐Compact
Yf Ft≤
Yf Ftb 64
≤
Stiffened Flange subjected to CompressionYf
Rolled Welded
Compact
Non‐Compact
YYf For
Ftc 3.159.16
≤
b 2123
Un‐Stiffened Flange subjected to CompressionNon Compact
YYf For
Ftb 2123
≤
Compact
Non Compact
Yf Ftb 127
≤
b 190
Stiffened Web subjected to Bending
Non‐CompactYf Ft
b 190≤
VI. ALLOWABLE STRESSESAllowable Bending Stress depends on section l ifi ticlassificationCompactN CNon‐CompactSlender (not covered in this course)
Shearqall. = 0.35 FY
VI. ALLOWABLE STRESSESTo qualify as compact section:
( )Must satisfy Local buckling limits (c/tf & d/tw)Must satisfy the following LTB limits:
bY
f
Y
fu C
FdA
Fb
L1380
&20
≤ Except box sections (other equations)
Section must be symmetric about the two axesI‐Shapes bent about minor axis, rectangular, and solid bars 0.72 FY
To qualify as non‐compact section:
Other Sections 0.64 FY
Must satisfy Local buckling limits (c/tf & d/tw)ALL UPN sections are Non‐Compactp
All non‐compact sections 0.58 FY or Fall., LTB (whichever is smaller)
VII. ACTUAL STRESSESActual Bending Stress:
X fMf ≤Uni‐axial bending .allX
Xact f
Zf ≤=
01≤YX MMfBi‐axial bendingActual shear Stress:
0.1,,
≤+=MyallY
Y
MxallX
Xact fZfZ
f
QShear in minor axis (I‐shape) .
,, all
netweb
Yacty q
AQq ≤=
QShear in major axis ., 5.1 all
flange
Xactx q
AQq ≤=
In web area calculations:•For rolled sections use full section height•For built‐up sections use web height
VII. ACTUAL STRESSESFor combined bending and shear:
22 1.13 allactactequ fqff ≤+=
This s found at support of continuous beams
.... allactactequ fqff
pp
VIII. ACTUAL DEFLECTIONActual deflection is computed for Live Load onlyFor simply supported beam under distributed load:
LL Lw5 4
δ
For simply supported beam under concentrated load:X
LLLLact IE384, =δ
p y pp
LLact IELP
48
3
, =δ
For other loading conditions:Get equivalent distributed load that gives the same M then
XIE48
Get equivalent distributed load that gives the same Mmax,LL , then estimate the deflection fro the distributed load case
8 M2max
.8
LMwequ =