Web Buckling and Web Crippling

• A heavy concentrated load or end reaction produces a region of high compressive stresses in the web either at support or under the load.

• This causes the web either to buckle or to cripple (or local bending).

(a) Web Buckling

(b) Web Crippling

WEB BUCKLING

• Web buckling occurs when intensity of compressive stress near the center of the section exceeds the critical buckling stress of web acting as a strut.

• At supports and under a concentrated load, certain portion of the beam acts as a column to transfer the load.

• Under this compressive force the web of the beam section may buckle.

• The load dispersion angle may be taken as 45 degrees.

• Hence the section should be checked for web buckling.

• The rolled sections are provided with suitable thickness for web so that web buckling is avoided.

• The probability of web buckling is more in the case of built- up sections having greater ratio of depth to thickness of the web.

• At the point of concentrated load web may be considered as a strut between the beam flanges.

• The load is assumed to disperse over the length of (b,+n,)., where (b,) is the stiff bearing length and (n,) is the dispersion of the load through the web at 45 degrees, to the level of half the depth of cross-section

• In case of built-up sections, it is necessary to check for buckling of web and provide web stiffeners.

• Effective web buckling strength is to be found based on the cross-section of web [(b,+n,)tw].

• Fcdw = [(b,+n,)t,]f.

• Fcdw = Web buckling strength

• f. = allowable compressive strength corresponding to the assumed web column corresponding to its slenderness ratio and effective length

• Effective length= 0.7d

• d= depth of the strut between the flanges= =[Total depth- 2*(tr + R1)]

• radius of gyration = r, = S(l,/A) = t/(2/3)

• slenderness ratio = 0.7d/r, = 2.42d/tw

WEB CRIPPLING

• Concentrated loads and reactions are resisted by compressive stresses in the web of steel beams.

• Therefore web is acted upon by large amount of stresses at these locations.

• Stress concentration occurs at the junction of the web and the flange, as a result large bearing stresses are developed. At these locations, web tends to fold over the flange.

• This type of local buckling is called as web crippling.

• Web crippling is the buckling of the web due to the compressive force delivered through the flange.

• So, the concentrated load should be transferred from flanges to the web on sufficiently large bearing areas.

• The root of the fillet is the most critical location for failure.

• In the case of web crippling at a support, the crippling strength of the web is given by: Fw = (b1+n2)twfyw/Tm0

• Where b1= stiff bearing length; fyw = yield strength of web

• n2 = dispersion length through the flange to the web junction at a slope of 1:2.5 to the plane of flange.

• At the point of application of a concentrated load,

  Fw = (b1+2n:)twfyw/Tmo

• Since the dispersion of the load takes place on either side.