][ STEEL STRUCTURES ][

Crippling of Beam Webs
P 292-293

Crippling = localized yielding of the beam

Cause:
=====
Excessive end reactions
Excessive concentrated load at any point

Formulas:
=======
The following are the maximum allowable loads for beams without stiffners!

Maximum End Reaction:
==================
(OVER SUPPORT)
R = 0.75 Fy x t (N + k)

Maximum Interior Load:
==================
(CONCENTRATED LOAD FROM ABOVE)
P = 0.75 Fy x t (N + 2k)

t = thickness of the beam web, in inches
N = length of bearing in inches
k = distance from outer face of flange to web toe
P&A 267 Table 8.1
Tension and compression on extreme fibers
Fb3 = 0.75 x Fy 
Fb3 = 0.75 x 36
Fb3 = 27 calculated
Fb3 = 27 from table

If the allowable loads are exceeded
the following solutions may be applied

Solution to the web crippling problem:
============================
reinforce with web stiffners
	plates welded in
increase bearing length
	distribute load over 
	greater length
select beam with thicker web
	S shapes have thicker webs
	than W shapes
	heaver members have thicker webs

CRIPPLING OF BEAM WEBS
Max. END REACTION
SAMPLE PROBLEM

A W 21 x 57 beam
of A 36 steel 
has an end reaction of 44 kips
that is developed in bearing over a length of
N = 10 inches 

A. GIVEN:
W 21 x 57
A 36 steel 
R 44 kips end reaction
N = 10 inches 

B. ASKED:
Is web cribbling critical?

C. GRAPH:
_________________ k = distance from flange
_________________ to web toe
N= 10"			         W 21 x 57
R1 = 44 kips

D. CALCULATION:
1. P&A 170 Table 4.1
	k = distance from outer face of flange to 
		web toe
	k = 1.38 in

2. P&A 170 Table 4.1
	t = thickness of the beam web, in inches
	t = 0.405 in

3. Maximum end reaction
	(OVER SUPPORT)
R = End Reaction over support
t = thickness of the beam web, in inches
N = length of bearing in inches
k = distance from outer face of flange to web toe

	R = 0.75 Fy x t x (N + k)
	R = 0.75 x 36 ksi x 0.405" x ( 10" + 1.38")
	R = 27 ksi x 0.405" x (11.38")
	R = 10.935 ksi" x 11.38"
	R = 124.4403
	R = 124 kips
 	
4. Actual End Reaction
	R1 = 44 kips

5. 124 kips > 44 kips
	R > R1
	o.k. 
	crippling is not critical

E. ANSWER
	The maximum allowable end reaction 
	is greater than the actual reaction.
	The beam therefor is NOT CRITICAL
	with regard to web crippling



Example Problem
CRIPPLING OF BEAM WEBS
MAX. INTERIOR LOAD

CRIPPLING OF BEAM WEBS
Max. INTERIOR LOAD
SAMPLE PROBLEM
A W 10 x 26
of A36 steel 
supports a column load of 70 kips
at the center of the span.
The bearing length of the column
on the beam is 10 inches.
Investigate the beam for web crippling
under this concentrated load.


A. GIVEN:
W 10 x 26
A36 steel 
P1= 70 kips
center of the span.
N= 10 inches.


B. ASKED:
Is web crippling critical
under the concentrated load.


C. GRAPH:
				 I  P1 = 70 kips
___________V_______
___________N = 10"__
				W 10 x 26


D. CALCULATION:
1. P&A 170 Table 4.1
	k = distance from outer face of flange to 
		web toe
	k = 0.88 in

2. P&A 170 Table 4.1
	t = thickness of the beam web, in inches
	t = 0.260 in

3. Maximum INTERIOR LOAD
	(OVER BEAM SPAN)
P = Max. Interior Load
t = thickness of the beam web, in inches
N = length of bearing in inches
k = distance from outer face of flange to web toe


	P = 0.75 Fy x t x (N + 2k)
	P = 0.75 x 36 ksi x 0.260 x [10" + (2 x 0.88")]
	P = 27 ksi x 0.260 x [ 10 + 1.76]
	P = 7.02 ksi" x 11.76"
	P = 82.5552 kips
	P = 82.6 kips

4. Pmax > P1
	82.6 kips > 70 kips


	
E. ANSWER:
	The maximum allowable interior load 	exceeds the existing load.
	The beam is safe from web crippling.






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Prof.
Dr. Hermann Gruenwald

(mail comments to:
HGRUENWALD@ou.edu)

College of
Architecture 

The University of Oklahoma

©Dr. Gruenwald 1996, 1997