Steel strucure lec # (12)

Steel strucure lec # (12)

1. Prof. Dr. Zahid Ahmad Siddiqi

STIFFENERS

It is usually necessary to stiffen the thin webs of
plate girders to prevent loss of strength due to web
buckling.

For riveted or bolted girders, angles are connected

to the webs while for welded girders; plates may
be welded to the webs as shown in Figure 6.13.

Stiffeners are divided into two groups:
a) Bearing Stiffeners.

b) Intermediate Stiffeners.

2. Prof. Dr. Zahid Ahmad Siddiqi

Stiffeners

3. Prof. Dr. Zahid Ahmad Siddiqi

Bearing Stiffeners

Bearing stiffeners are provided just above the
reactions or just below the heavy concentrated
loads.

These transfer heavy reactions or concentrated
loads to the full depth of the web providing a
uniform shear transfer.

Bearing stiffeners are always provided in pairs,
i.e., on both sides of the web.

4. Prof. Dr. Zahid Ahmad Siddiqi

Intermediate Stiffeners

Intermediate stiffeners are also called non-bearing
or stability stiffeners.

These are placed at various intervals along the web
to counteract against the buckling due to diagonal
compression.

These may also provide post-buckling shear
strength by the development of a mechanism called
tension field action, which will be explained later.

Intermediate stiffeners are provided in pairs on both
sides of the web or as a single unit alternatively on
each side of the web.

5. Prof. Dr. Zahid Ahmad Siddiqi

Behaviour Of Thin Webs With Stiffeners

Differential Element Close To
N.A. Where Flexural
Stresses Are Zero

Principal Stresses On
Element In Pure Shear

6. Prof. Dr. Zahid Ahmad Siddiqi

The produced diagonal compression causes the
buckling of slender web.

However, even after this buckling, the girder does
not fail due to shear.

The reason is that this buckled web can still carry
tension in a perpendicular direction and forms a
truss like arrangement consisting of flanges as
top and bottom chords, web acting as diagonal
tension members and stiffeners acting as vertical
compression members as shown in Figure 6.16.

7. Prof. Dr. Zahid Ahmad Siddiqi

Compression Zone or

Buckling Region Tension Field Stiffeners

Figure 6.16. Tension Field Action In Plate Girder Web.

8. Prof. Dr. Zahid Ahmad Siddiqi

The shear strength present after buckling of the web
is called post-buckling shear strength.

The phenomenon during which this strength
develops by the presence of a truss-like
configuration in the plate girder with intermediate

stiffeners is called Tension Field Action (TFA).

This gives better economy and provides a more
realistic idea of the actual strength of a girder.

Tension field action is always developed when a
girder with stiffeners is loaded beyond its initial
buckling stage.

9. Prof. Dr. Zahid Ahmad Siddiqi

During tension field action, a diagonal strip of the web
acts similar to the diagonal of a parallel chord truss.

The stiffeners prevent the flanges from coming closer
together and the flanges prevent the stiffeners from
moving towards each other.

The intermediate stiffeners, which before initial
buckling were assumed to resist no load, will resist
compression loads after web buckling and will serve as
the compression verticals of a truss.

With tension field action a web may resist shear equal
to two or three times that present at initial buckling.

10. Prof. Dr. Zahid Ahmad Siddiqi

The estimated ultimate shear that a panel of web
between adjacent stiffeners can withstand is equal
to the shear initially causing web buckling plus the
shear, which can be resisted by tension field action.

The amount of tension field action is dependent on

the proportions of the panels.

The resulting deflections may increase to several
times the values estimated by the usual deflection
theory.

11. Prof. Dr. Zahid Ahmad Siddiqi

Design Of Intermediate Stiffeners

The design of intermediate stiffeners involves
finding the required spacing and size of these
stiffeners.

C,= called shear coefficient without T.F.A.

and stiffeners. It is the ratio of critical
web stress to the shear yield stress of
web material.

fraction of shear strength available due
to web alone with respect to shear yield
strength of the compact web.

12. Prof. Dr. Zahid Ahmad Siddiqi

additional C, value for T.F.A. due to
stiffeners alone.

fraction of shear strength available with
respect to shear yield strength of web
due to stiffeners producing T.F.A.

k, web plate buckling coefficient.

Example 6.2: Design intermediate stiffeners for
the plate girder of Example 6.1.

13. Prof. Dr. Zahid Ahmad Siddiqi
Solution: Using the flow chart of intermediate
stiffeners design, one can proceed step by step as
under:

E Provide bearing stiffeners under concentrated
loads and over reactions.

= 230 > 246/E/F, =69.6

:. Intermediate stiffeners are required.
First Interior Stiffener In End Panels

No T.F.A.
Vis 1519 KN

u

14. Prof. Dr. Zahid Ahmad Siddiqi

Y, x1000
Required C, = a CSA,

1519 x 1000 o ad
09 x 0.6 x 250 x 2300 x 10
k,E

v

F

y

2
GE x me ) xF,
x 1.51E

0.489 x (230) x 250
302,000

Assuming h/t, > 1.37

= 21.41

15. Prof. Dr. Zahid Ahmad Siddiqi

k,E
hit, = 230 > 1.37 | OK

®
al a

Maximum % = = 0.552

Amax = 0.552x(2300/1000) = 1.27 m

260
Alt,

2
| xh = 294m

Also, ana = |

Let a = 1.2 m( Position of first
intermediate stiffener )

16. Prof. Dr. Zahid Ahmad Siddiqi
= 5+ BEE
(0.522)?
23.37
= 0.534

alh = 0.522

1.51x 23.37 x 200,000
(230)? x 250

0, Vp = 0.9 x 23000 x 0.6 x 250 x 0.534 / 1000

= 1656.9 kN
Spacing Of Other Intermediate Stiffeners
Portion AB

8. V, (x = 1.2m) 1519 - 76.9 x 1.2
1426.72 KN

17. Prof. Dr. Zahid Ahmad Siddiqi

Considering unstiffened webs with k, equal
to 5.0, check which formula for C, is
applicable for the selected panel.

hit, for this girder = 230 and aie

1370

a

1,510,000 1510,000
= = 0.114

Cy ( Y ) E, (30) x250

18. Prof. Dr. Zahid Ahmad Siddiqi
= ÿ, A, (0.6F,) C,/1000
0.9 x 2300 x 10 x 0.6
x 250 x 0.114/1000 = 354 kN

Check whether intermediate stiffeners are
required or not.

Vie so NG

hlt, < 260 OK
+. Intermediate stiffeners are required.

Z
ne Fa n = (29) x 2300/1000

2 3
{fs

= 2.94m

19. Prof. Dr. Zahid Ahmad Siddiqi

3.0 h/ 1000
90m (with TFA)

A, 1A,

2300 x 10
600 x 28

h/b; = 2300/600 = 3.83 < 6.0 (OK)

= 4.3172 262 (OK)

13. Total distance upto next bearing
stiffener = 7.00 — 1.20 = 5.80m

20. Prof. Dr. Zahid Ahmad Siddiqi

. a = 5.8/2 = 2.90m
alh = 2900/2300 = 1.26

5
k= St A = 8445
6)

Check which formula for C, is applicable here.
TEIN as
1.37 ee 110.6
k,E
IDS SHE] een,
E,

21. Prof. Dr. Zahid Ahmad Siddiqi

1.51%, £ 1.51x8.145x 200,000

ae (Ht, JF, (230) x 250
= 0.186

Tension field action will be considered to get
economical design.

tE: W018

1.15 h+&) 1151+ (26)

$y V, = 0.9 x Ay (0.6 Fy) (C, + C,’)/1000
0.9 x 2300 x 10 x 0.6
x 250 (0.186 + 0.440) / 1000
= 1943.7 kN

E:

22. Prof. Dr. Zahid Ahmad Siddiqi

19. V, = 1426.72kN< 4, V, OK
Portion BC
V, (x = 7.0m) = 230.7 kN

The shear capacity without tension field action is:

PA, (0.6 F,) C,/1000 = 354 kN as before > V,
and h/t, < 260

No intermediate stiffeners are required for
portion BC.

23. Prof. Dr. Zahid Ahmad Siddiqi

However if length of the portion is greater than
3.0 h, it is better to provide intermediate stiffeners
at spacing not exceeding 3.0 h.

20. Size Of Intermediate Stiffeners

F, +
(An GEXZ (coja)

V.

F E

V, = required shear strength and V, = 4,V,.

Using the minimum value of C,, the area of
stiffener will be critical. However, V, and o,V,,
should be used for the same region (x = 1.2 to
4.1 min this example).

24. Prof. Dr. Zahid Ahmad Siddiqi

1426.72
A.) = 0.15 x 2300 x 10 (1 - 0.186
sdmin ee ) 194370

- 18x 102 = 261.4mm?

For first interior stiffener, just as a check:

1519

(Amin = 0.15 x 2300 x 10 (1 - 0.534) ———

- 18 17,

0 (do not consider negative value)
*. Not critical

25. Prof. Dr. Zahid Ahmad Siddiqi

= V 7.4, m

46 mm

t = b,/15.8 = 3mm

st

This is to keep A, less than or equal to 0.56, #/ F',,

(15.8 for A36 steel).

jE A Bra:

amy
For first space, a / h 1.2/2.3 = 0.52, j= 7.25
For next spaces, a/h 1.26, j=0.5

26. Prof. Dr. Zahid Ahmad Siddiqi

(le) min 7.25 x 1200 x 10%
870 x 104 mm?
and 0.5 x 2900 x 103
= 145 x 104 mm? .. Not critical

24. Stiffener Size For Minimum
Moment Of Inertia:

Dar © K.),. #221 = 120 mm

te= by / 15.8 = 8mm
148 x 2503/12 = 1042 x 104 mm!

> 870x 104 mm!

27. Prof. Dr. Zahid Ahmad Siddiqi
Use 8 x 125 mm PLs
on both sides

25. Design Of Weld
PRE 0.0004 h kN/mm
0.0004 x 2300
0.92 kN/mm for four welds
0.23 kN/mm per weld

As before, let ft, = 8 mm

w

ei te 50 NN

28. Prof. Dr. Zahid Ahmad Siddiqi

1.26 kN/mm
50x1.26
0.23
274mm > 16 x web thickness +

€, = 210 mm

Use 8 x 50 mm welds 200mm c/c

29. Prof. Dr. Zahid Ahmad Siddiqi