Web stiffeners

Web stiffeners are required to improve the shear resistance of the web, for the attachment of transverse bracing and as support over bearings.

6.4.1 Intermediate transverse web stiffeners

Intermediate web stiffeners are usually made by welding a simple flat plate to one face of the web. The outstand proportions of the flat are limited by Clause 3/9.3.4, to ensure that its strength is not limited by local buckling.

The outstand and a portion of the web plate on either side form an effective Tee section (see Clause 3/9.13.2) that has its centroid just outside the face of the web.

Intermediate web stiffeners should be designed to resist:

• axial force due to tension field action

• forces and moments due to action with transverse bracing system

• direct loading (from a wheel at the stiffener position)

• bending about a longitudinal axis due to eccentricity of axial forces (in the plane of the web) relative to the centroid of the stiffener section

• destabilising influence of the web (buckling check only).

Most of the forces that the effective stiffener section is required to resist arise in the plane of the web, so they cause both axial and bending stresses in the effective section.

To restrain the web, the stiffener does not need to be connected to either flange, although it is usual to connect it to at least the top flange. Intermediate stiffeners are frequently not connected to the bottom flange. Where the stiffener is not connected, the code specifies a maximum clearance of five times the web thickness, but it is suggested that a clearance of not more than three times should be aimed for.

Where the stiffener acts as a connection for transverse bracing, it should be connected to both flanges. Practical experience from North America indicates that failure to provide such attachment may lead to fatigue cracking in the web at the point of curtailment of the stiffener.

Where bracing is attached to a web stiffener, the stiffener may need to be shaped to provide sufficient lap to connect the bracing members.

The fillet welding of the end of a stiffener to a flange does not introduce a lower class of fatigue detail than is likely to be present already, provided that the toe of the weld is at least 10 mm from the edge of the flange. Web stiffeners should be proportioned such that they are narrower than the flange outstand; shaped stiffeners for bracing may need to be notched at the end to ensure that the welds are not too close to the edge of the flange.

Intermediate stiffeners should normally all be attached to one face of each beam web. On the outermost beams, the stiffeners should be on the hidden face, rather than the exposed face, for better appearance.

Where a sloping flange changes direction (at the end of a tapered haunch), a transverse stiffener is required to carry the transverse component of force (Clause 3/9.13.1). Although all transverse stiffeners are strictly required to be full depth, it is common to make such stiffeners part height, provided that they are symmetrical about the web and the load can be

satisfactorily transferred into the web.

6.4.2 Load-bearing web stiffeners

Bearing stiffeners are effectively a special case of transverse web stiffener, and the design rules in Clause 3/9.14 contain many similarities to the rules in Clause 3/9.13. Again, flat plate stiffeners are often used, although more than one flat on each side may be needed, partly to provide sufficient area and partly to provide greater resistance to reactions that are eccentric along the beam.

Bearing stiffeners are usually provided on both faces of a web, and are usually symmetrical about the web, so that the centroid of the effective section is on the line of the web. In some cases, it may be convenient to provide two stiffeners on one face and a single stiffener on the opposite face of the web. This would give resistance to bending about a transverse axis while allowing a single lapped connection to transverse bracing. SeeFigure 6.4 for typical details.

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Figure 6.4 Typical bearing stiffener

Bearing stiffeners generally need only be checked for the ULS, in accordance with Clause 3/9.14, though the fatigue endurance of certain details must also be checked. The effective stiffener section must be able to resist:

• axial force arising from the support reaction

• forces and moments due to action in conjunction with cross-beams or bracing system (including effects of the couple needed to restrain the beam in torsion)

• bending (about longitudinal and transverse axes) due to eccentricity of bearing reaction relative to the centroid of the stiffener section

• direct loading (from a wheel over the bearing stiffener)

• destabilising influence of the web (this need only be considered for the check against buckling).

Axial load and bending due to bearing eccentricity are assumed to vary linearly to zero at the level of the top flange.

Some values to be used for eccentricity of bearings are given in Clause 3/9.14.3.3 but it must be noted that these are not exhaustive. They are applicable where radiused or flat-topped bearings are installed accurately relative to the beam steelwork. No values are given for elastomeric pot bearings, which are in common use; it is suggested that a value of 10 mm in either direction be used for the unevenness or inaccuracy of such bearings. Movements of the beam relative to the bearing due to changes in temperature are readily calculated. Further consideration should, however, be given to eccentricities due to fabrication tolerance, particularly on long viaducts, if it is not certain that the lower part of the bearing can be positioned accurately relative to the girder after erection, and to eccentricities due to shortening as a result of shrinkage of the concrete.

Three checks for adequacy are required:

• on direct bearing at the bottom of the web and stiffeners

• on maximum stresses on the effective stiffener section

• on buckling of the effective stiffener as a strut between flanges.

Bearing stiffeners should be ‘adequately connected to both flanges’ and should be ‘fitted closely to the flange…subject to a concentrated load’ (Clause 3/9.14.1). Close fitting to the bottom flange allows the compressive stresses to be transmitted in bearing rather than through welds, and should therefore usually be specified (ensure that it is marked on the drawings).

Close fitting to the top flange is not necessary and should not be specified (to ensure that it does not compromise fitting of the bottom flange); welding is an adequate connection to the top flange.

Although a fitted connection at the bottom will transmit compression in bearing, it should be noted that for the fatigue check all the variation in reaction due to the fatigue vehicle load is assumed to be transmitted through the welds (see Clause 3/9.14.4.1) in a Class W detail (see Part 10, detail 3.11). As for intermediate stiffeners, bearing stiffeners should be sized to ensure that the weld toes are not within 10 mm of the edge of the flange plate, or a lower class fatigue detail will result.

For further guidance on detailing and fabrication of bearing stiffeners, see Guidance Note 2.04.

It should be noted that when the main beam is designed as a compact section over

intermediate supports, plastic redistribution of stresses is assumed and consequently the check on yielding of the web plate (Clause 3/9.14.4.1) is inappropriate. (The combined action permitted by Clause 3/9.9.3.1 allows full longitudinal yield in tension and compression at the same time as up to half shear yield—the equivalent stress is then in excess of yield throughout the depth of the web, before any consideration of the transverse stress associated with the bearing stiffener.) The yield check may therefore be ignored at ULS.

6.4.3 Longitudinal stiffeners

The provision of longitudinal stiffeners on a deep slender web has a significant effect on the form of web buckling that can develop and, as mentioned previously, the code then requires each of the panels bounded by stiffeners to be checked separately. It requires also that the effective web stiffener (stiffener plus a width of plate) is an adequate restraint to the edge of the panel. This requirement is expressed in Clause 3/9.11.5.

Discontinuous longitudinal stiffeners (i.e. stiffeners that stop just short of the transverse stiffeners) are effective in restricting web buckling. They are commonly used to simplify fabrication and reduce costs. This is permitted, though the stiffener must not then be included in the effective section in bending (Clause 3/9.4.2.6), although, for the design of the stiffener itself, it is assumed to be at the same longitudinal stress as the unstiffened web at that position.

Further, the stress concentration that is caused by the discontinuity needs to be checked, particularly for fatigue.

Stiffeners that are continuous and assumed to be contributing to the bending resistance should extend beyond the point at which the contribution is needed (Clause3/9.11.6).

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