Лако-красочные материалы — производство

Despite the extensive use of adhesives to bond structural metal in both the aircraft and vehicle industries for many years their use in civil engineering has been relatively rare. Early writers believed that resin adhesives could eventually replace welding in secondary connections in structural steelwork(22). For example, welding stiffeners to girder webs or diaphragms to prevent buckling failure often raises problems of distortion and fatigue. In the aerospace industry it is now established practice to achieve similar stiffening by bonding honeycomb structures with adhesives. The advent of toughened structural adhesives brought the opportunity for replacing welding in box beam metal structures such as used in the construction of motor vehicles(23).

Studies into the use of adhesives in the fabrication of metal bridge parapets have been called for because it is difficult to obtain adequate welded connections between lengths of horizontal rail. The rails are generally rectangular hollow sections and for a welded joint different electrodes have to be used for the top and bottom

welds. Further, after completion the welds are ground flush to provide a smooth profile and a slight undercut can remove the weld completely. A simple bonded joint with either a splice or a sleeve could be designed to develop the full strength of the metal section.

In the United States some further possible applications were postulated in the form of bonded secondary steelwork connections on bridges(24). These included supports for concrete deck finishing machines (Fig. 6.16), sign support brackets (Fig. 6.17), lateral bracing connections (Fig. 6.18), drainage pipe supports (Fig. 6.19) and once again intermediate stiffeners to girder webs (Fig. 6.20). Experimental work had shown that, as a method for improving the fatigue life of bridges, adhesive bonds merited further investigation. However, it was carefully pointed out that a fundamental distinction needed to be made between short — and long-term projected uses. For example, the durability requirements for temporary construction attachments to girders are quite different from those for more permanent attachments such as vertical stiffeners.

For applications in new bridge construction a design life of 120 years is required(25). Although it may be possible to design out fatigue and creep by keeping working stresses low(26) the influence of the environment on adhesives over such a time span is very much

(a) Welded details

(a) Welded detail (b) Bonded detail

Fig. 6.19. Drainage pipe supports (Ref. 24).

(a) Welded details (b) Bonded details

Fig. 6.20. Girder web stiffeners (Ref. 24).

an unknown. It is for this reason that the application of adhesives in the repair and strengthening of steel structures is an area of more fruitful development. Design lives can be shorter, for example 30 years, in the knowledge that the bonded component may be replaced should deterioration occur. Indeed the Standing Committee on Structural Safety of the Institutions of Civil and Structural Engineers has consistently highlighted the lack of information on the long term performance of resins(27). They have warned against using resins in critical parts of a structure unless effective replacement can take place should structural deterioration occur.

One relatively recent application has been the bonding of existing Hobson’s patent steel floor arches to replacement steel edge beams on the Branbridges bridge over the river Medway in Kent(28). An adhesive joint was selected in favour of welding or riveting because of restricted working room, speed and economy. A two part epoxy adhesive was injected into the space between a steel trimmer plate

and the flange of the new edge beams (Fig. 6.21). A thixotropic filled epoxy was used to seal the gaps between the steel section prior to injection. The specification only allowed injection into the void between steel surfaces if their temperature was more than 3 °С above the dew point and the ambient temperature was in excess of 10 °С. To achieve this the contractor elected to sling a propane gas blower heater below the deck bridge.

The driving force for a change from traditional welding to adhesive bonding is thus the opportunity to avoid residual stresses. Possible applications that have been cited include large stiffened plate structures for both land and marine based industries(29). Such structures are designed to carry low to moderate forces mainly in shear and compression but may have to provide a substantial ultimate load carrying capacity as well as resistance to fatigue loading in service.

In Chapter 8 two potential applications for the bonding of structural steelwork will be considered in some detail. These are bonded web stiffeners and tension splices or cover plates to girder flanges.

Applications in repair and strengthening