This thesis is not directly studying or explaining the design of the weld to the Welda® plate.
Instead, the effect of the heat input during the welding process, which also is not defined precisely, is studied from the perspective of the adjacent material. The plate is designed to be used in joining steel members and transferring loads to a concrete structure. The advance of this method is that there are no base plate holes drilled to the concrete surface and there is no need for precisely positioned submerged threaded rods. This Welda® -method is providing significantly bigger tolerances regarding the positioning of the joint and a variety of alternatives for joining the members. (Welda® Anchor Plate Technical Manual 2018, pp.
4-6) Welding is producing locally extremely high temperatures which in the case of fire safety can be a problem at the construction sites. Flammable materials might combust due to a welding spark or the local high temperature leading to a fire hazard. Heat treated met als, especially steels can form new microstructures due to the heating and cooling cycle possibly altering the load bearing capabilities and increasing the probability of failure due to softening or hardening of the metal depending on the type of steel. Also, materials prone for melting at relatively low temperatures might be at risk when close to the welded plate. The plate is
however partially submerged to concrete as can be seen in Figure 25 so for example concrete combustion is not likely and the concrete functions as massive heat sink albeit with unknown efficiency. The fire hazards might occur due to multiple reasons one of which could be for example lack of cleaning of saw dust or similar flammable dust like material typically found from construction sites.
Figure 25. Welda® connecting plate principle (Welda® Anchor Plate Technical Manual 2018, p. 2).
The Welda connecting plate is available in a variety of material combinations. There are pre-designed material combinations done by Peikko and those are the materials studied in this thesis. The anchor stud of the Welda® connection plate is not considered because it is not directly affected by the welding of the joined member. The available pre-designed plate ma-terials with the corresponding numerical definition and standard are:
- 1.0577 S355J2+N Non-alloyed structural steel SFS EN 10025-2
o WELDA
- 1.4301 X5CrNi18-10 Austenite stainless steel SFS EN 10088-2 o WELDA R
- 1.4401 X5CrNiMo17-12-2 Austenite stainless SFS EN 10088-2 o WELDA A
The weldability of the S355J2+N is generally very good and not much harm can be done by welding it regarding the microstructure of the steel. However, the S355 is not exactly free of problems regarding welding that is the distortion due to welding must be considered espe-cially in long Welda® plates. An intermittent weld is a possible solution to reduce the heat input thus the distortion caused of a long weld, but it must be noted that SFS-EN 1993-1-8 stipulates not using intermittent welds in corrosive conditions (SFS-EN 1993-1-8 2005, p.
39). The distortion due to welding must not be neglected even though it might at first seem small but welding in this case happens from only one side which is enhancing the effect of plate distortion and therefor causing tension in the anchoring to the concrete.
Standard SFS-EN 1011-3 is presenting the recommendations for welding of stainless steels where the appendix A1 is specifically for the austenite stainless steels. The austenitic stain-less steels are rather well weldable when certain requirements are followed. The heat expan-sion coefficient of austenitic stainless steel is higher, and the heat conductivity is lower than of non-alloyed steels. Therefor the SFS-EN 1011-3 recommends low heat input due to the detrimental effects of heat to the shape and especially hot-cracking. Also preheating is not recommended due to its collaboration with welding heat input. The hot cracking is a major challenge to be considered when welding austenitic stainless steels. SFS-EN 1011-3 presents a list of actions for preventing hot-cracking. Preserving the ability to resist corrosion is an essential factor when welding stainless steels. Therefor it must be emphasized that the weld filler metal should be compatible with the process and the material. The compatibility of the filler metal should be in agreement with EN ISO 3581, EN ISO 14343, EN ISO 14174 and EN ISO 17633 depending on the welding process. (SFS-EN 1011-3 2018, pp. 10-14) Also welding dissimilar steel to the Welda® plate will cause special restrictions to consider. Gen-erally, can be stated that if dissimilar steels are welded the final properties of the completed joint must be assessed to figure out why the dissimilar joint is needed. It is always easier to weld similar steels with similar properties. In case of accidental welding of dissimilar steels, it can be safely assumed that the joint is not sufficient.
Concrete is cured during time and the curing process requires appropriate humidity. If the water is taken out of the process it affects locally the curing and eventually lowering the strength properties of the affected area. Problem due to the excessively hot zones in concrete is cracking. (Palmer 2020) Those factors together lead to a conclusion of possible problems in the load transferring capabilities of the connection plate to the concret e. Especially the concretes already low capacity of tension strength and vulnerability to cone fracture arises questions. This topic seems exceptionally viable for research specifically framed for the problem of local heating affecting the adjacent concrete structure generally and especially locally near the heat source. Nevertheless, it can be outlined that the phenomenon is indeed interesting yet out of the scope of this thesis.