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The first group consists of strengthening methods which will increase live load without increasing resistance. Increasing live load does not call for increasing the design values for actions’ effects, Sd. Without any increase in the resistance of the structure, Rd remains adequate. This can be achieved by decreasing dead load. This method is frequently applied, as it does not involve the complexities of an actual strengthening intervention in the full sense of the word. Decreases the dead load can be achieved in the buildings by replacing existing heavy floors and walls with lighter elements, or for bridges, by replacing heavy pavements with lighter ones. Reduction of dead load is often applied when additional stories are built on existing structures. In such cases, the additional dead load of the new construction should be taken into account. IT should also be noted that, due to the greater partial safety factors required for live load than dead load, the additional live load is less than the removed dead load.
Decreasing the span of existing structure is possible to allow an increase in live load that will be sustained by new supports. This method is applied when overall dimensions and aesthetics allow. Decreasing a span can be achieved by expansion of supports.
Changing the structural system is highly versatile. Using a new supplementary structure is possible to have a higher capacity of existing structure. Additional pre-stressing is an efficient way of strengthening of structures. When additional pre-stressing is applied, the stresses in existing structures should be precisely known. Care should be taken that tendons (ties) are sufficiently tensioned and corrosion will not develop. The second group of strengthening measures consists of those which increase the design values of resistance of structure Rd.
Increasing the cross section is a simple and frequently applied strengthening method;
Additional elements on the cross transversal section of elements is a very efficient method of strengthening. Additional reinforcement in the tension zone is a very efficient method of strengthening. Strengthening of circular cells in a steel silo has been successfully performed by adding reinforcement which has been protected with steel on outside of the silo. Additional bonded steel plates are good strengthening measures that have grown widely in recent times. Bond between the additional steel plate and the existing steel structure can be successfully achieved by welding or bolting. Instead of steel plates, other materials can be used. Bonded carbon-fiber or reinforced plastic plates have been used successively for this purpose.
Replacing structural members refers to strengthening of a steel structure by replacing structural members or by adding new members without removing existing ones. A convenient solution for strengthening a steel structure is to replace the existing deck. The new deck has been bonded to the existing girders by anchors and sealed with weld or with epoxy mortar. A footbridge with a suspended deck has been strengthened by new steel hangers, without removing the existing pressed ones. 3 Description of the experiment All these measures are considered due to a rational thinking and an engineering point of view. But, these measures need to be analyzed and simulated numerically and tested in the laboratory. The present paper makes a comparison between the results obtained in the laboratory and the analysis using finite element method.
First, it was tested a welded beam to column connection as shown in Fig.1. The force was applied normal to the column. The strain gauges gave the results for each step of 100kN. The final value of force P was 2000kN in order to avoid yielding in the assembly. This was necessary because the second test would have been performed on the same sample and the plastic deformations would have given inaccurate results. 1000P001P002P004P003 P003P004P005 Fig.1 Initial experimental connection The parts of the experimental connection are presented in Table 1. Table 1 Part Cross-section Length [mm] Material -0- -1- -2- -3- P001 HEB300 1300 OL52 (S355) P002 HEB300 150 OL52 (S355) P003 Pb140x10 240 OL52 (S355) P004 Pb100x10 100 OL52 (S355) P005 Pb300x25 300 OL52 (S355) P006 Pb240x10 1300 OL52 (S355) P007 Pb240x10 460 OL52 (S355) The strengthened experimental model is presented in Fig.2. The difference between the two models is the plates welded on the exterior of each flange denoted P006 and P007, in Table 1. 1000P001P002P004P003 P003P004P005P006P007 P007 Fig.2 Strengthened experimental model The strain gauges were put on the model as follows: two on the beam’s web, two on each of the beam’s flange, two on the column’s web for the strains developed normal to the force’s direction, two on the column’s web for the principal directions and one for strain in the direction of the force and there are three more strain gauges placed on one of the columns flange for the strain normal to the force’s direction. A sketch of the gauges placement is shown in Fig.3. Fig.3 Gauges position The main aim is to use the finite element method to observe if the results obtained in practice are the same with the ones simulated with computer programs. The advantage of s