**1. Introduction**

Generally, dynamic and static examinations are carried out to verify the bridge structure behavior versus design assumptions or to determine the actual load-carrying capacity of the bridge in connection to planned repair works. Conventional empirical experiments are normally conducted using gauges to measure accelerations, strains, displacements, and temperatures, as well as using wireless and geodesic techniques [1–5]. However, mounting the above-mentioned gauges requires the close contact with the examined construction. In some cases, there may be a problem with access to the testing objects, frequently needing the use of platforms, scaffolding, alpinists, cranes, etc. Consequently, this is the main motivation to examine and evaluate the chosen measuring techniques designed for non-contact observation on the bridge deflections.

Useful methods for observing the bridge deflections with interferometry, computer vision, digital image processing, a laser Doppler vibrometer, a Robotic Total Station, a GPS, and a video deflectometer are presented by Beben [6], Lee and Shinozuka [7], Meng et al. [8], Moschas and Stiros [9], Nassif et al. [10], Olaszek [11], Pan et al. [12], Pieraccini et al. [13], Pieraccini and Miccinesi [14], Psimulis and Stiros [15], Spencer et al. [16] and Zhang et al. [17]. Currently, some researchers are using new technology to monitor bridge behavior, such as for example the smartphone [18]. Most researchers, for example, Anigacz and Kokocinska-Pakiet [1], Schofield and Breanch [19], and Yu et al. [20], prefer a traditional way to do the object testing; i.e., they set a reference measuring grid both on the examined structure and outside. It requires signalizing the tested points on the structure. This way is completely appropriate but requires preparatory work in the field.

It should be emphasized that the latest achievements in measurement technologies allow obtaining a several million points set (called a point cloud) of the analyzed object. The point cloud can be obtained indirectly (photogrammetry) and directly (spatial terrestrial laser scanning (TLS)), as well by using both methods together. This allows receiving complete data about the object shape [21].

Jauregui et al. [22] reported an analysis of bridge displacements using the close-range photogrammetry method. The obtained results were compared to the total station readings and finite element analysis. Jiang et al. [23] presented the latest state-of-the-art on the close-range photogrammetry technique, including monitoring the bridge behavior and changes in its shape. However, the distance from the camera to the tested bridge was quite small. Yu et al. [20] described the latest measuring techniques and compared them to each other (Table 1). Nevertheless, the given data related to the TLS method is overestimated. This is because achieving precision up to 1 mm is practically impossible for the large-span bridges or limited to a short distance from the scanner to the bridge (in the case of small bridges). Yoon et al. [24] applied the TLS method to receive the crucial tunnel data.


**Table 1.** A set of displacement measuring techniques.

\* Depends on used instruments.

In a typical survey with a conventional measurement instrument, the creation of a measuring grid is related to the transfer of potential errors to the observed points. Beshr [25] and Osada [26] proposed methods to reduce such errors. However, it should be emphasized that these problems did not appear in an analyzed situation. This is because the photogrammetric and TLS examinations were conducted from the same position. The connecting points of the individual scans constituted white reference spheres and measuring targets, as well as clearly visible bridge details.

The main aim of the research was to examine and evaluate the measurement possibilities for determining the bridge deflections under various load scenarios. The preliminary research for this bridge is given in [27]. Information about bridge deflections, obtained during normal operating conditions, is crucial from a structural safety point of view. In addition, the adequacy of the design principles may be assessed using data on the bridge behavior under load. Furthermore, in the case of old bridges, it is important to know that the admissible deflections of the load-bearing structure are not exceeded. One of the main purposes of the study was to minimize the preliminary activities on the examined bridge. Therefore, it was resolved to apply white reference spheres, surveying targets and well-visible bridge details as the measuring points. In the tachymetry method (as the reference method), several surveying targets were also attached on the transverse beams of the bridge. For this reason, two measuring positions were constantly maintained throughout the test. This allows a direct comparison of results of the test sessions conducted using various methods.

In the present paper, a suspension bridge with a 165 m span was tested under static and dynamic loading. In the tested bridge, typical deflection sensors could not be used because the main span of the bridge is located over a large river. Therefore, the photogrammetry and TLS methods were applied, and the obtained results were compared to a classic measuring technique (tachymetry—used as a reference method). Until now, most photogrammetric measurements were conducted at a relatively short distance from the tested structures. In this case, a greater distance from the camera to the bridge (ca. 120 m) was examined. The laser scanning results were also presented in the form of color-calibrated maps of deviations to present bridge deflections and the most strained elements (important for the

bridge inspectors). This method of presenting results is new and highly useful because it supplies important information on deformation of the bridge elements. The most important results of the bridge deflections using applied methods were given and compared to each other and with appropriate values from the bridge standards. In addition, the benefits and drawbacks from each applied technique were also characterized.
