Ambient vibration tests were carried out to evaluate the dynamic properties of an asymmetric steel building with semi-rigid connections. The test case has many non-structural elements, constructed in the city of Tehran (Iran). The tests were conducted to obtain natural frequencies, mode shapes and damping ratio of the structure and then Fourier transform were used to analyze the velocity records obtained from the tests. The first and second natural periods of the building were obtained as 1.37 s and 1.28 s through the test and damping ratio for the first mode was calculated as 0.047. However, Natural periods obtained from finite element model have higher values from those gained from ambient vibration. Then the model was calibrated by modeling of the in-fill masonry panels at their exact locations and considering the boundary conditions by modeling two blocks near the block No. 3, but the differences were existed. These differences may be due to some hidden stiffness of nonstructural elements in the low range of elastic behavior, showing the structure stiffer than it is in reality.