Creaking noise when ascending and descending steel stairs in a building! I wonder if these stairs are okay...

consulting

This is a case in which the safety of the staircase was a concern for the concerned parties due to the creaking noise generated when ascending and descending the steel staircase. As a result of detailed analysis and evaluation of both noise and vibration, we were able to confirm the safety of the staircase, and thus allayed the concerns of the concerned parties.

Engineering Group, Division 1
Tianyang Sun

Issues in this case study

Steel staircases commonly produce creaking noises. The main causes are loose fasteners, friction between members, structural resonance, and changes in temperature and humidity. Squeaking itself does not necessarily indicate danger. However, if the structure has deteriorated over time or has been weakened, it may lead to accidents.
In this case, we were consulted about a creaking noise that was occurring when ascending and descending a steel staircase, and we were concerned. We measured vibration and noise, analyzed the data, identified the location of the creaking noise, and investigated the cause of the noise.

Measurement Overview

Noise Measurement

First, several people tried ascending and descending the subject steel staircase in different walking styles, and it was found that the creaking sound occurred at the tempo of ascending and descending of certain people. Furthermore, the creaking sound was heard near the waist wall on the way from the intermediate landing to the second floor. Therefore, in order to identify where the creaking sound was generated, noise measurements were taken while ascending and descending the staircase.

Noise measurement point

Five locations (S1-S5) were chosen near the waist wall from the intermediate landing to the second floor.

Vibration Measurement

On the other hand, to examine the vibration generated by the staircase, we confirmed its magnitude by measuring the vibration during stair ascent and descent. The results,Occupant Performance Evaluation Criteria for Building Vibration and Guidelines for Pedestrian Bridge DesignThe results were evaluated using

Vibration measurement point

Three locations (V1 to V3) from the first to the second floor.

V1 is the center area from the first floor to the middle landing, V2 is the center area of the middle landing, and V3 is the center area from the middle landing to the second floor.
Figure 1 shows the noise and vibration measurement points. Figure 2 shows an image of the measurement.

Figure 1: Noise and vibration measurement points

Figure 2: Measurement image

measurement results

noise

From the noise level results, when creaking sound was generated (ascending from the intermediate landing to the second floor), comparing the noise levels of S1~S5 in Table 1, the noise levels around S1 and S2 were slightly higher, and the area around S1 and S2 (the fifth waist wall from the intermediate landing to the second floor) was considered the source of the creaking sound.

Table 1: Noise level measurement results

It is one of the frequency analysis for the noise data of measurement point S1.FFT AnalysisThe results showed that the squeaking sound was generated with a peak around 700 Hz (Fig. 3). The results showed that the creaking sound had a peak around 700 Hz (Figure 3).

Figure 3. FFT analysis of squeaky sound (measurement point S1)

vibration

In addition, an FFT analysis was performed on the vibration data. The natural frequency of the staircase was estimated to be 15.6 Hz based on the constant microtremors at measurement point V3. The peak frequencies of the vibration data at measurement point V3 when creaking sound was generated (ascending from the landing to the second floor) were around 16.0 Hz and 25.0 Hz (Figure 4).

Figure 4: FFT analysis of the constant microtremors of the staircase (measurement point V3)

Figure 5: FFT analysis during stair climbing (measurement point V3)

A 1/3 octave band analysis was performed on the vibration data of the staircase when creaking sound was generated (ascending or descending from the landing to the second floor), and the results were evaluated using the occupant performance evaluation criteria and the pedestrian bridge design guidelines. What is the evaluation level of the occupant performance evaluation standard for staircases when creaking noise is generated (ascending or descending from the landing to the second floor)?
V1 became V-II, V2 became V-III, and V3 became V-I.

Table 2. Description of evaluation levels for vertical vibration

*The Architectural Institute of Japan (ed.), "Occupant Performance Evaluation Criteria for Building Vibration and its Commentary," 2018

　The results of the evaluation in the pedestrian bridge design guidelines are below the level of V1, V2, and V3, all of which are "slightly felt when standing on the spot" (Figure 6).

Figure 6. generated vibration during stair climbing [1/3 octave band analysis].

　The Steel Construction Association of Japan (ed.), "Pedestrian Bridges for the Future: Guidelines for Planning and Design of Pedestrian Bridges that are Attached and Friendly to People," 1998.

summary

The results of the noise measurements showed that the noise levels near S1 and S2 were higher than those at noise measurement points S1 to S5 when ascending and descending stairs, so the creaking sound was considered to be generated near S1 and S2 (the fifth waist wall from the middle landing to the second floor).
The analysis of the noise and vibration frequency components showed that the frequency component of the creaking sound was around 700 Hz. Since this is significantly different from the natural frequency of the staircase, 15.6 Hz, it was determined that the creaking sound was unlikely to be solid sound caused by resonance of the staircase itself. Judging from the site conditions, we also assumed that the creaking sound was caused by the staircase vibrating as it ascended and descended, and coming into contact with nearby materials (such as the steel frame in the waist wall).
The vibration during stair climbing was evaluated based on the occupant performance evaluation standard and the pedestrian bridge design guidelines. From the results, we confirmed that the measured vibration was at an acceptable level for a stairway structure and that there were no structural safety issues. We were able to report results showing that users can eliminate their sense of anxiety when ascending and descending stairs.

Sound measurement and consulting　
Vibration measurement and consulting