# Influence of Internal Pressure on Hollow Section Steel Members in Fire
## 5 Conclusions  
The study demonstrated that internal pressure caused by elevated temperature can in certain conditions significantly diminish the axial capacity of hermetically sealed hollow section members. Influence of the internal pressure on fire resistance of a particular hollow section member depends on temperature and geometrical parameters of the section. For RHS, the width-to-thickness ratio _c/t_ is in good correlation with the sensitivity of a section to the internal pressure. For sections with larger _c/t_ ratios, the loss of capacity emerges at lower temperature values. Regarding selection from the practical range of structural RHS sections, the temperature causing total loss of resistance without any remaining capacity for axial tension or compression may be as low as 580 \({}^{\circ}\)C, with \(\sim\)40% of all the analyzed cases failing in the temperature range from 580 \({}^{\circ}\)C to 750 \({}^{\circ}\)C. This temperature range deserves special attention, as it overlaps with a certain range of critical temperatures occurring in practical design.  
Assumed reasons why this phenomenon has not so far drawn attention in reported research reports may be the following: (a) the hollow sections in the tests are not always sealed hermetically; (b) the temperature in the particular test or critical temperature in the analysis did not develop up to the limit where the internal pressure might dominate the performance; or (c) the limit state is governed by a complex stress state, where the impact of the internal pressure is not differentiable.  
The present analysis focused solely on axial capacity, as it was conducted within the linear geometrical domain, neglecting local buckling effects in the compression state.  
Several issues require further consideration regarding the possible internal pressure effect in fire conditions:  
- The influence of internal pressure on the buckling capacity of compressed elements;  
- Local buckling and the capacity of sections belonging to class 4;  
- Evaluation of hoop stresses in the rounded corners of cold-formed RHS and SHS sections;  
- Assessing the capacity of welded joints in the RHS and SHS sections;  
- Examination of welded hollow sections and stress levels in the welds.  
Figure 13: Simulation results—Axial capacity diagrams for the RHS and SHS sections.  
From a practical perspective, one possible solution for the negative impact of internal pressure in tubular members might be relatively straightforward. Ventilation holes should be incorporated into the elements vulnerable to capacity reduction due to internal pressure. These holes should remain sealed during normal structural operation to provide corrosion protection and should activate only when a certain threshold of inner pressure is reached.  
Conceptualization, A.K. and I.T.; methodology, A.K. and I.T.; formal analysis, A.K.; investigation, A.K. and I.T.; writing--original draft preparation, A.K.; writing--review and editing, A.K. and I.T. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Data Availability Statement: Data are contained within the article.  
The authors declare no conflict of interest.