Source: Jinan Construction Engineering Survey and Design Quality Supervision Station, Editor: Yang Jianxing, Zhang Zhenkui, Review: Fu Anyuan
Stairs are an important transportation hub in a building, playing an important role as a vertical escape route in the event of an earthquake or fire. However, the review of construction drawings found that the designer did not pay enough attention to the staircase design, and the quality of the staircase design was uneven. The common problems in staircase design found in the construction drawing review are discussed as follows:
1. The force transmission path of the structure is inconsistent with the actual structural layout
In the projects reviewed by the author, it is found that the frequency of violations of "structural force transmission path is inconsistent with the actual structural layout" is high, especially in the design of staircase structure, such as the following:
Figure 1.1
Figure 1.2
As shown in Figure 1, there are ladder columns, platform beams and floor beams at both ends of the stair tread of this frame structure, and the accurate and reasonable calculation diagram should be input in the YJK or PKPM software calculation model into the interstory ladder column, the floor beam, of course, the design can also be used to approximate the action of the ladder column, and the load is first transmitted to the stair beams on both sides according to the conductive force mode on both sides, and then transmitted to the main beams around the periphery. According to the force transmission path in Figure 1, the load will be directly transmitted to the surrounding main beams, and the intermediate force transmission link is missing, resulting in the bending, shear and torsional load calculation of the ladder column support beam is small and unsafe.
Article 2.3.2 of the General Code for Engineering Structures GB55001-2021 emphasizes: "The calculation model used in structural analysis should be able to reasonably reflect the effect of the structure under the action of relevant factors. The boundary conditions set during the analysis should correspond to the actual conditions of the structure. ”
The model established by the structural analysis should accurately reflect the real response of the structure, and the simplified assumptions and boundary conditions introduced in this process should be consistent with the actual situation.
2. The layout of the staircase structure is unreasonable
Figure 2.1
Figure 2.2
As shown in Figure 2, stairwells and elevators are often arranged at the corners of buildings or protruding from buildings in residential projects, and the connection with the main structure is weak, and even some projects will have the unfavorable situation that both sides of the concrete wall are empty. In order to control the economic index, the design unit often arranges the walls of the elevator hall and stairwell too fragmented, which cannot form an effective cylinder, and the horizontal load cannot be directly and effectively transmitted reliably.
Article 2.4.1-1 of the General Code for Seismic Resistance of Construction and Municipal Engineering GB55002-2021: "The seismic system of engineering shall have a clear and reasonable transmission path of seismic action".
From the analysis of the reliability of the structural force transmission path, the author believes that the convex elevator and stairwell should strengthen the direct connection with the main structure, and it is not suitable to have a fragmented structural wall layout scheme such as small wall limbs and piece walls, and it is not suitable to be weakly connected with the main structure through the frame beam.
In the case of such architectural schemes, the structural designer should strengthen the consultation with the architectural profession, arrange as many concrete shear walls as possible to form an effective cylinder, or do not consider its stiffness contribution when judging the comprehensive index of the structure, as a safety reserve.
3. The structure of the staircase slab is inconsistent with the actual force
Article 3.6.6 and 6.1.15 of the Code for Seismic Design of Buildings GB50011-2010 (2016 Edition), under the action of horizontal seismic force, the staircase is more or less involved in the internal force distribution and deformation coordination of the main structure, and the ladder plate has a similar effect of diagonal brace, when the staircase is arranged inside the shear wall cylinder, the diagonal brace effect is not obvious, and the staircase plate of the frame structure will produce a large axial force. When the tensile (compressive) force of the staircase reaches or exceeds the ultimate tensile (compressive) strength of the concrete, tensile (compressive) failure will occur, and the destruction of the stairwell (including the staircase) will delay the evacuation and rescue work, resulting in serious casualties.
Therefore, attention should be paid to the structure of the staircase, the staircase of the frame structure should be designed by pressing (pulling) the bending member, the upper part of the staircase should be equipped with a long steel bar, the specific structure can be detailed in the atlas 22G101-2, and the staircase of the frame structure should be marked with the seismic grade.
4. The cross-sectional size of the structural column is inconsistent with the first grade of the building fire resistance grade
According to Article 2.1.5 of the General Code for Engineering Structures GB55001-2021, the structural durability design has become a strong article for the review of construction drawings, and the structural durability design needs to be considered from the perspective of the use and maintenance of the whole life cycle of the structure in addition to the durability of the material. In the event of a fire, when the column cross-section is too small, it will cause the components to fail to meet the requirements of the building fire resistance class and lead to premature failure of the components.
Article 5.1.2 and Appendix 1 of the Code for Fire Protection Design of Buildings GB50016-2014 (2018 Edition): "When the fire resistance level of civil buildings is Class I, the fire resistance time of the column shall not be less than 3 hours, and the minimum cross-section of the corresponding column is 200mmX500mm. ”
When the fire resistance level of the building is Class I, the use of 200mmX400mm for the stairwell structural column obviously does not meet the requirements; When the fire resistance grade is grade II and below, the stair column can be designed according to 200mmX400mm.
5. Other issues
1) If the whole cast staircase of the frame structure does not consider the influence of stiffness in accordance with the overall calculation of Article 5.1.3 of GB55002-2021, and the sliding bearing is not adopted, measures should be taken by choosing one of the two.
2) When the top end of the ladder column of the frame structure is connected with the upper frame beam, the weak connection is not indicated.
3) When the frame structure staircase is arranged close to the edge of the building, it causes the unilateral piercing column or the cantilevered beam as the out-of-plane support of the column.