ATTEN: Manager Liu
ADD: Longhua Development Zone, Jing County, Hengshui City, Hebei Province
Recently, Professor Chen Shao-bo, who is the main compiler of CECS102 of Technical Regulations for Portal Frame Light-weight Housing in China, took great pains at the age of 93 to put forward an important paper entitled "Designing Light-weight Portal Frame Beam-column Connections into Rigid Joints", pointing out that the major defects in the Regulations clearly indicate that rigid joints are made. The importance and necessity of points, and the relevant provisions are not complete enough, so that some design portal frames, beam-to-column connections can not meet the requirements of rigid joints, affecting the structural performance of the paper pointed out that this connection is usually regarded as semi-rigid connections. When used in multi-storey frames, it is considered that semi-rigid connections can adjust the bending length at the end of the beam and in the middle of the span, so as to achieve roughly equal material saving. This kind of connection is considered to have good rotational ability, which is beneficial to the seismic performance of the frame. If the whole end plate connection is designed, it can also be made into a rigid joint. This is how the Code treats the connection of beams and columns. "If the rotational stiffness of the beam-column connection is far from the ideal rigid joint, and the internal force is still analyzed according to the ideal rigid frame, and the calculation length of the column is determined according to the provisions of the Regulations, the reliability of the portal frame will be insufficient, and even an accident will occur."
The above comments are accurate, pertinent and timely. No one has pointed out before that they have made us deeply instructed and worthy of consideration by the members of the compilation team. It is also a rare learning opportunity for the majority of structural designers. The paper points out that semi-rigid connections not only increase the displacement of the frame, but also decrease its bearing capacity. The self-weight and roof load of light portal frame are very light, and the seismic action is very small, so the semi-rigid connection with strong energy dissipation ability is not needed. The quasi-static test of the shaking table model shows that the joint is still in the elastic stage under 8 and 9 degree earthquakes, and the bearing capacity has a large reserve. It is inappropriate to allow the connection stiffness of light portal frames to be insufficient. It is necessary to design the light portal frames as rigid joints. This paper summarizes the past research work and current situation, and puts forward some interesting facts. The thickness of end plate is calculated according to the strength of the plate area supported by members and stiffening ribs under the action of bolt tension, without considering the stiffness requirement. When a triangular vertical stiffener is added to the extension part of the end plate, the end plate enjoyment decreases significantly, which may lead to insufficient stiffness of the joint.
One experimental study completed two experiments of portal frame joints, one is the end slanting beam-column connection, the other is the beam splicing joint at the roof. The former does not meet the requirements of rigid joints, and there are many differences. The thickness of end plate is 14 mm, which is less than the diameter of bolt (16 mm). The main reason is that the thickness of end plate is estimated to be too small. Another experimental study completed seven point tests of extended end plates, simulating the edge-column joints of multi-story frames, and concluded that they are semi-rigid connections. The relative angular displacement between beams and columns is mostly determined by the shear deformation of the joint area. An analysis shows that the maximum gap of the end plate at the edge of the niche is 0.7mm, mainly because there is no mule bolt in the middle of the end plate and the end plate deforms under the pulling force of the ridge. There is a research and analysis of three kinds of portal frames with different spans and two different column bases. The conclusion is that the oblique stiffening ribs in the joint area can be used as rigid joints for design calculation. Another analysis suggests that the oblique stiffeners in the joint area are necessary and should be treated as necessary structural measures rather than as required by the Code when the shear strength requirements are not met.
There is also a study that although the overhanging end plate can meet the requirements of rigid joints through reasonable design, it does not point out the reasonable design method. The teacher used a series of facts to illustrate that the design of light portal frame according to the "Regulations" can not meet the design requirements of rigid joints. In the past, the semi-rigid connection of strong chirped end-plate connection and its ability to adjust bending moment and energy dissipation were considered rigid when the internal force was small and semi-rigid when the internal force was large. In the previous versions of the MBMA low-rise building manual, the actual displacement of this structure is much smaller than the calculated value, which makes us think that the stiffness of end-plate connection is not a problem. According to the standard form of beam-column connection provided by MBMA, the design pays attention to the skin effect of the panel, ignores the stiffness requirements of the frame itself, and does not think of the stiffness of the joints. What problems will exist? Li He-bang, former chief engineer of Northeast China Architectural Design Institute, questioned the stiffness of the top column joints of portal frames several years ago. He doubted whether the formula of checking bearing capacity of the joint area in the Regulations was sufficient to ensure the flexural stiffness of the joints. He intuitively felt that it could not bear the bending moment of the frames, and there would be stiffness requirements, and could not put forward any stiffness criteria. 。
At that time, we proposed that oblique stiffening ribs could be set when the bearing capacity of the joint area is insufficient, which is also a problem.