ATTEN: Manager Liu
ADD: Longhua Development Zone, Jing County, Hengshui City, Hebei Province
In the design process of portal frame, it has always been a concerned problem for designers to determine the out-of-plane effective length of inclined girder of portal frame, because only the principled clauses are given in the existing design standards, such as the oblique girder of solid-web rigid frame pointed out in Technical Regulations for Steel Joint of Portal Frame Light Building 6.1.6,2. The distance between the lateral support points should be taken as the calculated length of the exit plane. When the distance between the lateral support points of the two flanges of the inclined beam is not equal, the distance between the maximum compressive flange lateral support points should be taken. But what kind of point can be used as the lateral support points? Although the Regulation also points out that the spacing of lateral supporting points can be set up at the corner and the support joint between columns, whether setting the corner can play the role of lateral supporting points and under what circumstances the role of lateral supporting points is not clearly specified in the specification. 2. In the Guidelines for the Design of Light Steel Structures, the relevant regulations specify in detail many methods for determining the out-of-plane gauge length of portal rigid frame beams in practical projects.
1. The effective length of upper flange Ly usually has three methods: (1) The pitch of the lateral support of upper flange is determined by the span of the rigid frame and the column spacing. Usually, the multiple of purlin distance, such as 1.5m purlin distance, can be taken as 3.0, 4.5 and 6.0m support pitches. The support pitch directly affects the cross section and overall stability of the beam. (2) The distance between the corners. The knee brace is connected to the lower flange with elastic lateral displacement by purlin, so it can not be used as the lateral fulcrum when the upper edge is compressed. In some cases, it can be used as the lateral fulcrum of the lower flange under compression. In the design of portal frame of many light-duty houses, two purlin spacing is taken, and three m is taken when the purlin spacing is 1.5 m. If the above two purlins and three sandalwood bars are not in the transverse support joints, all purlins bend along the upper flange of the beam and can not play a supporting role. Only purlins located at the support point can play the role of lateral support (i.e. take the pitch of transverse support). Taking 3m, a reasonable cross-section of the beam can be designed to give full play to the role of strength.
When the roof stiffness is good and the connection with sandalwood bar is reliable, considering the actual skin effect of the roof, two purlin distances, namely 3m, are taken. According to the spirit of regulation 6.1.4, in practice 2. The effective length of the lower flange Ly usually has two ways to choose the spacing between the corners, and the spacing between the corners is not greater than (b1 is the width of the compressed flange), most of which is 34.5m. Another viewpoint is to take the pitch between purlins connected at the upper wing green transverse support node, that is, the pitch of transverse support. The precondition is related to the bending moment figure. Normally, the end of the beam is short in negative bending and the middle of the span is short in positive bending. Considering that the cylindrical wind load makes the bending point of the beam move inward, it is safe to take 1/5 of the bending point from the end of the beam and calculate it by using the calculation formula of the out-of-plane length of the lattice rigid frame. However, if the effect of wind load is greater than that of constant load, negative bending moment will occur in time. At this time, the effective length of the lower flange of the beam span should be taken. Some scholars suggest that the effective length of the lower flange of the beam should be 2 times as long as the distance between the reverse bending point of the beam and the end of the beam. The additional effect of the corner brace on purlin should be considered when the beam is placed near the end of the column. When the lower flange of the beam is compressed, the lower flange of the beam is connected with the sandalwood bar from the side to the supporting point through the corner support. In practical engineering, the additional influence of the corner support on the sandalwood bar is generally not considered by the designers. There are few relevant literatures about the influence of the corner support on purlin.
In some cases, the influence of corner support on purlin can not be neglected. Purlins are generally considered as pure flexural members. After setting up the diaphragms, the vertical bearing capacity has a certain beneficial effect. However, the existence of diaphragms will inevitably produce longitudinal and axial pressure inside purlins. At this time, the force model of purlins will be changed from flexural members to compressive and bending trusses. Purlins are usually formed by 2rnr~3r_n steel sheet. Under the compression condition, the height-thickness ratio of purlins is difficult to satisfy. In the connection of purlins, the shear resistance of bolts and the local pressure of purlins are difficult to satisfy because of the large axial pressure of the corners.