by The calculation of the transmission line tower is generally based on the truss system, and the members are the two force bars hinged at both ends, that is, the axially loaded members that can only bear tension and pressure.
The program calculates the internal force and bearing capacity based on this basic assumption.
However, in fact, the pole connection of the transmission line tower is bolted connection.
When the angle steel is connected on one side, the force transfer surface and the centroid of the angle steel will inevitably be eccentric.
In addition, when there is one bolt at both ends of the member, the connection is close to the ideal hinge, but when there are multiple bolts, the actual performance of the end connection is between hinge and rigid connection, which belongs to semi-rigid connection.
The existence of end eccentricity and restraint will affect the compressive stability bearing capacity of the member.
The slenderness ratio shall be corrected by considering the corresponding correction factor in the specification.
In this paper, the analysis and discussion are carried out.
Relevant provisions of 15486-2020 row standard 5486-2020 row standard for the correction factor K of the slenderness ratio of axial compression members are as follows.
It can be seen from the provisions of the specification that when the slenderness ratio L0/r is less than 120, the eccentricity correction is made according to the eccentricity of both ends of the rod.
(1) When the center of both ends is compressed, it is not corrected because it is consistent with the calculation assumption, and the correction factor is 1; (2) When the center of one end and the other end are eccentrically compressed, the correction factor is greater than 1; (3) When both ends are eccentrically compressed, the correction factor is greater than 1, and its value is greater than the case (2).
It can be seen from the provisions of the specification that when the slenderness ratio L0/r is not less than 120, the slenderness ratio is modified in three cases according to the constraint conditions at both ends of the member.
(1) When there is no constraint at both ends (single bolt), the correction factor is 1 because it is consistent with the calculation assumption; (2) When one end is constrained (two or more bolts), the correction factor is less than 1; (3) When there are constraints at both ends (two or more bolts), the correction factor is less than 1, and its value is less than the case (2).
The specification considers that the end eccentricity mainly affects the members with small slenderness ratio (short and thick bars), and the end constraint mainly affects the members with large slenderness ratio (thin and long bars).
The limit of slenderness ratio for eccentricity and constraint correction is 120.
Relevant provisions of American Standard 2ASCE/SEI10-15 ASCE/SEI10-15 for the correction factor K of slenderness ratio of axial compression members are as follows.
It can be seen that for the slenderness correction factor K of axial compression members, the provisions of the Chinese and American standards are completely consistent.
3 Theoretical basis of steel structure stability The theoretical basis of end eccentricity and end constraint correction is steel structure stability theory.
Please refer to Professor Chen Ji’s “Theory and Design of Stability of Steel Structures” (5th Edition).
Some chapters are taken as follows: 4 Calculation of the eccentric correction factor K for the slenderness ratio of axial compression members For the convenience of calculation and comparison, the correction factor for eccentric compression at one end of the center and the other end is recorded as PK1, and the correction factor for eccentric compression at both ends is recorded as PK2.
During calculation, the range of slenderness ratio L0/r before correction is 0-120.
From the numerical point of view, both PK1 and PK2 are not less than 1, that is, multiply the slenderness ratio before correction by an amplification factor to reflect the reduction effect of the eccentric end of the member on the compressive stability bearing capacity.
The following is the calculation process of PK1 and PK2 eccentric correction factors.
In order to better explain the degree of impact, the data are plotted into curves.
In order to better calibrate the curve, when drawing the curve, set L0/r between 20-120, and the curve is as follows.
From the above curve, it can be seen that the eccentric correction curve decreases with the decrease of the slenderness ratio L0/r before correction, and PK2 is always greater than PK1, which is equal to 1 when L0/r=120.
In order to better explain the impact of eccentricity at both ends, the data of PK2/PK1 is plotted as follows.
The following curve shows that eccentricity at both ends has a greater impact than eccentricity at one end, and its impact degree decreases with the decrease of L0/r.
5 Calculation of the slenderness ratio constraint correction factor K of axial compression members For the convenience of calculation and comparison, the correction factor with constraint at one end is recorded as YK1, and the correction factor with constraint at both ends is recorded as YK2.
During calculation, the range of slenderness ratio L0/r before correction is 120-250.
From the numerical point of view, both YK1 and YK2 are less than 1, that is, multiply the slenderness ratio before correction by a reduction factor to reflect the improvement effect of the end restraint of the member on the compressive stability bearing capacity.
The following is the calculation process of YK1 and YK2 constraint correction factors.
In order to better explain the degree of impact, the data are plotted into curves.
From the above curves, it can be seen that the constraint correction curve decreases with the decrease of the slenderness ratio L0/r before correction, and YK2 is always smaller than YK1, which is equal to 1 when L0/r=120.
In order to better explain the impact of constraints at both ends, the data of YK2/YK1 is plotted as follows.
The following curve shows that the constraint at both ends has more influence than the constraint at one end, and its influence increases with the decrease of L0/r.
6.
Software input methods and precautions The above analysis is the requirements of the specification.
For the calculation of iron tower, it is necessary to accurately input the software according to the input rules of the software and the specific structure of the rod end, otherwise the calculation error will be caused.
(1) The software input rules take SMARTPOWER software as an example, and column M is the input of end eccentricity and constraint conditions to realize the correction of slenderness ratio.
So how to identify the eccentricity and constraint of the rod end and input it into the software accurately.
This requires the designer to be fully familiar with the structure of the rod end.
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