by [Eagle language] Eagle has been reviewing a steel structure project for a long time recently.
One of them is about whether it is reasonable to deduct camber from the calculated value of vertical deflection of heavy-duty roof.
He thinks it is quite representative, so he posted this article on his official account to communicate and discuss with building structure peers.
Hawk noted that there are few documents about the influence of pre camber on the deformation of structural members.
The current steel structure design standards refer to those items.
Whether the camber can be deducted in the calculation of deflection may have deviation in understanding, which is paradoxical.
During the discussion with peers, there are also different views, some say it can be deducted, some say it cannot be deducted, and some say it can be partially deducted.
Hawk has his own thoughts on this issue.
However, his views are not necessarily correct due to his limited level.
He hopes to introduce new ideas and clarify the relationship.
He also welcomes criticism.
Main contents: 1.
Proposition of problems 2.
Contradictions between the current steel structure design standards and the provisions on springing 3.
Analysis and discussion on the understanding of springing purpose and the impact on component stiffness 3.1 Understanding of springing purpose 3.2 Analysis on the impact of springing on component stiffness 3.3 Calculation examples and analysis 4.
Several aspects of deflection control 5 Conclusion Can the deflection limit of “rigid demand” be relaxed through arch camber? Xiamen Eagle (Helidao Examination Company) I The problem is raised.
A single layer (concrete column+steel truss) bearing system is adopted for the structure of a project submitted for approval.
The column height is 21m, the truss span is 29.5m, the average column spacing is about 6.5m, the accessible roof, the floor bearing plate is 100mm thick, and there is a room on the roof where more than ten tons of equipment are placed in a local frame.
The roof dead load is 6.0KN/m2, and the live load is 2.0KN/m2.
Top through square steel truss is used for longitudinal and transverse stress.
The midsagittal height of the truss span is 1.6m.
The calculation diagram is shown in Figure 1.
3DS software is used for calculation.
Figure 1 During the preliminary review of the steel truss calculation model diagram, the eagle’s intuition was that the truss’s 1.6m rise height would not be too small? This is an accessible concrete roof with a span of nearly 30m, and it contains heavy equipment.
It obviously belongs to the category of heavy roof.
The maximum rise height of the truss mid span is only 1.6m, and the height span ratio is nearly 1/19.
Can the required stiffness meet the requirements? At that time, I was impressed that the rise height of the 30m span pipe trusses in the National Standard Drawings of Light Ladder Type Pipe Truss had reached 3.0m, and the height span ratio had reached 1/10, while the rise height of the heavy roof in this project was only half of that of the light roof in the atlas.
Therefore, the preliminary review comments put forward: “The truss span is up to 30m, and the roof contains concrete slabs and columns on the beams.
The load is large, and the truss height is only 1.6m, which seems to be small.
It should be reviewed according to the stiffness calculation results.”, The designer is required to provide deformation deflection value, deflection span ratio and other stiffness calculation information.
The calculation results provided by the design review and modification are shown in Figure 2.
The 3D3S calculation shows that the maximum deflection value in the middle of the span is 128mm.
If the deflection span ratio is 1/230 according to this result, it obviously does not meet the provisions of the current steel structure design standard [2] Article 3.4.1 and Appendix B Table B.1.1 Item 4 Truss allowable deflection value L/400.
The calculation sheet provided by the design considers arch camber.
The calculated deflection is L/500 minus 60mm arch camber value, and the final deflection span ratio is 1/434, which seems to meet the allowable deflection value requirements of the steel standard [2].
Figure 2 The screen shot of the calculation result of deflection after deducting arch camber at the preliminary review stage shows that the total deflection is 128mm, and 60 mm arch camber will be deducted to offset nearly half of the total deflection.
Is this reasonable for the stiffness control requirements required for use? However, in the current steel structure design standard [1], Appendix B, Table B.1.1, Note 2 does indicate that “camber shall be subtracted if there is camber”.
The steel label [1] says in black and white that it can be deducted.
What can we review? The design insisted that the camber value should be deducted.
Therefore, in order to be prudent, the review opinion required that the design should be further reviewed according to the actual stiffness of the truss and the comfort of the accessible roof.
II The contradiction between the current steel structure design standards and the provisions on arch camber.
With regard to the deflection value of flexural members, the current “Steel Structure Design Standards” [1] Article 3.4.1 has been clarified, and the allowable value of member variation in Appendix B can be adjusted, but it is conditional, that is, in the principle of “not affecting the normal use and appearance”.
Eagle understands that arch camber is also a way to adjust the deformation of structural members, which should comply with the above principles.
As for arch camber, Article 3.4.3 of the Steel Standard [1] stipulates that “the transverse stressed members can be arched in advance, and the size of arch camber depends on the actual needs.
The deflection value generated by adding the standard value of dead load plus 1/2 of the standard value of live load can be taken.
When only to improve the appearance conditions, the deflection of the member shall be the calculated value of the deflection under the standard value of dead load and live load minus the arch camber value”.
Note that the camber value deducted by deflection specially emphasizes “only for improving appearance conditions”.
What is the “appearance condition” is not clearly defined in the current steel standard, which can be interpreted literally as the deflection and sagging of members that affect the appearance.
However, as mentioned above, the steel standard [1] in Appendix B.1, Table B.1.1, Allowable Values of Deflection of Bending Members, is “Deflection Caused by Permanent and Variable Load Standard Values (If there is camber, camber shall be deducted)”, and there is no precondition in Article 3.4.3, “When only improving appearance conditions”.
As long as camber is cambered, camber can be deducted from the total deflection, as in the above reviewed project.
Or that question, is it appropriate for the stiffness requirements of normal use? This is worth thinking and discussing..
