Concrete column design fundamentals
Column design settings
Prior to running column design you should review the design settings by selecting Design > Settings > Concrete > Column.
- Reinforcement layout, e.g.
- Minimum and maximum bar sizes
- Minimum and maximum spacing values
- Confinement reinforcement
- Detailing settings
- General parameters (depending on the regional code being designed to)
For further details, see: Column design settings
While you can apply both normal and lightweight concrete in the column properties, column design using lightweight concrete is currently beyond scope.
Apply rigid zones
Unless you have chosen not to apply them, rigid zones are created at concrete column/beam connections.
In most situations in order to get an efficient design you would want rigid zones to be applied. You can however choose not to consider them by checking the Rigid zones not applied option that is provided in Model Settings, this will deactivate them throughout the model. You can also selectively deactivate rigid zones at specific column/beam connections by unchecking the Apply rigid zones option that is provided in the column properties under the Design control heading.
- For example, you might choose not to apply them if you encounter problems with short members and rigid zones which cannot be overcome by modifying the physical model.
- When rigid zones are not applied, the positions of releases in analysis model are affected, and member start and end points for design are also adjusted.
The Apply rigid zones setting is located under the Design control heading in the column properties.
Design parameters (Eurocode only)
Located under the Design parameters heading in the column properties, the following parameters relating to shrinkage and creep can be specified for individual members.
Permanent Load Ratio
The permanent load ratio is used in the equation for determining the service stress in the reinforcement, which is in turn used in table 7.3N to determine the maximum allowable centre to centre bar spacing. It is also used to calculate the effective creep ratio which appears in the column slenderness ratio calculations.
It is defined as the ratio of quasi-permanent load to design ultimate load.
i.e. SLS/ULS = (1.0Gk + Ψ2Qk) / (factored Gk + factored Qk*IL reduction)
If Qk is taken as 0 then:
SLS/ULS = (1 / 1.25) = 0.8
Hence, setting the permanent load ratio to 0.8 should provide a conservative upper bound for all cases.
When determining this ratio more precisely, consideration should be given to the amount of IL reduction specified, for example (assuming Gk = Qk and Ψ2 = 0.3):
For 50%IL reduction,
SLS/ULS = (1 + 0.3) / (1.25 + 1.5*0.5) = 0.65
For no IL reduction,
SLS/ULS = (1 + 0.3) / (1.25 + 1.5) = 0.47
Typical input range 20 to 100%
Age of Loading
This is the age at which load is first applied to the member.
Located under the Confinement reinforcement heading in the column properties, the Provide support regions setting determines the way each stack is divided into regions for the purpose of designing the confinement reinforcement.
- Checked - confinement reinforcement is designed separately in three regions.
- Cleared - the same confinement reinforcement is designed for the whole stack.
Located under the Slenderness heading in the column properties, the significant parameter within the slenderness criteria is a choice of how the column is contributing to the stability of the structure.
- bracing - provides lateral stability to the structure.
- braced - considered to be braced by other stabilizing members.
The second slenderness parameter is the effective length factor, which is either input directly by choosing the User input value option, or it is calculated in accordance with the requirements of the selected design code.
The stiffness settings affect the calculation of clear height, also referred to as the unsupported or unrestrained length (depending on the regional code being worked to) which is the clear dimension between the restraining beams at the bottom of the stack and the restraining beams at the top of the stack. The unsupported length may be different in each direction.
Effective concrete beams
An effective concrete beam is one which provides stiffness at a restraint position. A concrete beam is only considered effective if it is "fixed" at the position where it joins to the column. Concrete beams are only effective in a direction if they are within 45° of that direction, and therefore no concrete beam can be effective in both directions. A concrete beam is only effective if its angle to the horizontal is 45° or less.
A concrete beam only restrains the end of the stack if it is within the depth of the stack section from the end of the stack, and if its centre is nearer to this end of the stack than the far end. Therefore, at a node at a stack join, if the top of the beam is below the node by a dimension greater than the depth of the stack below the node, it is not considered. Similarly, if the bottom of the beam is above the node by a dimension greater than the depth of the stack above the node, it is not considered.
Effective flat slabs/other types of slab
When determining the unsupported length, if no effective beams are found at the end of a stack, Tekla Structural Designer considers whether there is a flat slab restraining the stack at that end. The Use slab for calculation... upper/lower, major/minor options, (which are located under the Stiffness heading in the column properties), are used to indicate whether any such slab should be considered as a restraint.
If there are no effective beams and there is no flat slab, the program looks for any other type of slab panel at that end. If a panel is found, then provided it has the Include in diaphragm property selected, it acts as a restraint at the position, in the same way as a flat slab.
A flat slab or any other type of slab panel only restrains the end of the stack if it is within half the slab depth from the end of the stack, and if its centre is nearer to this end of the stack than the far end.
If, at an end of the stack, no effective beams, flat slab or other slab panel that acts as a restraint is found, then the unsupported length includes the stack beyond this restraint, and the same rules apply for finding the end of the unsupported length at the end of the next stack (and so on). If there is no stack beyond this restraint (i.e. this is the end of the column), the unsupported length ends at the end of the column.
By default all stacks of all columns are taken into account in the sway/drift, wind drift and seismic drift checks.
Located under the Sway and drift checks heading in the column properties, these parameters provides a facility to exclude particular column stacks from these calculations to avoid spurious results associated with very small stack lengths. You can either clear the checkbox located under 'All Stacks' to exclude the entire column, or you can exclude a particular stack by clearing the checkbox located under that stack only.
In the column properties, the nominal concrete cover is the distance between the surface of the reinforcement closest to the nearest concrete surface (including links and surface reinforcement where relevant) and the nearest concrete surface.
The type and grade of vertical and confinement reinforcement to be considered for the design are specified under the Reinforcement heading in the column properties.
Where possible bars are arranged in a single layer:
Using a 2nd layer of reinforcement
- In Autodesign: for rectangular, circular and parallelogram sections, if a
single layer of reinforcement is not sufficient, design will attempt a two
- Rectangular and parallelogram sections: a second layer is added in line with the first bar along the adjacent length.Note: At least 2 intermediate bars are needed in each direction for a 2nd layer to be added.
Circular sections: if a second layer is required you can control the minimum layer spacing via Design Settings (a larger spacing will be used if needed).
- A link is added along the second layer for all 3 section shapes.
- In Interactive Design: you can choose to toggle the second layer on or off in the Interactive Design dialog.
- When using a 2nd layer, design checks are unchanged, except for the addition of a layer spacing check for circular sections.