Insufficient stress block
18
Tekla Connection Designer
Environment
British Standard
When designing a base plates subject to moment, in order to develop the rotation and potential resistance in the tension bolt group, we need to provide a reasonable compression zone between the extreme edge of the stiff baseplate cantilever length ‘c’ and the concrete grouting compound. The design model always assumes there is a (compression) stress block and cannot consider moment without one. The minimum length of compression stress block has been set as 10 mm.
Image
Small design forces
For the situation of relatively small loads (relative to size of connection) and axial + small moment the fail can be considered an anomaly. You may decide to increase the moment to develop a sufficient stress block or ignore the moment (set to 0 kNm), in which case the program checks for the condition of axial load alone. For instance in the example file:- Combination 01 with axial value = 12 kN and moment value = 12 kNm , returns x = 9.1 mm = FAIL
- Combination 02 with axial value = 12 kN and moment value = 15 kNm , returns x = 11.6 mm = PASS
Large design forces
However, the situation can also occur with uplift + significant moment. In which case it is not correct to ignore the moment. The stress block limit is intended to ensure that for highly loaded connections where considering a stress block < 10 mm length might be unsafe since it is simply impossible logically to develop that amount of force over a length of <10 mm - though the mathematical model of the Green book/program may allow it.
The design model can only consider pure tension, or moment (with stress block) hence it may not be correct either to increase the moment (which helps with x dimension and hence check) or to ignore it.
Therefore the issue is load combination dependant and must remain a matter of engineering judgement.