Residual Creep in SLS Stress Summary Table

[Anonymous]

Hi,

I was wondering if the stresses that result from the effects of residual creep are accounted for in the SLS analysis of pre-tensioned beams. In the stress output, I can see the stresses associated with differential shrinkage however not residual creep. Differential shrinkage will tend to increase both the compressive stress at the top of the beam as well as the tensile stress at the bottom whilst residual creep will have the opposite effect. Therefore the resultant of the two won't be large but as per AS5100.5 appendix E the two should be considered in combination.

Are the residual creep stresses hidden somewhere else within the SLS stress summary table or have they been conservatively omitted?

Thanks

[david.geeves]

Hi khigginson92,

The effects of shrinkage and creep on the beam stresses is dealt with in three place, as can be seen in the output calculations:

The first part is for effects in the loss of prestress due to shrinkage and creep before the section I composite

The second part is for effects in the loss of prestress due to shrinkage and creep after the section is composite

The third part is for differential shrinkage (which is modified by residual creep)

The differential shrinkage effects are split into two parts

1) primary stresses

2) secondary stresses if the structure has redundancies

The primary stresses are calculated automatically based upon the shrinkage an creep parameters set on the analysis form, which include the residual creep factor:

If you analyse for differential shrinkage you can see how the stresses and restraining moments and forces are calculated:

Clicking on "Results" gives the detailed calculations and you can see the effect on how the stresses and restraining moments and forces are affected by the residual creep factor.

Because the residual creep factor has the effect of reducing the retraining moments and forces, which are used in a structural analysis to determine the secondary differential shrinkage moments and forces, the effect of residual creep has been taken into account for both primary and secondary differential creep stresses.

I hope this has answered your query.  If so could you please mark your post as solved as it makes it easier for others to search for solutions.

Kind regard

[Anonymous]

I might be missing something here but from what I gather, the above solution still only covers the effect of differential shrinkage in accordance with section 8.10.3.2.3 of AS5100.5-2017 (or Appendix E.3.2.3 of AS5100.5-2004 shown below):

However, what seems to be missing from the SLS stress summary, and what I think @Anonymous was referring to in the original question, is the effect of residual creep on composite members calculated to section 8.10.3.2.2 of AS5100.5-2017 (or Appendix E3.2.2 of AS5100.5-2004 shown below):

Have these calculations perhaps been covered elsewhere in the SLS stress summary?

[dave_geeves]

Hi,

I think you are right.  The overall effect of residual creep does not seem to be included in the AS5100 calculations as indicated in E.3.2.2 of 2004 version (I don't have the 2017 version).  I'm not sure if this was intended or an oversight but I don't remember seeing any other comments on this. Hopefully the developers will notice this post and provide more feedback.

I know this effect is included in the Eurocodes calculations (I'm more familiar with Eurocodes as I'm in the UK) as a new load case was introduce - call the "Construction on Centring" load case which reflected the stress condition if the structure was built in one stage (ie monolithically) with full prestress applied at the same time.  The stress summary table then determines the difference in the stresses between the normal build up of stresses and the "Construction on centring" stresses and then multiplying these by an appropriate factor to determine the residual creep effect.  The terminology and factors are slightly different but the effect is the same.  

I'm sure that this procedure could be introduced into the AS5100 Calculations in a similar way but Autodesk would need to add this to their development plans.

I'm afraid I don't have any suggestions for a temporary workaround to get this effect included.

Kind regards

Dave Geeves.

Kind regards

Dave Geeves