Dead loads, construction sequence, Load cases and Compilations

[nicoCTQNU]

Hi,

Ok I don't fully understand the program's use of dead loads.  When it adds it and when it doesn't and what needs to be carried over to the beam design module.  So to preset this, I assume the following:

• The "Design Beams" module is separate from the other parts of the program in the sense that you have to transfer the results there.  If it is not transferred, then it does not take it into account including dead loads.
• The "Design Beams" construction sequence loads that are added are calculated and the program takes this into account when showing the results for the transferred loads that you imported from analysis.  I.E stresses incurred during construction stages remains and the transferred stresses are added.

Is this correct?

Now on to the next:

I have a grillage (Post tensioned, precast beams that will be transferred to site and installed, after which the deck, surfacing and barriers will be installed).

As can be seen below, I have the following construction sequence loads:

1. Beam Self weight
2. 2 slabs, (one on top and one below between the beams
3. Surfacing
4. Barrier loads

Is it necessary to add the loads for SDL below? I would assume that it is not required since all the dead loads are already taken account by the construction sequence loading?

If this is true, then is the reverse also true where I add all the self weight loads here (slabs, surfacing and barriers) and only do construction sequence for the first two steps (tensioning the beam and then grouting in order to check SLS compression limits of the concrete)?

Is one better than the other?

Any help or insights will be appreciated.

[dave_geeves · Accepted solution]

Hi,

Yes, you are correct in that the design beam ULS and SLS design calculations are based only on the values defined in the loading tables for the design beam. These loading tables (moments shears and axial forces at various positions along the span) can be calculated in three ways

1. Typing in the values manually
2. Using the generate button to define simple load cases on simply supported beams.
3. Transferring the values from a structural analysis (either single loadcase, combinations or envelopes)
4.  Reading in (Importing) from an .sld file (ASCII file created externally.

The calculations in the post-tensioned design beam will use the appropriate tables for the design condition being considered at each point of interest.  Values will be interpolated if the POI does not fall exactly at a point in the load table.  This includes all components of load that need to be considered including the primary and secondary effect of prestressing.  The primary prestressing effects will be defined by the prestressing data, and at what stage they are applied, and can be seen as a sub-table of the appropriate construction stage loading table.  These are only visible if add prestress is present in the construction stage definition.

The construction stage definition in the design beam will define what load cases need to be considered at each stage and what level of prestress to include.

Nothing is transferred automatically from the structural analysis - this has to be done explicitly.

If you carry out the design calculations for  for "construction stage 3"  then it will include all loads applied in the construction stage definition up to and including stage 3 (as defined by the load case table applied in the stage construction definition table).

In your table you show 6 construction table where the last four stages do not change the beam definition but just add loads.  This is fine if you want to examine the stresses after the application of each load.

For your second question, I don't fully understand the question but at a guess I can say that if you have included the  SDL loads as various construction loads then you should not include then again by defining an SDL load case (just leave this table blank.  This should work ok if the basic loads have had the correct load factors applied and is useful when you know the sequence of applying the surfacing etc.

I hope this helps

Kind regards

Dave Geeves

[nicoCTQNU]

Hi Dave,

Thanks for the reply.  You answered most of my questions.  Just to get 100% confirmation of my second question, I have created a small picture with 2 options.  In my view they should deliver the same results if the load factors are correct. Can you verify please?

Also is there a reason to do one over the other in your view?

Thanks again for the assist

[dave_geeves · Accepted solution]

Hi,

I'm afraid I don't understand option 1 at all.  I can see that you have created various dead loads in your structural analysis but where are the resulting moments being included in the design beam load tables?  I can only assume that you have added these dead load effects into each live load combination (structural analysis compilation) and transferred this compilation into the tables "Live Load BM1" and "Live Load BM2".  This would work but I'm not sure if the load factors would be correct. 

It is better to put the structural live load analysis results into the Live Load table and the dead loads of the beam and the slabs into the various construction load tables - as you have done in option 2.  However you have included the SDL loads into the construction load table - which is OK as long as the correct load factors have been used, and have not created an "SDL" load table.

The alternative is not to have construction stages 4 and 6 but to create  SDL load case tables and apply the surfacing and barrier moments and forces into these table.  Doing it this way the SDL loads will not be included in the analysis of the beam for construction loads but will be included when you analyse for any live load case.  The benefit of doing it this way is that you can have multiple SDL load cases (for example 1.  one span with surfacing one without; 2. both spans with surfacing; ....etc)  In this way the program decides which of the SDL load cases gives the most adverse effect for the point of interest being considered - which of course may vary for different locations along the girder.

In the method you have chosen you will see the effects of the SDL Loads at the appropriate construction stages, which is what you may have intended,  but it will assume that there is no variation of the SDL unless you create the variation in additionally defined construction stages (ie "surfacing span 1", "surfacing span 2", "Barrier span 1", "Barrier span2"... etc.

When analysing for live loads I would expect the results to be the same.

I hope this helps

Kind regards

Dave Geeves