Hyperstatic moments in concrete slabs 4

[927927]

My question is:
When calculating final moments for a slab, are all hyperstatic moments subtracted from moments at limit state? Or are only the moments at supports subtracted and not the ones at midspan e.g.?

 

[ishvaaag]

You design (it is surmised) everything to meet the forces reulting from a series of hypotheses mandated by the code. Between them there may be some that when the effect of some prestressing action becomes a positive factor in resisting the applied forces, the code may mandate that even if enforcibly present you count only at service level or say 90% of it. It can as well specify that since the inverted curved section of strand over supports augments the moment they are to be taken at the max, value, i.e., factored loads level as your question seems to ask; they are not contributing to the reduction of the reinforcement at supports.

And note that I am not talking of the Hyperestatic moments, but of the full application or not of what produces worse or better effect, and hence, a stronger structural element, or a weaker one.

This is no novelty, traditional combinations mandate all the live loads treated as nonexistent or zero when a positive factor to reduce the maximum effects of forces, but mandates the spans at both sides of a support fully loaded to get the maximum effect.

For limit states and only in the cases where a satisfactory check of the service level behaviour is warranted, the limit strength check may be a bit more lenient on what the standing deformations at such limit state level are. For example the full passive and active rebar may be directly taken at their respective design strengths whereas a more proper check would be one in compatibility of deformations, that in the end and for a strength check may not differ that from some simplification of that of all the steel at its design strength.

For a general design of structure where the slabs act conjointly with the columns (as must be done typically) the hypothesis at hand defines what to count and what not for the check; this, on the other hand, needs to be compatibilized with the particulars -as specified by the guiding code allowing it- of some simplified mode of calculation when using it. You will find as well minimums of rebar to place, maximum negative moment redistribution, particular specs for the geometry of the rebar etc.

 

[rapt]

927927,

Depends on the design code and the calculation being done, flexure/shear.

 

[hokie66]

927927,

You haven't gotten a lot of answers because most of us don't remember (if we ever knew) what hyperstatic means.

 

[msquared48]

?

Mike McCann
MMC Engineering

 

[asixth]

Back when I was still a university student I was at a bookfair when I came across a book authored by academics from Monash and Melbourne University (in Australia) titled "Hyperstatic Structures: An introduction to the theory of indeterminate structures". I bought the book for a couple of dollars knowing that it was published in 1960, around the same time that FE was being introduced. Saying that, I never referred to the book because I was taught in the "computer-analysis" era so my only assumption is that hyperstatic refers to a system which is not statically determinate.

Yes, the live load should be placed where it will generate the greatest action. I find the best way to do this is to generate load cases with loads placed on each span, and filter through each load case to see how the load position will affect the shear and moment diagrams. This will give you a good idea on how you need to pattern the loading to give the greatest design actions. Experienced designers who have a good understanding on how structures will behave can do this by inspection.

 

[JAE]

Yes, hokie66, you are the brave one here to admit not knowing what we all didn't know either. way to go!

 

[slickdeals]

Here is a paper on Hyperstatic moments by Allan Bommer.

 

[BAretired]

927927,

Hyperstatic moments? You must be kidding! What exactly is your question?

BA

 

[BlastResistant]

To all who are ripping on 927927 because you don’t know what hyperstatic moments are:

Hyperstatic actions are caused due to external constraint to displacement of a member under prestressing forces. Bending moments caused by external constraints of post-tensioning (PT) actions, aka “hyperstatic moments” will cause failure in PT slabs if not accounted for properly in design. Hyperstatic is common terminology in advanced PT design universally, and the term is plain English and very straightforward if you are familiar with the topic. Perhaps we should refrain from making fun of others if we don’t understand what they are talking about; possibly there are many others out there who do understand and can help. We have all asked dumb questions before. And this is not a dumb question.

To 927927:

I am not sure what country you are in or what code you use, but in the US (where I am) when modeling PT tendons we use the post-tensioning values when checking for Service Level (serviceability) effects; i.e., deflections, stresses, and cracking. When checking for Ultimate Level (strength and safety) effects, we use hyperstatic (secondary) values (with a load factor of 1.0). Modeling a PT tendon as an applied loading necessitates the introduction and consideration of hyperstatic (secondary) actions as a separate design load for the safety check of post-tensioned members. If checking a particular limit state for hyperstatic effects, all hyperstatic effects must be accounted for – if checking flexure at supports, include hyperstatic moments acting at the location of that particular section; if checking flexure at midspan, include hyperstatic moments acting at that specific section.

 

[BAretired]

I still don't know what hyperstatic moments or hyperstatic structures are. BlastResistant claims the term applies to PT structures, yet there are numerous articles on the internet which do not mention PT structures. For example:

http://www.jcem.vgtu.lt/upload/civil_zurn/kudzys et al.pdf

So far I have not come up with a consistent definition for the term hyperstatic and for me it is not intuitive.

The OP mentioned slabs, but he did not mention PT slabs, so I am not sure if we are on the same wavelength or not.

BA

 

[slickdeals]

BA, read the paper that I attached. Hyperstatic moments are secondary moments induced due to post-tensioning in continuous structures.

 

[BAretired]

slickdeals,

I read the paper by Allan Bommer. I understand his definition of hyperstatic, but it appears others have a different definition and I don't know which one the OP had. I find it difficult to respond to a question which I do not understand.

BA

 

[slickdeals]

Totally understand. I don't know what the OP's definition is either, but from what I know the secondary moments due to post-tensioning are often referred to as hyperstatic moments.

I am wondering where RAPT is?

 

[BAretired]

It has been a few years since I designed post tensioned slabs. There were cases where external constraints played a role, but I found that the best technique was to avoid or at least minimize the constraints rather than design for them. Admittedly, this may not always be possible.

BA

 

[JAE]

Hey, BlastR.. - no one is, was ripping the OP or making fun.

Re-read the posts. We were making fun of ourselves.

 

[hokie66]

To be fair to BlastR, he commented before BA's first post was edited a bit.

 

[JAE]

Oh -OK...didn't see that.

 

[927927]

Thank you all for your comments.

My understanding of hyperstatic moments was limited. That is now changed due to your comments.
My apologies for not stating my question more clearly. The only reason I can give is my own ignorance on the matter.
To sum up, I did mean post-tensioned slabs, when I unclearly stated concrete slabs. Again sorry for all the confusion.
I will be more vigilant when posting questions in the future.

 

[rapt]

In prestress design the uplift/downward forces in the columns from the prestress loading are often called Parasitic or Hyperstatic or Secondary Prestress effects. We tend to call them Secondary Prestress effects as it is easier to spell!

These are included in service design as part of the full prestress moment

Mp = Pe + Msecondary,

with a factor of 1.

In Ultimate design, the secondary moments only are included in the design as applied moments. The Pe part of the prestress is not included on the applied moment side of the strength equation as it is used in the strength side.

Most codes include the secondary moments in the ultimate applied moments with a factor or 1. So it is included even if it helps to reduce the applied moment. There are arguements for and against this, but, in the end, it is only a redistribution of moments as, if it increases the +ve moments in a span, it will reduce the -ve moments in that span. The total moment, wl^2/8 is maintained.

So as long as sensible levels of secondary moment are calculated (some people grossly over-estimate them (eg Adapt PT in beam design if the beam flange is made excessively wide as Adapt recommend (or used to)), there is no problem including them like this. At one stage Eurocode was suggesting that they should have a variable factor, 1.2 where they hurt and .8 where they help but I think that was changed to 1 in the final release. RAPT computer program allows designers to niminate a variable factor if they want to.

Alan Bommer's paper has added extra effects into this calculation that are not generally considered to be part of this because they are not caused by the prestress uplift forces. The extras are caused buy restraint to shortening. As such they are not Parasitic or Hyperstatic or Secondary effects. They are restraint effects.

From his point of view, I assume it is nice and easy to include them as part of the Secondary moments so that he does not have to split the effects up, but he should as they are a separate effect. Secondly with this, the prestressing of another floor will effect the prestress shortening in the floors below it, as will cracking of columns, and creep of columns over time, so the whole building construction sequence would have to be modelled to include this, it cannot be done floor by floor. It should be treated separately.