Hyperstatic moments in concrete slabs
Hyperstatic moments in concrete slabs
(OP)
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.?
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.?






RE: Hyperstatic moments in concrete slabs
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.
RE: Hyperstatic moments in concrete slabs
Depends on the design code and the calculation being done, flexure/shear.
RE: Hyperstatic moments in concrete slabs
You haven't gotten a lot of answers because most of us don't remember (if we ever knew) what hyperstatic means.
RE: Hyperstatic moments in concrete slabs
Mike McCann
MMC Engineering
RE: Hyperstatic moments in concrete slabs
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.
RE: Hyperstatic moments in concrete slabs
RE: Hyperstatic moments in concrete slabs
RE: Hyperstatic moments in concrete slabs
Hyperstatic moments? You must be kidding! What exactly is your question?
BA
RE: Hyperstatic moments in concrete slabs
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.
RE: Hyperstatic moments in concrete slabs
ht
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
RE: Hyperstatic moments in concrete slabs
RE: Hyperstatic moments in concrete slabs
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
RE: Hyperstatic moments in concrete slabs
I am wondering where RAPT is?
RE: Hyperstatic moments in concrete slabs
BA
RE: Hyperstatic moments in concrete slabs
Re-read the posts. We were making fun of ourselves.
RE: Hyperstatic moments in concrete slabs
RE: Hyperstatic moments in concrete slabs
RE: Hyperstatic moments in concrete slabs
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.
RE: Hyperstatic moments in concrete slabs
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.