Long Term Deflection Estimates
Long Term Deflection Estimates
(OP)
Hi all,
I know full well the ins and outs of long term deflection - creep, shrinkage, cracking, f'c, mix design, restraint, P/A, compression reo and the list goes on...
But what I'm asking here are people's 'back of the envelope' deflection estimates.
For example, I've seen people get a quick figure by factoring up the elastic uncracked deflections - 3G + 1.5Q. This can be back calculated from various code equations.
I've also seen a RAM Concept file with the load combo - 3.35G + 1.64Q (note the creep factor being set to 3.35).
Anyone have any reference material that quotes some of these elastic uncracked deflection multipliers, purely just to get ballpark figures?
Or do people more opt for span/depth ratios to give them a feel for a particular design?
I know full well the ins and outs of long term deflection - creep, shrinkage, cracking, f'c, mix design, restraint, P/A, compression reo and the list goes on...
But what I'm asking here are people's 'back of the envelope' deflection estimates.
For example, I've seen people get a quick figure by factoring up the elastic uncracked deflections - 3G + 1.5Q. This can be back calculated from various code equations.
I've also seen a RAM Concept file with the load combo - 3.35G + 1.64Q (note the creep factor being set to 3.35).
Anyone have any reference material that quotes some of these elastic uncracked deflection multipliers, purely just to get ballpark figures?
Or do people more opt for span/depth ratios to give them a feel for a particular design?






RE: Long Term Deflection Estimates
https://spiral.imperial.ac.uk/bitstream/10044/1/19...
RE: Long Term Deflection Estimates
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Long Term Deflection Estimates
RE: Long Term Deflection Estimates
Anything more precise than that, without taking account of reinforcement ratio, concrete tensile strength, loss of tension stiffening, etc is just fooling yourself, and to use factors to 3 significant figures is just ridiculous.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Long Term Deflection Estimates
Reductions for compression reinforcement such as those in SLABS are grossly un-conservative in most cases.
I have never found a relationship to reinforcement ratio like Kootk's to have any relevance!
RE: Long Term Deflection Estimates
To clarify these are used with cracked section properties by Inducta.
The paper I linked to in my earlier post agrees with RAPT's 6 times uncracked short term deflection suggestion.
RE: Long Term Deflection Estimates
I'm surprised to hear this rapt. I first saw the method proposed by David Fanella (snippet below). I thought that it was an interesting proposition and began checking 0.5 rho_b along side my normal deflection calcs. Low and behold, I found it to be a very reliable rule of thumb deflection check.
Conceptually, I'm sure that we can agree that any concrete bending member proportioned such that it can be lightly reinforced is likely to be in pretty good shape from a deflection standpoint. It's never the singly and lightly reinforced members that have deflection problems. Rather, it's the member with four layers of tension steel and some compression reinforcing thrown in to boot that cause headaches.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Long Term Deflection Estimates
Re your conceptual thinking,
- with 4 layers of tension steel and extra compression steel, I would assume it is a transfer beam, and if dimensioned properly for shear and flexure I would normally not expect deflection problems in that type of beam. They are normally relatively low span/depth ratios and are approaching deep beams and I find that shear normally controls, not deflection.
And with an RC beam requiring that much reinforcing I would use bonded PT anyway!
- I find that relatively lightly loaded slabs and beams are more likely to have deflection problems according to my long term deflection calculations!
RE: Long Term Deflection Estimates
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Long Term Deflection Estimates
What's this 1/2_rho_b business? Please elaborate. Many thanks!
RE: Long Term Deflection Estimates
@rapt/IDS:
Consider two mildly reinforced beams where the following is true:
1) All parameters are identical except the reinforcing ratio and beam depth.
2) Beam #1 has a reinforcement ratio of 0.25 times the balanced reinforcement ratio and the depth is made as small as possible.
3) Beam #2 has a reinforcement ratio of 0.75 times the balanced reinforcement ratio and the depth is made as small as possible.
Do you guys really have any doubt at all that:
1) Beam #1 will be deeper than beam #2 and;
2) Beam #1 will deflect less than beam #2?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Long Term Deflection Estimates
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Long Term Deflection Estimates
RE: Long Term Deflection Estimates
With the question as posed, if the design is controlled by deflection, then immediately after removal of formwork they will both deflect the same. Within a few days under most circumstances the lightly reinforced beam will deflect more, because the factors that the code simplified methods do not deal with adequately affect lightly loaded beams more than heavily loaded beams.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Long Term Deflection Estimates
That should read lightly reinforced and heavily reinforced, since in this example the loading is the same.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Long Term Deflection Estimates
RE: Long Term Deflection Estimates
In that case, as I said, although they start off with the same deflection, it is probable that the more lightly reinforced beam will deflect more than the more heavily reinforced one over time.
The point is that in my experience lightly reinforced beams and slabs often have deflections greatly exceeding what would be expected from a simplistic analysis.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Long Term Deflection Estimates
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Long Term Deflection Estimates
But for less extreme changes in steel ratio, the effect will depend on whether you effect the change in steel ratio by changing the beam depth or beam width.
What it gets down to is there are too many variables involved to base it on a simplistic rule like this.
Why not just calculate expected long term deflections based on a rational and proven model and get rid of the guesses.
I still stand by my statement that I have seen a lot of designs where long term deflections were a problem for lightly reinforced slabs and beams which indicates that the % reinforcement rule does not actually work for all cases.
RE: Long Term Deflection Estimates
OK, if you have two beams of the same span, and concrete type, and with the same reinforcement stress at every cross section, the one with the lesser depth will have a greater curvature, and will deflect more.
Does this mean that it is true to say "deflection problems are rarely encountered in beams with ro = 0.5ro.max"?
No it doesn't. I have encountered many deflection problems in beams and slabs with a reinforcement content much less than that. It may be true that if the beam/slab had been designed with a shallower depth and more reinforcement then the problems would have been even worse, but in that case it is likely the problem would have been recognised at the design stage, by application of the code simplified deflection calculations, which are reasonably accurate for heavily reinforced sections, and grossly inaccurate for lightly reinforced sections.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Long Term Deflection Estimates
Per the criteria that I specified initially, the mental experiment involves modifying only the member depth and reinforcement ratio.
I disagree. For ordinary conditions, I've had excellent result with this algorithm.
Two reasons come to mind:
1) I understood the subject of this thread to be "rules of thumb" rather than "detailed deflection analysis".
2) Some folks, like me, rarely perform the detailed design of beams. Rather, I spot check the work of others. For that kind of deflection check, I find the 0.5_rho_b business invaluable. The usual situation is much simpler than the scenarios that you and IDS have been describing. Usually, it goes something like this:
a) Someone sizes a podium slab beam using normal span to depth ratios etc.
b) Because podiums see a lot more load than other floors, flexural design is challenged.
c) Because a prior depth commitment has been made, no beam depth change is possible.
d) Designer solves problem by widening the beam and packing it full of rebar including some top steel.
e) KootK reviews said beam and discovers that it's at 0.7 rho_b.
f) KootK grabs designer and says "are you sure this thing isn't a frickin' wet noodle?".
g) Designer scurries away and does the detailed, long-term deflection check that should have been performed in the first place.
h) KootK is right and something about the beam gets changed.
Of course it doesn't work for all cases. Neither do the span-to depth ratios that some engineers find mildly useful. That's why it's a "rule of thumb". If it worked for all cases, we'd be calling it a "kick ass, all encompassing, stand alone deflection check".
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Long Term Deflection Estimates
As I mentioned in my latest response to Rapt, my experience indicates that it generally does. Are there some exceptions to the rule of thumb? Well... yeah. However could there not be?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Long Term Deflection Estimates
I see deflection problems with lightly reinforced slabs more frequently than heavily reinforced slabs.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Long Term Deflection Estimates
The KootK/Fanella Story
Flexural members proportioned such that flexural reinforcement is kept within a reasonable range seldom have deflection issues.
The rapt/IDS Story
Deflection calculations for lightly reinforced members can often be inaccurate and/or un-conservative.
I believe both of these stories to be generally true and feel that they are not mutually exclusive in any way.
Applied to conventional flexural members, the 0.5_rho_b business is essentially just an indirect way of encouraging the use of appropriately deep members. And, surely, we don't need to argue about whether or not structural depth is a good thing when it comes to limiting deflections? Right? Or does structural depth also not "have any relevance" when it comes to deflection control?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Long Term Deflection Estimates
2 Problems with your argument above,
1 The members we are saying have most problems and are handled worst by simplified deflection check methods are the ones that fit into your OK definition. Not sure how your logic is compatible with this.
2 The steel ratio logic does not require a depth increase. It will also be satisfied by a width increase! And depth is far more efficient than width in reducing concrete member deflections.
RE: Long Term Deflection Estimates
You may be erroneously assuming that the domain of consideration here is lightly reinforced members. It's not. The domain here is members that are lightly reinforced AND properly designed for flexure. There's a significant difference there.
With both of those criterion considered in tandem, the steel ratio logic does require a depth increase if one is even remotely interested in efficiency. This is because of the following relationship in which bending resistance varies with the square of depth:
Mr = rho x b x d^2 x constant (valid up to about 2/3 rho_b).
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Long Term Deflection Estimates
Which could be written:
Mr = ast x b x d x constant
or for member supporting only its own weight:
Mr/M* = ast x b x constant
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Long Term Deflection Estimates
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Long Term Deflection Estimates
Just that the required steel area is proportional to d, not d^2, for constant load, or if the load is proportional to d then the required steel is constant.
But going back to your previous comment:
So am I, and I presume rapt is too. Why would we concern ourselves with sections that didn't work for flexure?
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Long Term Deflection Estimates
Ok, I like algebra as much as the next guy. However, none of that changes the fact that moment resistance is proportional to d^2 once the constraint on reinforcement ratio is introduced.
I have no idea IDS. You guys really haven't given me much to work with here and I'm not clairvoyant. So far, I've provided anecdotes, a reference, an intuitive behavioural explanation, a mental experiment, and some math to support my claim. In contrast, you and rapt have weighed in with:
A) you don't like my idea and;
B) you know of some lightly reinforced members with deflection problems.
In the interest of debate parity, I'd love some additional info:
1) were the offending slabs properly designed for moment?
2) were the perceived deflection issues field problems or analytical shortcomings?
3) to what do you attribute the tendency for deflection issues in lightly reinforced members?
4) why are simplified deflection checks in AU unconservative for light reinforcing?
5) do you know of any publications that expand upon your concerns?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Long Term Deflection Estimates
Ief = Icr + (I - Icr) (Mcr / Ms)^3
Where Ms = the service moment.
I believe that other Codes have the same or similar formulas.
I think that what RAPT and Doug are getting at is that if Mcr is close to Ms (ie. lightly loaded) this formula can be non-conservative and that a check based on 1/2_rho_b will not pick up the problem.
RE: Long Term Deflection Estimates
You were talking about design efficiency, so the point is that at constant steel area the moment resistance only increases in proportion to the depth, not depth squared, and if your load is proportional to depth the over-design factor doesn't increase at all. The structures I have been talking about were nowhere near the limiting reinforcement content anyway.
Well that's a pretty blinkered account of who has provided what, but never mind, I'll answer your questions anyway:
Yes. In fact the flexural design was satisfactory for the maximum moments occurring during construction, and also completed structure + live load, which are of the order of double the dead load only moment.
Construction was in accordance with specified requirements; so it's an analysis problem
Codes provide rules for "tension stiffening" after concrete cracking, but these rules tend to be un-conservative at moments just above cracking, and shrinkage effects tend to reduce the cracking moment to well below the value given by codes. Differential temperature can also have a significant effect.
Because our code requirements are based on those in ACI 318. The Eurocode rules work better.
Also the projects I have in mind are not just Australia. Also UK (worst case), Middle East, and USA and Canada.
Yes lots. Many papers by RI Gilbert, a fairly recent book by Gilbert and Ranzi, quite a bit around 2005 from Beeby, also just found this paper from some other Australian guy:
http://www.interactiveds.com.au/Publications/Defle...
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Long Term Deflection Estimates
RE: Long Term Deflection Estimates
Also, is anyone going to the Gilbert seminars 'Cracks and Crack Control in Concrete Structures' in Aus run by the concrete institute this month? Gonna get my ticket tomorrow
RE: Long Term Deflection Estimates
Thanks for indulging me. The additional information is a great help.
I believe this statement to be in error. If designers assumed a particular A_s and then chose their member proportions accordingly then, yes, moment would be a linear function of depth. However, that's not how design generally unfolds. And it's certainly not how I would expect the 0.5_rho_p method to be applied. Instead of proportioning to a target A_s, designers would proportion to a target rho. In that scenario, moment increases with the square of depth per the relationship that I stated previously. Think of it this way:
1) If you just adjust "b" to hit the target rho, Mr is increasing as a result of additional steel.
2) If you adjust "d" to hit the the target rho, Mr is increasing as a result of additional steel and additional lever arm on that steel.
Now I see that you guys aren't concerned about lightly reinforced members, you're concerned about very lightly reinforced members. I guess another ride-along assumption in the 0.5_rho_b check might be that it only applies to members that actually require significant flexural reinforcing. I'll revise the domain as shown below which should still permit a fairly wide range of applicability.
This certainly holds true when all other parameters are held constant. But, again, I'm not suggesting limiting reinforcement. I'm suggesting encouraging appropriate depth selection by limiting reinforcement ratio. There's a difference.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Long Term Deflection Estimates
1 If you are going to control it all by adjusting depth, then there must be an appropriate width that should be used. What is it? Though not economical, width can be used to control the steel ratio to suit your initial requirement of limiting the steel ratio to less that .5. The rule is too simplistic to use reliably.
2 The paper you quoted a section out of required a steel ratio = .5. NOT <= .5. Agreed that as long as the width /depth ratio is logical (not sure what that is defined as) any member designed for strength with a steel ratio = .5 will probably be ok for deflections.
Our argument is that members with significantly lower steel ratios than .5 often have deflection problems, even if designed to the simplified code deflection rules (not simple span/depth ratios which should be removed from all codes). A major reason for this is things like Branson's formula for tension stiffening which is very un-conservative for lightly reinforced members and leads to gross under-estimation of expected deflections in those cases. Long term multipliers that allow for compression reinforcement effects cause similar problems. Combine the 2 with a lack of understanding or output from analysis software and you get gross under estimates of expected deflections which lead people to distrust calculated deflections.
RE: Long Term Deflection Estimates
http://www.concreteinstitute.com.au/Cracking-and-C...
Not just Ian Gilbert talking about how to do the calcs, but also Michael van Koeverden talking about how to make concrete behave nicely and follow the calcs once you have done them.
Sydney people have missed it though, you'll have to go to Melbourne or Brisbane.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Long Term Deflection Estimates
It would be whatever it takes to resist the moment in conjunction with the selected depth.
Sure, one could go out of their way to produce a horrendously inefficient beam for which the 0.5_rho_b check would be unsuitable. As I said above, however, I don't see rare exceptions to a rule of thumb as being valid justification for wholesale abandonment of the rule of thumb. It's also important to remember that I'm a fairly capable engineer/human and it's not as though I'm putting my brain on ice while I'm performing these checks. If I see something stupid, I'll flag it as stupid no matter what my rule thumb tells me.
Is there any doubt in your mind that was a misprint? There's not a shred of doubt in my mind. It's hard to imagine that Dr. Fanella would intend for us to be designing all of our beams to exactly 0.5_rho_b.
I certainly accept that argument and have learned a good deal about it through the course of this discussion and my related Googling. That said, I still do not accept the notion that reinforcement ratio is an irrelevant parameter when it comes to assessing deflection. Unless I have misread our discussion woefully, it seems to me that we agree that reinforcement ratio checks will correlate well with acceptable performance over a fairly broad range of application.
Certainly, it was not my intention to single out Australia as the only place on the globe where deflection problems arise. Still, I find it curious that:
1) I'm the only non-Aussie involved in this thread.
2) When I Google this issue, most of what is returned comes from Australian sources.
Did you guys experience some extraordinary deflection problems at some point in the past? Or do you simply benefit from having a local academic that has made this stuff her life's work?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Long Term Deflection Estimates
Answering your points in order (briefly in most cases)
1 That is the problem if there is a depth limitation, someone can control it with a wider beam - - we all agree this is illogical but it happens
2 See 1
3, No I do not think it was a misprint and I would not make that assumption. I would fined out from the author or use it if It fitted in with my observations.
5 With our history in PT design using Partial Prestress in Australia since the early 1970's, we have had to do proper deflection checks, allowing properly for cracking, tension stiffening, creep and shrinkage for PT design. So our designers have been exposed to those types of calculations for a long time. The PT industry in the USA went a different way and have not until recently made any attempt to do proper deflection calculations (until RAM Concept was basically forced to do it to sell internationally).
Because Australian designers were exposed to this for PT design and the same design tool was used for RC design (RAPT), they then did the same thing for RC design. Basically an L/D ratio is the starting point in deflection design in Australia, not the final design requirement.
Looking at other countries, UK and all countries using BS8110 essentially base all of their deflection calculations on L/D ratios, hardly ever calculating a real deflection, so they do not know what they are getting.
USA and countries designing to ACI318 basically use L/D ratios and for RC and PT design where they do calculate deflections still use long term deflection multipliers for even for PT design even though the ACI code specifically says not to.
As well, investigations into buildings with deflection problems for designs done by simplified methods by several people like IDS, Peter Taylor, Ian Gilbert and others showed a lot of the deflection problems with buildings designed using simplified deflection design procedures.
That is not to say that all Australian concrete designs are done properly and do not have deflection problems, but there is a greater awareness here of the shortcomings of the simplified methodologies and the solutions.
RE: Long Term Deflection Estimates
Taking this in a different direction (sorry, Trenno), I wonder if those here who have knowledge of in service deflection problems can share what types of structural systems were involved, and what caused the deflection issues. I don't have a lot of anecdotal examples, just things I have heard and tried to avoid on my own jobs.
I suspect that most deflection issues in recent times have been caused, at least in part, by pressures to cheapen construction. The main type system to which this applies is flat plates. It used to be that when the thickness of a flat plate was selected, the first criteria was to satisfy punching shear. When punching shear was good, there was generally enough depth that deflections were satisfactory, provided the design was respected during construction. Now the desire for thinner flat plates, along with the use of those accursed studrails, means this insurance is no longer there.
Another important factor, which was addressed in Doug's paper, is the use of higher strength reinforcement. Because ultimate flexural strength has been allowed to drive designs, the reduction in amount of reinforcement has adversely affected serviceability, i.e. deflections and cracking. I don't think we needed 500 MPa steel when 400 worked just fine.
Lack of attention to restraint, both in design and construction, is a big contributor to direct tension cracking, leading then to greater deflection.
Poor placement of top reinforcement is a problem that will never go away without adequate inspection. Many contractors and site workers don't even know that top steel needs to be at the top, rather than just with minimum cover, so they don't use the right height chairs. Another construction problem is speed...too early stripping, haphazard and premature removal of back propping, etc.
RE: Long Term Deflection Estimates
Most of the cases I have heard of have been flat plates/flat slabs as you suggest, but many of the Australian ones were designed in the 1960/70's, well before stud rails were invented. Design was typically based on L/D ratios and often took advantage of compression reinforcement rules to reduce the slab depth.
Spans were often 8*8m or 9*9m and final deflections on the bay were up to 80-90mm. In the mid 1970' I had consultants who were putting out RC flat slabs with L/D rations of 35 arguing that they did not need PT because their slabs worked (according to their L/D calculations). These same consultants 15 years later were telling me that they never do RC flat slabs any more because the deflections are far worse in practice than is being predicted by the code rules. I then proceeded to show them that we were predicting 80-90mm long term deflections for these slabs, similar to what they were getting in practice!
A lot of the later cases I have heard of tend to be designed using FEM software with automated design routines attached. I have seen wondrous deflection predictions by consultants using this type of software and they have proceeded to build the buildings based on these designs, not realizing that the software they are using is predicting short term uncracked deflections while the codes are talking about long term cracked deflections. This does not apply to all FEM software and does not mean that the FEM software is wrong, the designer simply has to understand what type of numbers the software is giving in each case and treat them accordingly. But if the designer does not understand the difference then there will be problems. A lot of designers from a steel design background will accept the deflections by well know 3D building analysis/design software because it is pretty good for steel buildings, not realizing that it is not allowing for cracking and long term effects in its RC design. It is doing exactly the same elastic design for concrete as it does for steel. Unfortunately, concrete cracks, creeps and shrinks, unlike most steel members.
In the simplest case, if the deflection is short term uncracked for RC slabs, multiple it by about 6!
I think the most problems I have seen is in buildings designed by engineers used to working with design codes that are predominately based on L/D ratios who then try to optimise the design by moving to FEM, and simply not understanding what results they are getting.
This has been happening with PT flat slabs over about the last 15 years also, where a lot of the early PT FEM software was also predicting short term uncracked deflections and/or still using RC multipliers to predict long term deflections in PT slabs. Contacts in PT companies are forever telling me about super thin slabs they are being asked to build and the design is always based on a FEM analysis that has been misinterpreted by the designer.