w L^2 / 7.9

[RonRoberts]

OK, I'm taking a poll here.

As more of the hairs turn gray and the list of projects where the owner wants to add new loads or cut new openings gets longer or where I have to figure out a fix for the positive steel the plumber just cored thru, I find much more comfort in providing sufficient positive moment steel so that all concrete beams can act as simply supported spans.


Am I alone? Are you guys all designing for w L^ / (11, 14 or 16)?????

 

[msquared48]

I try and design for the worst case scenario I can see with the loadings, then provide at least 15% more steel than is required, also considering the maximum steel ratio.

Sometimes I do analyze situations as simple span conditions where they are critical, near mechanical shafts. With bidder designs, it's hard to predict their piping solutions most of the time.

Mike McCann
MMC Engineering

 

[hokie66]

Well, I think that I am probably more conservative than most, but have seldom taken that approach. I am more likely to use more negative steel than is required than to beef up the positive reinforcement.

 

[asixth]

I use a frame analysis packages that accounts for the loss of stiffness due to flexural cracking when the design moments exceed the cracking moment of the concrete.

For a two-way flat slab system (with drop panels), this results in a hinge mechanism developing mid-span and therefore the system acts more like two cantilevers opposed to one continuous member.

 

[csd72]

Ron,

You have touched on a very good point that I have often considered.

As engineers we are usually pushed to give a lean economic design, but on site any mistake will mean that this lean economic design ceases to work and therefore would require additional work to rectify.

Is it not more reasonable to provide a design that allows for these mistakes?

Just a thought.

 

[RonRoberts]

csd72,

It's not just mistakes during construction. I just finished helping a contractor repair the 3 tendons the plumber cored thru on a 10 year old building and I've had more than one owner come and ask if they could use a bay of an office building for file storage or an office library.

And it's not just concrete. Check out the attached photo of the dry rot we are repairing. Insulating both the top and the bottom of a timber deck roof creates microbe poop really fast.

 

[Ron]

There's no reasonable way we can design for potential construction mistakes....there are too many possibilities and combinations. My approach is to be relatively conservative in the design and don't allow compromises during the construction (doesn't always work...but I give it a shot!)

 

[rapt]

asixth,

What about the hinges at the supports which occur first?

 

[asixth]

rapt,

Let's take the example of a typical internal span.

The positive midspan moment is equal to wl^2/24 while the negative support moments are wl^2/12. Therefore the moment that occurs at a support is twice that which occurs midspan.

Now let's say I have provided drop panels over my columns which are twice the depth of the slab. Now when calculating the stiffness in my column strip, the Z value is proportional to d^2, so if I have twice the depth of concrete, I should have 4 times the cracking moment, therefore cracking will first occur mid-span.

I also had an isolated case the other day of a large point load located midspan of a transfer beam. Now in this situation the positive midspan moment will be equal to the negative support moment (Pl/8), and I was getting the same occurance in my bending moment envelope, that being larger support moments than midspan moments.

Therefore what I can assume is that the midpsan cracks first and transfers stress to the positive support moments.

It is something that I have noticed quite alot with my designs, when patterned live loading is taken into consideration, I generally get hogging occuring midspan of flat slab floor systems and I need to provided minimum flexural reinforcement in these zones (0.025bd for my governing code).

I know it is unethical to ask questions midway through another persons thread, but what is your interpretation of patterned loading for ultimate flexural design (i.e. 75% factored live load or 100% factored live load on alternate spans).

 

[Lion06]

asixth,
I was with you for a bit. I don't think,however, that cracking at midspan constitutes a hinge that will cause the slab to act as two cantilevers.
Once it cracks at midspan it still has PLENTY of moment capacity. Concrete is designed to crack. Once the moment capacity is reached at midspn, then it will act as two cants (provided there is enough steel at the supports such that the hinge will not occur there first).
All of this is also predicated on the idea that the sections have enough curvature ductility such that the first hinge has enough rotational capacity to allow the second hinge to form. It is a given in steel, but not in concrete.

 

[rapt]

asixth,

I missed the reference to drop panels in you earlier post. So, yes, midspan will probably crack first in this case. In any other case, the support sections will crack first.

Then the stresses will be attracted to the uncracked section until it cracks and then you have a mechanism going so it does not act like 2 cantilevers.

What it finally comes down to is it is best to put your reinforcement where the elastic stresses and moments say it should be. Then you will have adequate strength, crack control and a logical load path.

RE pattern loading of 75% or 100%, I assume you are refering to AS3600 clauses 7.5 and 7.6. I would normally use 75% for buildings other than storage/libraries where I would use 100%.
This is being changed in the next version of the code and will be 100% in future, mush to my displeasure. In my opinion it should have been a variuable factor depending on building usage, but I lost that argument.