2 WAY CONCRETE SLAB WITH REINFORCING 45 DEGREES TO AXIS OF GRID

[Rweave]

Has anyone come across a 2 way concrete slab system with drop panels at the columns but the reinforcing of the concrete slab is 45 degrees from the axis of the column grid and not the typical perpendicular direction? We are going to be renovating the entire existing building. They want to change the use of the building and therefore are going to be putting all kinds of new plumbing penetrations through slab. They are asking where they can place these penetrations (best locations)? Also I have to analyze the building for seismic/wind lateral loading and based on what I saw in the field and the as builts the only thing I think that is holding this structure up is the slab and columns. Therefore I was going to look at the building as concrete moment frames.
So I would like to know how to come up with capacities for the slab system and how to take in account the new penetrations?

 

[KootK]

Any chance the rebar might be radial, like this: Link?

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.

 

[Trenno]

Principle bending moments in flat plates almost never follow traditional X/Y strips, yet your reinforcement almost always does.

Just need to make sure the amount of steel passing through a given design cross section (with arbitrary orientation) satisfies the bending moment (Mx, My AND Mxy).

On second thought: why can't you simply disregard the grid lines and take strips @ the 45 deg angle?

 

[hokie66]

The flat slab will take the most load in the shorter direction regardless of how the reinforcement is placed. Assuming equal spans and equal reinforcement in both directions, the top steel direction should not matter much, but I think the bottom mat direction does if the reinforcement has been placed in column and middle strips diagonally. It just needs to be assessed with force vectors.

Flat slab moment frames are not very efficient at the best of times. Any chance of introducing some shear walls?

Just curious, how old is the building and where is it located?

 

[Rweave]

The reinforcing is not radial and they def have column strip reinforcing at columns at the 45 degree angles.


KootK (Structural)1 Jul 15 04:32
Any chance the rebar might be radial, like this: Link?
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.

 

[Rweave]

Trenno that was my thought initally was just look at the slab and system at the 45 degree angle. But when I was searching online I was finding some old articles about a "4 Way" slab system. They started taking like the sin of the angle of all the reinforcing and converting it to an equivalent reinforcing in the parallel direction of the column grids. Has anyone came across this before?



Trenno (Structural)1 Jul 15 05:25
Principle bending moments in flat plates almost never follow traditional X/Y strips, yet your reinforcement almost always does.

Just need to make sure the amount of steel passing through a given design cross section (with arbitrary orientation) satisfies the bending moment (Mx, My AND Mxy).

On second thought: why can't you simply disregard the grid lines and take strips @ the 45 deg angle?

 

[Guastavino]

Rweave,

Any chance this was a C.A.P. Turner system?

http://www.mbjeng.com/wp-content/uploads/2011/03/NL13_Oct2010-Meghans-article.pdf

There are several antiquated two-way systems that it might have followed that might make your life easier if you can find the information on them.

 

[KootK]

Age and location of the building would be very useful parameters here.

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.

 

[Rweave]

The building was built in 1927 and it is located in New York.


KootK (Structural)1 Jul 15 13:41
Age and location of the building would be very useful parameters here.

 

[Rweave]

No the system doesnt appear to look like that at all. There are square drop panels at these columns.


njlutzwe (Structural)1 Jul 15 13:19
Rweave,

Any chance this was a C.A.P. Turner system?

http://www.mbjeng.com/wp-content/uploads/2011/03/N...

There are several antiquated two-way systems that it might have followed that might make your life easier if you can find the information on them.

 

[theonlynamenottaken]

taking like the sin of the angle of all the reinforcing and converting it to an equivalent reinforcing in the parallel direction of the column grids. Has anyone came across this before?

I have used that approach several times and I believe it is discussed in Park's, "Reinforced Concrete Slabs" book.

John Klein, P.E., M.L.S.E.

http://www.johnkleinpe.com

 

[Ingenuity]

Keep in mind that flat plates designed back in the early 1900's were permitted to be designed for less than conditions of equilibrium and statics!

Nichols' total static moment concept of wl[sup]2[/sup]/8 was fought by many, including distinguished professors.

I think the ACI-318 provision of 85% of the total static moment stayed in the code until 1963 or 1971! Long live Newtonian mechanics!

 

[Brad805]

Chapter 2 goes thru the design steps to detail rebar in non-orthogonal directions using mohr's circles from simple linear FE analysis. I suspect you might be looking for something simpler, but it might be helpful. There is a copy on Scribd. I am not a fan of scribd, but it is a lot faster than ordering from Switzerland.

http://www.fib-international.org/practitioners-guide-to-finite-element-modelling

 

[rapt]

Backing up Ingenuity's comment, flat slabs before about the 1950's in USA were designed using 50% of the load in each direction. So do not expect it to work to the current design codes.

The reinforcement can be at any angle. The principal stress will be at different angles at every point on the slab so we cannot put the reinforcement in at the principal stress angle, so we normally use the angle that is easiest to build, orthogonal and perpendicular to the slab edges, and resolve the moments to suit in each direction.

I once had a square slab that was shown as 11.3m * 5.7 perpendicular to the edges. But if you rotate everything 45 degrees, it was actually an 8m * 8M grid, but with 45 degree sloping sides! You could design it either way, but an 8m flat plate suited much better than an 11.3 * 5.7m beam and slab arrangement.