Analyzing One-Way Slab 1

[DMWWEngr]

What am I doing wrong here??

I have a 55 year old building with a doubly-reinfocved one-way slab (only one bay wide). I'm trying to analyze it to determine that largest load that can be placed (anywhere) on the floor during some construction/restoration work and I'm having some trouble getting a reasonable maximum moment Here's what I have:

6" Thick slab
Steel in the direction of span:
#4's at 6" - TOP
#4's at 6" - BOTTOM
Span is 9'
Long Direction (S&T) Steel is:
#4's @ 9" - Top and Bottom
Long Direction is ~140' long

(Imagine a 9' wide hallway)

I'm being conservative and assuming:
fy=40ksi
f'c=3ksi
d=4.5"
d'=1.5"

When I run my calculations I get:

Mu: 5.09'-k - When neglecting top steel layer.

Mu: 5.83'-k - When analyzing both layers of steel.

Is this what you guys get?? I've been tripping over this all day and I can't figure it out. My problem with these numbers is they are too low. The moment from the wieght of the slab alone would be around 6.8'-k.

I'm just coming out of school (4-months). Is a one-way slab analysis appropriate or would there be a better choice?? This is what I'm still learning

Any help would be greatly appreciated!!
---
Andrew

 

[DMWWEngr]

In another forum (ACI) someone has pointed out that I calculated my dead load moment wrong. I have made that change.

Any other comments would also be appreciated!!
---
Andrew

 

[steve1]

I calculate same capacity as you, but I get a different dead load moment. For a 6 inch slab dead load is approx. 75 psf. Therefore dead load (factored) moment is equal to:

1.4 X 75 X 1.0 (Trib. width) X 9 X 9 /8 = 1063 ft pound

 

[DMWWEngr]

Thanks Steve!!

That's were my error was. Big error too!!

 

[austim]

Hi, DMWWEngr.

I just want to congratulate you (yes, seriously).

Many long-experienced engineers would not have been game to expose their blunders for everyone to see as clearly as you, and you are on the side on the angels.

Many years ago, Karl Terzaghi set out some rules for professional activities in engineering. I regret that I can't quote the exact reference, so here they are (quoted very second-hand):

"1. Engineering is a noble sport which calls for good sportsmanship. Occasional blundering is part of the game. Let it be your ambition to be the first one to discover and announce your blunders. If someone else gets ahead of you, take it with a smile and thank him for his interest. Once you begin to feel tempted to deny your blunders in the face of reasonable evidence you have ceased to be a good sport. You are already a crank or a grouch.

2. The worst habit you can possibly acquire is to become uncritical towards your own concepts and at the same time sceptical towards those of others. Once you arrive at that state you are in the grip of senility, regardless of your age.

3. When you commit one of your ideas to print, emphasis every controversial aspect of your thesis which you can perceive. Thus you win the respect of your readers and are kept aware of the possibilities for further improvement. A departure from this rule is the safest way to wreck your reputation and to paralyse your mental activites.

4. Very few people are either so dumb or so dishonest that you could not learn anything from them."

[Austim comment - not a bad set of guidelines to live by, even though they were written for a bygone era].

So, DMWWEngr, as far as Rule 1 is concerned at least, you get 10/10.

 

[DMWWEngr]

Thanks for the compliment. But I make plenty of mistakes...I know, I'm just coming out of school and can still remember my grades

Since this was just an analysis of a building, that was built 31 years before I was BORN, I wasn't as concerned about my mistake. I knew I was wrong because the floor was still there!! If this was a design I did then I would be more concerned. I guess I'm starting to work on the correct "projects".

At least I got one rule down. I still have 40 years to work on the rest of the them.

Regards
---
Andrew