Modeling Simple Slab in STAAD - Need help with the results

[Awre]

I wanted to model an 8.67' X 31.5’ concrete slab with a 16,000 lb. load applied at the center and nothing else.

I did not apply any self-weight or load factors in this exercise

The slab will have fixed ends at all sides.

Can anyone explain why I am getting apparently a very low maximum moment of 2.3 k-ft/ft.?

As a “one-way slab”, the expected max moment utilizing manual calculations is about 17.3 k-ft/ft; by using the formula (PL/8 for fixed ends), where L=8.67’, P=16 kip, and by neglecting the self-weight

** Does the moment I am getting sounds right? And is it acceptable to design for?

I appreciate any help... Thanks

 

[Settingsun]

Your hand calc should be 17.3 k-ft, not k-ft/ft. The model is saying the effective width is 7.5 ft.

 

[Awre]

Thanks, @steveh49 ... So if I design for the 2.3 k-ft moments (of course after I apply the load factors and approperiate load combinations), is it acceptable?

Where does it say in the model that the effective width is 7.5 ft?

I am new to STAAD and still experimenting with it ... Thanks for your help!

 

[Settingsun]

The PL/8 calculation gives you the total bending moment, not the bending moment for a unit width of slab. To get 7.5 feet, I assumed the model was correct: 17.3 k-ft / 2.3 k-ft/ft = 7.5 ft. This doesn't seem unreasonable for the 8.67' span. I would have used a smaller effective width if doing it by hand, but you've done a more detailed model which is an acceptable basis for design provided it's giving correct results.

I don't know whether your supports are truly fixed but the reality is usually less than the ideal case. The sagging moments will therefore be bigger than you've calculated so a bit more bottom steel is my recommendation.

 

[JoshPlumSE]

I threw together a pretty quick model and got about 6 k-ft / ft as the max moment for simply supported edges and about half that for fully fixed edges. That makes sense to me.

Now, I can get something approaching your 17 k-ft/ft if I leave the long edges free and use simply supported edges along the short edges.

This leads me to believe that you're probably comparing hand calcs to a model that is created with different edge assumptions.

 

[JoshPlumSE]

Standard Caveat:
I'm not a STAAD expert. I created the model in both RISA and SAFE (programs that I'm more familiar with). While it doesn't apply to this post (because I'm not giving any opinions of the various software programs) I have worked for the companies that sell RISA and SAFE.