ACI 8.6.4

[slickdeals]

"It shall be permitted to analyze solid or ribbed
slabs built integrally with supports, with clear spans
not more than 10 ft, as continuous slabs on knife edge
supports with spans equal to the clear spans of the
slab and width of beams otherwise neglected."

What is the reason that spans longer than 10' are not allowed to be analyzed using a pin-support? How do you design one ways slabs with spans of 12' or more supported on beams?

 

[hokie66]

You need to consider the full moment diagram, including the moment at the centre of support, and also at the edge of support. For short spans, it doesn't make much difference.

 

[StructGen]

What year of ACI-318 you are looking at?

 

[hokie66]

The clause is actually in ACI318-08, but it is 8.9.4, not 8.6.4

 

[slickdeals]

The clause is in existence in various ACI editions since 95 or so. I had referred from ACI 318-02.

If I use a knife edge (based on center of support) and design my interior beams for compatibility torsion, then why couldn't I take the moments based on a pinned model and take it at the center of the support?

It appears that I am missing something, but otherwise am I not conservative in my calculations?

 

[hokie66]

That method gives higher positive moments and lower negative moments, because it neglects the depth of the beam. Like neglecting drop panels in a flat slab.

 

[DaveAtkins]

You CAN assume knife edge supports for spans greater than 10 feet. What this clause says is that for spans less than 10 feet, you can actually assume the beams have a width of zero. So if you have a slab-beam system with center to center spans of 12 feet and two foot wide beams, you can assume for slab design that you have a series of ten foot spans on knife edge supports.

I would never do this, however. It would throw off your reactions, which you need for beam design.

DaveAtkins

 

[slickdeals]

Well, the comment is really from a building reviewer. We have some one way slabs that are spanning more than 10', and we used a knife edge as support taking the center to center distance between supports.

The comment states that this is not an appropriate method and I was curious what the intent of the code really is in this regard?

 

[DaveAtkins]

Your plan reviewer is misinterpreting this section of the Code. Explain to him what it really means, and why you are not violating the Code.

DaveAtkins

 

[hokie66]

Your method is certainly conservative for the positive moments, but how do you get the moments at the beam edges?