Pan Joist question

[shemp]

Section 8.11.4 of ACI 318-99 says, "joist construction not meeting the limitations of [monolithic construction and certain rib width, depth, & spacing limitations] shall be designed as slabs and beams. What exactly are the differences between the design of slabs & beams vs. joists? I see in section 8.11.8 that you get a 10% bonus in shear strength for joists but, I am not aware of any other differences.

 

[JAE]

That is one difference (8.11.8)

Also, for joists, you can use 3/4" clearance instead of 1 1/2" (See Chapter 7 - section 7.7.1(c)). And joists get another break with minimum shear reinforcing (see 11.5.5.1(b))

References are from ACI 318-02.

 

[shemp]

Thanks JAE. Gosh, wouldn't it be nice if the index of ACI 318 under "joists" actually referenced those sections that involved joists???

Wonder why with wood design you get a 15% bonus in flexural strength for repetitive members but nothing for shear. And in concrete design you get a 10% bonus in shear strength for repetitive members but nothing for flexure?

 

[JAE]

Perhaps: Wood is much more resilient in flexure (can take higher short term loads without failing), and the wood shear is a very high variable....

And for concrete, the ability to engage multiple joists at one time and spread out flexural moments is more difficult due to the high rigidity of concrete....and for shear... I dunno.

Or its just how the codes morphed and evolved politically....who knows...