Hi guys!
I found this instruction manual for software called SlabWOrks (made by mr. Eric Green) wich is used for designing stiffened slabs on grade on expansive clays.
Manual is located here:
I will quote some sections here and then hopefully discuss with you.
![[bigears]](/data/assets/smilies/bigears.gif "[bigears] [bigears]")
page 22/95
"Overall structural performance of stiffened slabs is generally independent of the
performance of the thin slab in the areas between beams. This portion of the
foundation slab is generally intended only to acts as a separator between the
building and the soil below. However, if thermal or shrinkage cracking is noted
in these areas, many owners will perceive the foundation is in a failed state. This
is particularly important if the owner anticipates the use of tile or stone finishes.
Therefore, performance expectations with respect to slab cracking should be
discussed with the owner and architect prior to design.
In the past, many engineers have relied on the minimum temperature and
shrinkage steel requirements from ACI-318 (0.18% steel). These guidelines are
intended for elevated structural slab and are not applicable to slabs-on-grade.
This is discussed in the commentary to the latest version of ACI-318.
The engineer should instead refer to ACI 224 “Control of Cracking” for guidance
on controlling cracking of slabs-on-grade. Generally, cracking in stiffened slabs
is controlled with bonded reinforcement, and control joints are not used.
According to ACI 224, 0.50%-0.60% steel is required to control cracking with
steel alone. Control joints can be installed, with the control joints located midway
between the stiffening beams. Control joints near beams will not be effective because the
beams restrain the concrete from movement."
I found an interesting thread here on the forum on MINIMUM reinforcement requirements for members in bending.
It seems that ACI codes made this a bit confusing but generaly it was agreed that a ratio of 0,18% is the minimum reinforcement that should be provided anywhere in the member where tension occurs.
If you have moment appearing in the top and in the bottom of a slab you should provide 0,18% ratio in the top portion of the slab and also 0,18% in the bottom of the slab.
This wll ensure ductile behaviour of a member because of a slightly higher moment capacity o the cracked section vs. uncracked section.
The sentense "These guidelines are intended for
elevated structural slab and are not applicable to slabs-on-grade" is where I get confused and if you read further in the manual, page 89/95, Quote:
"As discussed in the commentary, ACI 318-05 minimum steel requirements
(0.18%) for elevated slabs are not applicable to slabs-on-grade. Instead, refer to
ACI 224 “Control of Cracking” for directions on controlling cracking of slabs-ongrade. Generally, cracking is
controlled with a combination of steel and control joints. According to ACI 224,0.50%-0.60% steel is required
to control cracking with steel alone."
0,5-0,6% ratio is VERY HIGH reinforcement amount and I cannot understand with this.
So why doesnt rule apply also to elevated slabs wich are not resting on the ground and wich usualy have higher bending moments?
Would this rule also then apply to raft structures since they are also constructed as jointless, let say for residential house a 12inch raft, plan dim. 50ft x 35ft would need a 0,5% reinforcement ratio if poured all at once (no joints that is).
Please correct me couse I dont wanna belive that this is rational design and Ive designed raft with 0,18% both sides many times.
Was I wrong?
