Confused by new AASHTO (5th Ed.) crack control equations

[umrce]

We were having a discussion in my office about AASHTO's (LRFD 5th Ed.) new method regarding crack control 5.7.3.4. Many people here believe it is counter intuitive (and thus incorrect) for you to have a tighter max bar spacing for more concrete cover (or a lower allowable stress in the bars). I see this as being incorrect and agree with AASHTO's method. I would simply like to state why and get other's opinions.

Having more cover essentially means more unreinforced concrete past the flexural reinforcement (higher dc value) and as the beam deflects you have more rotation and thus crack can start earlier. If you have more reinforcement (i.e. smaller spacing) you have lowered the stress in the bars and thus created a stiffer beam which provides less opportunity for cracks to start and propogate. Does this make sense to anyone or am I losing it?

 

[rapt]

ryanokc,

I have not seen the new ASSHTO rules, but in general,
- a higher cover with the same steel stress will result in a larger crack width.
- A smaller bar spacing will result in a smaller crack width with the same stress in the reinnforcement (smaller bars at ssmaller spacing with then same area of reinforcement)

So, with larger cover you would need to

- add extra reinforcement to reduce the steel stress to reduce the crack width resulting in the same bar size at closer spacing

- reduce the spacing of the reinforcement sufficiently to reduce the crack width without the reduction in steel stress (this will require a larger reduction in spacing than the 1st option).

 

[umrce]

I couldn't agree more. I continued my discussion since posting this with my peers and concluded we weren't on the same page regarding the assumptions we were making. Thanks.

 

[a7x1984]

ryanokc, I am curious of your application and the need to worry about crack control.

The crack control 'freaks' tend to be in the fluid-retaining structures niche, e.g. ACI 350.

On a side note, at the IBC conference in Pittsburgh, a presenter described the general philosophies of steel protection used in Chinese bridges. They do not epoxy coat as they just provide greater amounts of cover. I would tend to think that in overhead soffit applications, the additional crack width at the surface does little to hurt the protection of the bottom bars.

rapt is very much correct regarding smaller bar diameters. The ACI 350 has the environmental factor for concrete that is applied over the fluid LL and gets quite large (on the order of 3xLL, including 1.6F).

All in all, I would get a hold of that ACI publication to get a better feel for crack control application & philosophy.

"Structural engineering is the art of modelling materials we do not wholly understand into shapes we cannot..."...ah...screw it, we don't know what the heck we are doing.