dumb question about beam cracks on tension side

[delagina]

why do all concrete beams in good condition that i see in buildings, everywhere dont have cracks on the tension side.

shouldnt it crack before rebar engage.

 

[rowingengineer]

Not necessarily, a lot of beams will have a fair amount of strength as an uncracked section. This is even more the case if the are PT.

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[frv]

There's also the possibility that the cracks are nearly imperceptible.

 

[Teguci]

If you are looking at the exterior, wait until it rains to look for cracks. Another possibility is the pretensioning in precast concrete will usually be designed to not crack at service loads (a lot of exterior concrete tends to be precast). Finally, I will size a beam dimensionally based on the worst case loading. For other beams I tend to use the same dimensions and just modify the rebar.

 

[haynewp]

The rebar is engaged prior to cracking because it goes thru deflection along with the rest of the cross section under bending.

 

[Forensic74]

Because f't~500psi and E=29000 ksi. If its loaded enough to exceed the cracking moment, the engagement of the rebar results in very little rotation of the section (ie small crack widths).

 

[TXStructural]

The other reason is that very few structures are loaded to their design loads. A beam might be part of a 90 mph wind-resisting frame and have been designed for an unreduced, code-required 80 psf. It has likely never seen that kind of wind nor gravity loading, and then there are load factors.