Precast Cocnrete double T roof in Arkansas with SDC C. Does it require a concrete topping 1

[Delmarva_Struct]

I'm designing a Precast Concrete double T roof in Arkansas with SDC C. Does it require a concrete topping? as far I understand only SDC D and onwards need one.

 

[haynewp]

What Code are you using?

 

[Delmarva_Struct]

IBC 2018
ACI 318-14
ASCE 7-16

 

[haynewp]

I have the same situation but under IBC 2015. ACI 318-14 has the below. I don't see where untopped diaphragms are not allowed in SDC C or worse. The ACI is just saying it does not have provisions for untopped in SDC D, E, and F.

Other related with example:

https://nvlpubs.nist.gov/nistpubs/gcr/2017/NIST.GCR.17-917-47.pdf

https://www.fema.gov/sites/default/...seismic-provisions-examples_p-1051_7-2016.pdf

and ACI 550.5.


R12.5.3.6 This Code does not contain provisions for
untopped diaphragms in buildings assigned to Seismic
Design Categories D, E, and F. Diaphragm shear in untopped
diaphragms can be resisted by using shear-friction reinforcement
in grouted joints (FEMA P751). Required shear-friction
reinforcement is in addition to reinforcement required
by design to resist other tensile forces in the diaphragm, such
as those due to diaphragm moment and axial force, or due
to collector tension. The intent is to reduce joint opening
while simultaneously resisting shear through shear-friction.
Alternatively, or additionally, mechanical connectors can
be used to transfer shear across joints of precast elements.
In this case, some joint opening should be anticipated. The
mechanical connectors should be capable of maintaining
design strength under anticipated joint opening.

 

[Delmarva_Struct]

Please see page 13-50 of FEMA seismic provisions that you listed. It says that the connections between double T members can be designed to carry the seismic forces and that topping may not be required for low to moderate seismic areas.

 

[haynewp]

I am going with untopped for my SDC C. I think the FEMA document is saying it is required in SDC D or worse since ACI does not directly address it those cases. The NEHRP documents has the below which says it will need AHJ approval if using untopped in SDC D or worse.

 

[Delmarva_Struct]

Thank you for your help Heynewp..I am going to with untopped precast double T's as well.

 

[JohnRwals]

I wonder what detail you use for chord reinforcement.
This is another big issue for pre-topped double tees.

JRW

 

[haynewp]

The PCI Manual has some info on it.