Discrepancy between 510.2 commentary and 510.4

[chrisrog]

Working on a new building with 2018 IBC.

Basement parking with ground floor apartments in a Type 1A structure under Floors 2-5 apartments - construction type to be determined:

510.2 Horizontal Building Separation allows 4 more floors of Type 5A construction above the 3 hour assembly on top of the 1A structure.

510.4 seems to add the restriction that the max number of floors above the Type 1A construction begins counting not at the 3 hour horizontal assembly, but at the level of the floor above the parking.

This seems clear until you look at the commentary for 510.2 which uses a Group R over Parking example but seems to contradict the requirements of 510.4 and states that you start measuring from the horizontal separation.

I don't have commentary for 510.4 so I can't see if that commentary contradicts the 510.2 commentary. I also might be reading Section 510.4 incorrectly.

 

[RLGA]

These are two separate requirements that are used independently of each other (See Section 510.1) and not for the same structure--you can only use one or the other.

Section 510.2 - Height in stories for the building(s)* above the horizontal separation is measured from the horizontal separation. Height in feet for the building(s)* above is measured from the grade plane. If you can make the building(s)* above work within those two requirements, you can have any number of stories between the grade plane and the horizontal assembly of the Type IA construction building below.
* Multiple buildings are permitted above the horizontal separation.

Section 510.4 - Parking is limited to one story above grade plane. Height in stories for the building above is measured above the parking story for the construction type of the building above the parking garage story. Type IA, IB, or IV can be used for the parking story, and any construction type is permitted for the Group R building above. Per Table 508.4, a minimum 1-hour separation is required between the parking garage story and the Group R building if the building is sprinklered throughout, or a 2-hour rating if it is not (which you can't, anyway). Per Table 601, the floor between the parking garage and the Group R building must be 2-hour-rated for Types IA and IB, and HT for Type IV.

 

[chrisrog]

Thank you for the direction. I missed the language in 510.1 that limited applicability to a single section. I interpreted it as an additional restriction when building above parking, but in fact it appears that 510.4 is actually intended to allow additional height when the parking portion of the building is taller.