Which types of mortar are required for masonry providing the lateral force resisting system in seismic design category D2?

In seismic design category D2, structures are required to have stronger materials to ensure stability and resistance to lateral forces, such as those that arise during an earthquake. Type S mortar is specifically designed to provide the necessary strength and flexibility to withstand these lateral forces. It contains a higher proportion of Portland cement which gives it enhanced tensile strength and adhesion properties compared to other types of mortar.

Type S mortar has a compressive strength of at least 1800 psi (at 28 days), making it suitable for use in structural applications such as load-bearing masonry walls which contribute to the lateral force resisting system. This strength is critical in seismic-prone areas where buildings must endure significant stresses.

In contrast, other mortar types like Type N and Type O do not have the same strength characteristics necessary for seismic applications, with Type N being suitable for non-load-bearing structures and Type O being used only in non-structural applications. Type M, while strong, is generally not recommended for use in all mortar applications because its rigidity might not accommodate the slight movements of masonry in seismic conditions, potentially leading to cracking or failure.

Therefore, Type S mortar is the appropriate choice, as it fulfills the requirements of both strength and flexibility essential for masonry in seismic design category D2,