Manasara 16.74

16.74

kuryādadhaḥpradeśe syānmadhye vṛttamuttaropari
mahābhāraṃ tulāṃ vāpi kṣudradaṇḍamathāpi vā

One should make [it]; in the lower region it should be circular in the center above the architrave; / a main heavy beam, or a cross-beam, or a small rafter;