The underarm of the basement civil air defense door is usually added to the open side of the door. According to the personal library, most civil air defense buildings use armpits, which can ensure the stability of the structure. Adding an armpit to the side is adding a support, similar to a T-beam. Axillary brace is to increase the area of two intersecting sections at the corner of the whole structure at the same time, which is generally made into a triangle and poured at the same time with the intersecting structure.