In a 1979 article in PMLA, John D. Niles examined the “ring composition” of Beowulf. He argued that each of the major battles (Grendel, Grendel’s Mother, the dragon) are arranged chiastically, and that the poem as a whole also has a chiastic shape.
He outlines the initial fight with Grendel in this way:
A. Preliminaries Grendel rejoicing (Grendel laughs before he enters Heorot).
B. Grendel devours warriors
C. Grendel’s wishes to flee once Beowulf gets hold of him: “fingers cracked”
D. Uproar in hall; Danes stricken with terror
HEOROT IN DANGER OF FALLING
D’. Uproar in hall; Danes stricken with terror
C’. Grendel’s “Joints burst”; Grendel forced to flee
B’. Grendel slinking back toward fens
A’. Aftermath: Beowulf rejoicing
The overall structure places Beowulf’s fight with Grendel’s mother at the center, a decision that according to Niles is not “casual”: “It is at this point in the narrative that the young hero Beowulf has his closest brush with death; he is in fact given up for dead by the Danes, who think that the blood welling to the surface of the pool is his. Insofar as Beowulf is marked out as ‘a mythical figure of death and resurrection,’ as Albert B. Lord has maintained, ‘it is here that he can be said to suffer symbolic death’.” Niles finds parallels with the Odyssey and Aeneid, both of which place a journey through the underworld at the structural center: “Like Homer and Vergil, the Beowulf poet had the narrative genius to develop his story around its point of greatest mystery.”
For the Beowulf poet, there’s an additional reason, a theological one. Beowulf’s struggle with Grendel’s mother “called to mind the greatest story of Christendom as well. By repeatedly associating Grendel and his dam with the creatures of hell, he presents Beowulf’s descent in terms that call to mind Christ’s legendary harrowing of hell, as re- counted in the apocryphal Gospel of Nicodemus.”
(Niles, “Ring Composition and Structure in Beowulf,” PMLA 94:5  924-35. Thanks to Kelly Kerr for passing on the article.)