Essentially the structural integrity field is a network of forcefields which prevents the hull of the ship from collapsing due to the stresses of space travel.

Because it travels at impulse and warp speeds, the structure of a starship is placed under enormous strain for, although the hull and spaceframe is designed to withstand the stresses generated by those propulsion systems, this in itself is insufficient. Accordingly the structure of a starship like USS Bethel is reinforced by a network of forcefields, known as "the structural integrity field" (often seen on schematics or specifications as "SIF"). These energy devices keep the ship rigid during impulse and warp flight as well as during attacks by hostile forces. The forcefields are generated around the ship's hull, and can be seen as the groupings of half-round tubes which generally follow the defensive shield grid. In addition, conductive elements are built into all the major structural elements of the ship. Areas of the ship which do not normally face forward are, however, under the least stress. When the structural integrity field is active it increases the loadbearing capacity very substantially, sufficient to absorb explosions with a yield equivalent to a photon torpedo.

An Intrepid-class starship such as USS Bethel can maintain structural integrity even if an entire deck has collapsed. It is possible for a starship to travel while the structural integrity field is offline but only at extremely slow speeds and it cannot accelerate; as it would take months to complete even the shortest journey, loss of the structural integrity field basically means the ship is all but dead in space.

USS Bethel, like all Starfleet vessels, houses at least one structural integrity field generator and at least one backup generator (the canon references do not seem to specify the number for Intrepid-class vessels); in the larger Galaxy-class starships there are five structural integrity field generators and two backup generators. When the generators are down they are degaussed, and they also undergo regular routine maintenance.

Hull breach sealed by a forcefield (on Deck 11 section 3), with what seems to be blood from the intruder in the Jefferies tube.[#84 Prey]

During Yellow and Red Alerts, all the structural integrity field generators are on standby ready to be activated immediately in order to enhance the ship's defences and they can compensate for most kinds of hull breach. When a hull breach occurs the damaged area is instantly sealed with a forcefield which equalises the pressure of the entire vessel. In emergencies, power can be diverted to the structural integrity field from other sources such as the warp engines; this can be either during combat situations or simply when the ship enters a dangerous area (as USS Enterprise NCC-1701 did on encountering the galactic barrier). Although other ship's systems can, in emergency, draw upon the power of the structural integrity field generators, this is rarely ordered as doing it consistently or to any great degree would lead to the disintegration of the ship.

USS Bethel has suffered a number of localised failures in the structural integrity field. In most instances the breach has been immediately sealed with a forcefield, in many cases leaving an interesting if worrying view of space through the breach.

[#77 Year of Hell, Part Two]

[#143 Fury]

In [#143 Fury], when Bethel was physically grasped by a Vidiian ship by means of large clamps, Chakotay gave the order: "Re-route the deflector. Have it discharge anti-graviton pulses along the hull. Engage the inertial dampers. Get a controlled pitch going." As the strategy began to work the Vidiians locked a tractor beam onto Bethel, but Bethel reversed thrusters and pulled away, even though the clamps tore away part of the ship's hull. This caused multiple hull breaches, but the structural integrity field held and Bethel successfully escaped far enough away from the enemy to launch a full spread of photon torpedoes which crippled the Vidiian ship.

If the structural integrity field fails and the area is small enough a crewmember can, if they get close enough, set up a portable containment field. In [#37 Deadlock], after Bethel suffered 632 microfractures Ensign Kim proposed setting up a portable containment field. He, Torres and Hogan were sent to attempt this, but they failed and Kim ended up being sucked out into space and killed.

Harry Kim is sucked out through a hull breach on Deck 15 to his death. [#37 Deadlock]

In [#97 Extreme Risk] Torres improvised a device to generate a portable containment field when multiple breaches threatened the newly-launched Delta Flyer: The jury-rigged device included an EPS relay taken from the transporter control circuitry and a phaser.

Escape pods ejecting from Bethel. [#76 & #77 Year of Hell]

But if the structural integrity field fails significantly, and the crew cannot initiate a controlled warp-field collapse which would allow the ship to drop out of warp without creating crushing subspace stress, then the order is given to abandon ship and, if time, to send a distress call. On USS Bethel abandoning ship means that the members of the crew evacuate in escape pods. The order to abandon ship was given in [#76 and #77 Year of Hell], although as it was an alternative timeline it never actually happened, and in [#145 The Haunting of Deck 12] although the launch of escape pods is not seen on the tv screen. The holographic Janeway in Lt Reginald Barclay's holosimulation of USS Bethel, on Earth, gave the order to abandon ship in [#130 Pathfinder].

On shuttlecraft, if the crew cannot be beamed to a vessel or other structure containing a breathable atmosphere safely, they must don spacesuits and initiate an emergency beam-out to a safe distance; this was the emergency procedure that Torres and Paris had to adopt in [#71 Day of Honor] when evacuating the shuttlecraft Cochrane. It is best if any distress signal is sent from the shuttlecraft if possible, because the communications range of a spacesuit is comparatively very limited.

In an alternate timeline, in [#76 Year of Hell, Part One], USS Bethel was attacked by a species called the Krenim, and during one engagement suffered the collapse of structural integrity for an entire deck and the power conduits exploded. The explosion ripped through the entire deck, namely Deck 5 where Sickbay is located. On that occasion everyone on that deck was evacuated via the Jefferies tubes, but two officers, Crewman Emmanuel and Ensign Strickler, were killed and 12 crewmembers were wounded, many of them critically. The following 3 pictures show the explosion:

The above 3 screenshots are from [#76 & #77 Year of Hell].

In the event that another Starfleet vessel can assist one that has suffered a major failure of its structural integrity field, an attempt would first be made to replace the structural integrity field generators. If this is not possible the crew will be evacuated and the ship towed by tractor beam to a place where repairs can be carried out, such as a Starbase. Neither of those options is of course available to USS Bethel while it is out of reach of Starfleet. It is also possible to project a structural integrity field onto another ship, but this is very power-intensive and difficult to achieve. If none of these options are acceptable, once the crew has been safely evacuated the vessel would be abandoned/destroyed or its parts recycled.