[PK]

One easily overlooked way to create force walls in GURPS is to use the Wall enhancement on Innate Attack. Specifically, you could do this as Crushing Attack (Area Effect; Wall) to create an actual, physical bridge with HP and DR.

The only downside is that there's no specific correlation between "bridge HP/DR" and "bridge weight limit." However, you could approximate this by assuming that even the bumpiest ride over a bridge will cause no more of a downward impact than a 1-yard fall would (in other words, a truck driving over a bridge is certainly going to stress it less than if you dropped that truck onto the bridge from a 1-yard height!) and making sure the DR can absorb this. It's intentional overkill, so the GM can feel confident in never needing to worry about it.

Since a 1-yard fall is Move 5, assume that vehicles "inflict" (HPx5)/100 dice, aka HP/20 dice, which multiplied by 3.5 is about HP/5.7 points of damage. Since each level of Crushing Attack gives DR 3 (and 5.7x3=17) that means the Crushing Attack level (CAL) must equal HP/17, or in plain English, one level of Crushing Attack per 17 HP of vehicle weight. Because the stress is going to almost always come from all vehicles traveling carefully over the bridge, it's fair to lump their weight together.

Technically the travelers may be some combination of living, unliving, and homogenous, but for simplicity just use the "Unliving/Machine" column on p. 558, and you get this:

CAL -- Total Weight Supported
1 -- 76 lbs.
2 -- 614 lbs.
3 -- 2,072 lbs.
4 -- 4,913 lbs.
5 -- 9,595 lbs.
6 -- 16,581 lbs.
7 -- 26,330 lbs.
8 -- 39,304 lbs.
9 -- 55,962 lbs.
10 -- 76,765 lbs.

Note that the formula for deriving weight is Weight in lbs. = (HP/4)^3 = (CAL*4.25)^3 (round down, or just round to 2 or 3 significant digits).