Utility Boxes and Bridges — Eruvin 89a
תוספות עירובין דף פט/א
במחיצות שאינן ניכרות - כשהבתים מחוברין דאין מחיצות הבתים ניכרות כדפירש בקונטרס ונראה דהוא הדין אם הגג בולט חוץ למחיצות הבית דחשיב נמי מחיצות שאינן ניכרות
From The Contemporary Eruv:
Many problems may arise when gud asek lechayayim (short rods or sticks attached to the side of a telephone pole) are used. Often a utility company box may be attached to the utility pole between the lechi and the overhead cable. If the lechi is built all the way up to the box, or to within three tefachim of it, then the box may be considered part of the lechi. The halachic mechanism of “lavud” connects them. If, however, the lechi ends more than three tefachim beneath the box, then the box interrupts the gud asek (Mishnah Berurah 363:112 based on the Taz there se'if katan 19. The scenario discussed there concerns a roof positioned between the lechi and the overhead cable, but it seems that the same Halacha applies to this case). Just as in our case the roof "overrides" the walls underneath, the utility box overrides the lechi.
Due to this, and other considerations, “state of the art” eruvin do not utilize gud asek lechayayim, but, rather, build the lechayayim all the way up to the overhead cable. No imaginary lines need then be drawn, as an actual structure extends all the way to the wire.
Moreover, it is generally accepted that walls underneath a bridge - even artificial walls - are not meaningful vis-á-vis the bridge itself, as the bridge overrides the walls, and that whatever rectification is ultimately required must be mounted above and across the bridge’s roadway. The nature of that rectification is contingent on whether the bridge is or serves an area classified as a reshus ha'rabbim (in which case delasos are required) or a carmelis (in which case a tzuras ha'pesach is required). See the discussion in this regard in the Chazon Ish, siman 108; Igros Moshe, Orach Chaim, 1:139:1; and, No’am, ibid., pp. 224-228. See the discussions in No’am there as to: a) whether extant structures on the bridges may be considered halachically valid tzuros ha'pesach; and, b) whether drawbridges require an additional rectification.