What are girders in construction?

Girders are the primary horizontal members of a steel frame, the ones the other beams land on. Infill beams frame into the girders and deliver their load to them, and the girders carry it down to the columns, which is why a girder line that is missed or double-counted moves the tonnage more than almost any other single member. For the estimator they are a weight line: pounds per foot from the shape callout times the member length, summed and divided by 2,000 for tons.

Both are horizontal members, and in steel they are often the same wide flange shapes. The difference is hierarchy: beams carry the deck or joists and deliver their load to girders, while girders carry those beams and deliver the load to columns. On a framing plan the girders run along the column lines and the infill beams frame into their webs, which is why girders are typically the heavier shapes in any bay.

The shape designation gives it to you directly. A W24x76 is a wide flange, nominally 24 inches deep, weighing 76 lb per linear foot, so a 30 ft piece weighs 30 x 76 = 2,280 lb. Multiply weight per foot by length for each member, sum the pounds, and divide by 2,000 to get the tons that fabrication and erection are priced on.

Not from the shape weight alone. The per-foot weight covers only the rolled member, so estimators add a connection allowance, commonly in the range of 8 to 12 percent of net weight depending on the connection types, to cover clips, plates, bolts, and stiffeners. Moment connections and heavy gusset work push that allowance higher, which is one reason two bids on the same plans can show different tonnages.