I'm a high school student who is trying to investigate the forces that keep a bridge from collapsing - specifically the Fern Hollow Bridge in Pittsburgh that collapsed a few months ago. We estimated the mass of the bridge to be 6,488,185 kilograms and there were approximately 20,865 kilograms on the bridge at time of collapse. The bridge has two supports that meet the bridge at 140.5 feet away from the edge and there is a distance of 166 ft between the supports. The supports are at 60 degree angles. Would the forces on the bridge that kept it from collapsing be tension and compression, and how would you calculate those forces?

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  • $\begingroup$ Hmm. we're going to have trouble framing the answer at the level of high school physics. But, let's have a go. Suppose we replace the continuous beam that forms the bridge with 3 beams, connected by 2 pin joints, and allow the end of the two end beams to move laterally. Then draw a free body diagram for each of the 3 beams and the 2 legs. TLDR: you can't really answer that question with the information you have $\endgroup$ Jan 8 at 3:18

1 Answer 1


I've gathered the preliminary reports from 3 sources. Note that all reports consistently indicated no significant deficiencies were found on the supporting metal frame members, rather, the deterioration of the concrete deck could be the culprit. For such as case, I suggest evaluating the shear strength/capacity of the deck near the support abutment and piers and anticipating a shear-type failure rather than bending; also the integrity of the deck to support connections needs to be examined. Without having the construction details, no calculation can be attempted.

CBS News

The Fern Hollow Bridge collapsed on the morning of Jan. 28, injuring several people and making national headlines.

The inspection and the report conducted by state and city inspectors had been withheld by PennDOT since the collapse but were released Friday after a right-to-know request by our news partners at the Post-Gazette.

The report is redacted in parts but cites exposed rebar, as well as corrosion and holes in support beams and decay in the deck, allowing water to seep through.

NTSB Investigation

The bridge was an uncoated weathering steel, three-span, continuous rigid “K” frame structure with two welded steel girders, welded steel floor beams, and rolled steel stringers. 1 The ends of the structure rested on reinforced concrete caps on stone masonry abutments. Each girder was additionally supported by two inclined, welded steel frame legs, also made of uncoated weathering steel, which rested atop reinforced concrete thrust blocks. Although certain areas of the welded steel girders were identified as being fracture critical, no primary fractures were found in these areas.

Thus far, the investigation has not found any evidence of widespread deficiencies with rigid K-frame superstructure types. All aspects of the collapse remain under investigation while the NTSB determines the probable cause, with the intent of issuing safety recommendations to prevent similar events.


The preliminary report was released on February 7.[14] The collapse apparently initiated at the west end of the structure.[14] Surveillance video recovered from the bus showed the bridge had already fallen off the western abutment when the eastern expansion joint began to separate.[40] No primary fractures were found in the critical areas of the welded steel girders.


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