Six lanes of traffic pass daily across San Francisco's famous Golden Gate Bridge, oblivious to activity below as the bridge is torn apart and rebuilt. Three dozen steel erectors working for the US wing of Britishcontractor Balfour Beatty are busy on the bridge's northern approach. Piling rigs are increasing the size of foundations. Work involves pulling out steel sections and bolting in new ones to strengthen the lattice structure bridge deck against earthquakes. More dramatically, four steel towers supporting the approach are being replaced and their foundations beefed up. The work also links separate elements of the deck longitudinally to make the structure stiffer. TY Lin is the main consultant for the retrofit design for client the Golden Gate Bridge Authority. Balfour Beatty's work is the first phase of an ongoing seismic upgrade that will also see the lattice deck strengthened on the south approaches and on the main span.