This is not even the stress of an earthquake causing these problems, and as someone mentioned, the bridge is supposed to withstand an 8.5 magnitude quake...
It is standard to bake the bolts at about 600f after plating to remove the hydrogen
May have been hard if they did not have a 18 foot long oven
When you torque a bolt, even the toughest alloy, it stretches almost like a rubber band. It has to hold that state of tension without elongating beyond a certain point and loosening, or breaking, and it must hold this property over time. Design involves choosing the proper material for the expected load but sometimes the best calculations fail in practice.
The GG Bridge was built in the early 1930's. In the 1980's, for the 50th year celebration, they allowed a mass of people to walk the bridge, completely filling it with people (jammed together weighing much more than any load of vehicular traffic). The bridge flattened out from it's arch but everything held together, and engineers were able to relax from their fear of a collapse (politicians wanted the people walk). That's 1930's U.S. steel! Nowadays, replacement steel pieces regularly break on the Oakland side of the Bay Bridge, some having damaged cars. Part of the reason for the new section.