The problem faced by Galloping Gertie, the failed bridge across the Tacoma Narrows, was caused because the engineers did not follow the perform tests on the prototype/testing/ step in the engineering design process.

1) Perform

2) Prototype

Explanation:

The prototype testing phase of the design process to test the readiness of the design for the intended use

The prototype which is a first or early model of the project made so as to ensure proper functioning of the concept is to pass through general function test as well as design performance evaluation in a testing or lab setting

Other tests include, excessive stress test, testing to failure and other design tests.

Therefore;

The problem faced by Galloping Gertie the failed bridge across the Tacoma Narrows, was caused because the engineers did not perform tests on the prototype in the engineering design process.