"By the 1970s the use of thin shell concrete had all but disappeared... This change was due to a combination of [economic] factors... This disappearance was also caused by the design challenges [of] thin shell concrete. Shell structures, because of their thinness, must be shaped to conform to the forces present in the structure. Until recently this shell form-finding could only be done... with specialized computer expertise, or through... physical model testing and measurement. ...In recent years ...simple computational models ...have been adopted ...to explore rapid form-finding in ...early design phase."