Diamonds is described as 'is the perfect software for structural design analysis of beam grids, floor slabs, rafts, walls, frames and multi-storey buildings, ... in concrete or steel. Development of calculation engineering design software with powerfull tools en simple user interfaces'. There are more than 50 alternatives to Diamonds for Windows, Linux, Online / Web-based and Mac. The best alternative is midas Civil. It's not free, so if you're looking for a free alternative, you could try SkyCiv Structural 3D or SkyCiv. Other great apps like Diamonds are midas Gen (Paid), RISA-3D (Paid), RISA-2D (Paid) and Etabs (Paid).
midas Gen is one-stop modeling, analysis and design solution for building and general structures. It features an intuitive UI, advanced analysis features, practical output and powerful design capacities using various standards
SkyCiv Structural 3D is a powerful structural analysis and design software on the cloud. Nothing to install or download and users can subscribe on a month-to-month basis so there's no life long commitment. It's also free to get started.
For nearly 30 years, ETABS has been recognized as the industry standard for Building Analysis and Design Software. Today, continuing in the same tradition, ETABS has evolved into a completely integrated building analysis and design environment.