Tekla Structural Designer is described as 'Analyze and design buildings efficiently Tekla Structural Designer is revolutionary software that gives engineers the power to analyze and design buildings efficiently and profitably'. There are more than 25 alternatives to Tekla Structural Designer 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 ClearCalcs. Other great apps like Tekla Structural Designer are RISA-3D (Paid), midas Gen (Paid), RISA-2D (Paid) and SAP2000 (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.