Sometimes I get these “stretch” lines in a surface when I combine 2 meshes.
In many cases setting auto-smooth enabled is a fix. But other times the stretch stays visible.
Blendfile: surface-strech-lines.blend (2.2 MB)
This is caused by the angles of the edge lines to the vertices. A Weighted Normals modifier can knock out some of those shading issues. Better is to use the Knife tool to cut new edges at 90 degree angles from column to column, and column to platform edge, then dissolving the acute and obtuse ones.
Each column needs 2 connecting edges. One to platform edge, one to another column.
Side note, unless you’re planning to add a sub-d modifier, you don’t need those support loops on the platform. A bevel modifier would be more efficient.
If your “pipes” are evenly placed, you may also subdivide the top to even rectangles, and where the pipes are, you can use the build-in Loop tools addon (may need to turn it on), to form circles, then you can extrude those up
If need be, you can of course move around the circles somewhat afterwards.
The first one, I had to delete the surface and created a new one, so that there wouldn’t be any of connected edges.
The second one: I did a duplicate of that model, removed those pillars and made a new ones by using a knife tool. Also I’ve added a Weighted Normal modifier to make it looks without any of shadows bugs.