Why compression force in tension members and why tension force in compression members developed?

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Here is how the generated member forces can be explained.

If you look at the axial forces in the inclined braces, these are developing significant axial tension ( specially I am referring to the extreme 3 on each side ). The amount of axial tension in these braces increase significantly as we go from the center towards the two ends. As you can imagine, the axial tension in these braces is going to introduce axial compression at the center of the top chord. Now as you start moving away from the center and go towards each end, the tension in these braces would cause more and more tension to develop in the top chord eventually resulting in axial tension at the end members of the top chord.

Similarly for the bottom chord, the member at the center is subjected to axial tension and then as we start moving towards to the two ends, the axial compression being introduced into the bottom chord by tension force in these braces, starts reducing the axial tension in bottom chord members, eventually resulting in compression at the ends.

On a different note, you do have a duplicate PERFORM ANALYSIS command in your file which you may want to take out.      

Here is how the generated member forces can be explained.

If you look at the axial forces in the inclined braces, these are developing significant axial tension ( specially I am referring to the extreme 3 on each side ). The amount of axial tension in these braces increase significantly as we go from the center towards the two ends. As you can imagine, the axial tension in these braces is going to introduce axial compression at the center of the top chord. Now as you start moving away from the center and go towards each end, the tension in these braces would cause more and more tension to develop in the top chord eventually resulting in axial tension at the end members of the top chord.

Similarly for the bottom chord, the member at the center is subjected to axial tension and then as we start moving towards to the two ends, the axial compression being introduced into the bottom chord by tension force in these braces, starts reducing the axial tension in bottom chord members, eventually resulting in compression at the ends.

On a different note, you do have a duplicate PERFORM ANALYSIS command in your file which you may want to take out.