<p>By using only the bare minimum materials produced from factories, Mies sought to cast off what he found to be the main problem with contemporary architecture: that overly decorative structures with no function were wasteful uses of space. </p>
<p>Why do I need a colon after architecture?</p>
<p>[Colons</a> with Formal Appositives](<a href=“http://englishplus.com/grammar/00000100.htm]Colons”>Colons with Formal Appositives)</p>
<p>The part after the colon describes the “main problem”</p>
<p>Colons are used to list things, or to expound upon the main idea, which in this case, as was stated above, is “the main problem”.</p>