Your tax query is incorrect, field should be the field you want to query on: term_id, name, or slug.
$posts_array = get_posts(
'posts_per_page' => -1,
'post_type' => 'fabric_building',
'tax_query' => array(
'taxonomy' => 'fabric_building_types',
'field' => 'term_id',
'terms' => $cat->term_id,
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