_layout.scss

.row {
  @include clearfix;
  clear: both;
}
@for $i from 1 through 12 {
  .col-xs-#{$i} {
    @media screen and (max-width: $screen-xs-min) {
      width: percentage($i/12);
      float: left;
    }
  }
}
@for $i from 1 through 12 {
  .col-sm-#{$i} {
    @media screen and (min-width: $screen-xs-min) {
      width: percentage($i/12);
      float: left;
    }
  }
}
@for $i from 1 through 12 {
  .col-md-#{$i} {
    @media screen and (min-width: $screen-sm-min) {
      width: percentage($i/12);
      float: left;
    }
  }
}
@for $i from 1 through 12 {
  .col-lg-#{$i} {
    @media screen and (min-width: $screen-md-min) {
      width: percentage($i/12);
      float: left;
    }
  }
}

CSS Layouts.md

CSS Layouts