Edit in JSFiddle

<div class="my">
<div class="ui-grid-a">
    <div class="ui-block-a">test a</div>
    <div class="ui-block-b">test b</div>
</div>
</div>

$(function(){
    var $window = $(window);
    $window.resize(function(){
        if ($window.width() < $window.height()) {
            //portrait
            $(".my").addClass("ui-responsive");
        } else {
            //landscape
            $(".my").removeClass("ui-responsive");
        }
    });
});

.my {
    margin-top:40px;
}