var alturaDocumento = document.getElementById('main1altura').offsetHeight + 10;

document.getElementById("besq").innerHTML = '<div id="besq" name="besq"><img src="imgs/spc.gif" width="10" height="'+ alturaDocumento +'" /></div>';
document.getElementById("bdir").innerHTML = '<div id="bdir" name="bdir"><img src="imgs/spc.gif" width="10" height="'+ alturaDocumento +'" /></div>';
document.getElementById("upgo").innerHTML = '<div id="upgo" name="upgo"><img src="imgs/spc.gif" width="1" height="10" /></div>';