A multilayer interlayer structure having a first and second polyvinyl acetal (poly(vinyl acetal)) layer and a cellulose ester layer having a thickness of at least 10 mils disposed between the first and second poly(vinyl acetal) layers. The cellulose ester layer can have a higher storage modulus and/or higher Tg than at least one of the poly(vinyl acetal) layers. The interlayer structure is useful to make glass panels having high stiffness and which possess good optical clarity for a variety of applications, including outdoor structural applications.