tubing welded together in such a manner that all members  of the truss can carry both tension and compression loads.  [Figure 1-15]  In some aircraft, principally the light, single- engine models, truss fuselage frames may be constructed of  aluminum alloy and may be riveted or bolted into one piece,  with cross-bracing achieved by using solid rods or tubes. The monocoque (single shell) fuselage relies largely on the  strength of the skin or covering to carry the primary loads.  The design may be divided into two classes:  Different portions of the same fuselage may belong to either  of the two classes, but most modern aircraft are considered  to be of semimonocoque type construction.  The true monocoque construction uses formers, frame  assemblies, and bulkheads to give shape to the fuselage.  [Figure 1-16]  The heaviest of these structural members are  located at intervals to carry concentrated loads and at points  where fittings are used to attach other units such as wings,  powerplants, and stabilizers. Since no other bracing members  are present, the skin must carry the primary stresses and  keep the fuselage rigid. Thus, the biggest problem involved  in monocoque construction is maintaining enough strength  while keeping the weight within allowable limits. To overcome the strength/weight problem of monocoque  construction, a modification called semimonocoque  construction was developed. It also consists of frame  assemblies, bulkheads, and formers as used in the monocoque  design but, additionally, the skin is reinforced by longitudinal  members called longerons. Longerons usually extend across  several frame members and help the skin support primary  bending loads. They are typically made of aluminum alloy  either of a single piece or a built-up construction. Stringers are also used in the semimonocoque fuselage. These  longitudinal members are typically more numerous and lighter  in weight than the longerons. They come in a variety of shapes  and are usually made from single piece aluminum alloy  extrusions or formed aluminum. Stringers have some rigidity  but are chiefly used for giving shape and for attachment of  the skin. Stringers and longerons together prevent tension  and compression from bending the fuselage.  [Figure 1-17] Figure 1-16.  An airframe using monocoque construction.  Monocoque Type Semimonocoque Type Figure 1-17.  The most common airframe construction is  semimonocoque. Figure 1-15.  truss-type fuselage. A Warren truss uses mostly  diagonal bracing. 1-9 1. Monocoque 2. Semimonocoque Skin Former Bulkhead Skin Stringer Bulkhead Longeron Longeron Vertical web members Diagonal web members