Machine Design

Tent maker has the Army covered

Most branches of the U.S. military use vehicle maintenance shelters (VMS) to house and service equipment.


The VMS frame is modeled with Algor beam elements that represent 8-ft maximum lengths.

The tent fabric is modeled with membrane elements in about 2-ft. squares. Wind and snow loading is included, along with nine dead weights to represent tools and equipment hung from the rafters. The cross supports or X-cables that stiffen the sides are also visible.

An Algor Mechanical Event Simulation let the Anchor team consider the nonlinear effects of the X-cable tensioners by applying loads in 20-sec. intervals to let transients settle.

Anchor's VMS is big enough for a 2.5-ton truck and yet the structure disassembles into a container with inside dimensions of 99 34 38 in.

But what the Army had considered a lightweight structure was 1,600 lb and took at least 16 people to carry and assemble. What's more, it needed a 276-page instruction manual.

Engineers at Anchor Industries Inc., Evansville, Ind., thought they could do better and so set design goals for the next-generation VMS. They included a weight reduction of the tent's frame and covering, and keeping stresses below 60 to 70% of yield for the aluminum structure. The team also wanted to make it easier to assemble by eliminating external parts such as pins, guy ropes, and tie-downs. And to cut costs, the design would use extrusions already in the company's inventory rather than custom parts.

Senior Structural Engineer Richard D. Cook started on the redesign with a finite-element model of existing extrusions. He built the frame of several parts, including arches and purlins (horizontal beams), based on square aluminum tubes. Beams forming the arches were modeled using aluminum extrusions with channels. Fabric panels then slide into those channels, eliminating the need to secure the vinyl panels to each other. This made for faster, simplified installation as well as a smooth, weather-tight fit and maximum stability.

Cook simulated legs, arches, and purlins with FE beams, assigning them to 1 to 2-ft lengths. Splices are critical stress points, so they were meshed as separate elements. The frame included eight cross supports or X-tensioners made of 0.25-in. steel cables for stability. They were modeled with truss elements. The walls and roof were modeled with membrane elements and assembled to represent the 0.02-in. -thick vinyl-coated fabric. The complete model had about 1,200 membrane elements for the covering and 600 beam elements for the frame. A rectangular mesh covered the sides, roof, and lower ends of the cover, and a triangular mesh served for the apexes of the ends. He then uniformly meshed the structural components with smaller elements at splices on the vertical legs.

Loads representing hanging accessories inside the structure were modeled as constant forces of 100 lb evenly spaced at the center of each rafter and the peak, for a total of 900 lb. He also applied a 7-lb/ft 2 wind load and a vertical 10-lb/ft 2 snow load on the roof. Load configurations included three separate wind loadings, one snow load and a combination of wind and dead loads.

Mechanical Event Simulation from Algor Inc. let Cook consider the non-linear effects of the X-cable tensioners. Although the forces were steady, each loading lasted 20 sec consisting of 1 sec of rest to let the initial tension in the X-cables distribute itself, 17 sec of increasing load, followed by 2 sec of rest.

Cook ran models with 8 and 16-ft bays, and refined the 16-ft model with lighter aluminum extrusions for the frame and purlins and then repeated the analysis, checking to see if the extrusions withstood loads and remained within the goal of 60 to 70% of yield. After several iterations, he arrived at a model that remained within yield standards.

The final concept included two side purlins to withstand the loads from side winds. The final purlin was a 2-in.-square aluminum tube with a 0.125-in.-thick wall. Purlin splices were 1.68-in.-square aluminum tubes that slip inside the purlins. Frames were made of aluminum extrusions with channels. The leg splices are steel, as are the eve and ridge weldments.

Field tests on portions of the frame verified the simulation. "In the worst case, one purlin splice approached the maximum yield strength under the snow load. I was anxious to see if the splices were as strong as the FEA software predicted and found through direct application of weight that the software was correct. What's more, the analysis pointed to beams that are 40% lighter than initially expected."

The company later built a VMS prototype based on the simulation. The biggest cost savings came from the 40% reduction in aluminum.

Algor Inc.,
(412) 967-2700,
Anchor Industries Inc., (812) 867-2421,

TAGS: Defense
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