Why it matters
The horizontal distance to the windward edge provides context to the loading calculations about where the truss design is located within the building and is used to set the internal and external pressure coefficients. Generally, the closer the component is to an edge of the building that receives wind, the greater the pressures from the wind will be. A truss that is a distance of 0 from the windward edge is going to receive the most pressure from the wind on that side. Setting the distance too low for a truss that is far away from a windward edge could result in wind loading that is much greater than necessary for that truss.
The horizontal distance to windward edge is the shortest perpendicular distance from a truss to an edge that receives wind (note that the wind direction is considered perpendicular to the edge). Here, truss design 5 appears twice, so its horizontal distance to windward edge is the minimum of the two–equal in this case, 5’ 10-1/12”.
For the girder, truss design 1, the horizontal distance to windward edge also happens to be 5’ 10-½”, which is the shortest perpendicular distance to a windward edge from its location.
For Truss Design 7, of which 3 are pictured, the shortest distance to the windward edge from any of the trusses, 7’ 10-½”, should be used for all of the trusses. The worst case gets used for all of the designs.
The king jack, truss design 2, may be the most confusing case. The short answer is that its horizontal distance to windward edge is 0 because it intersects a windward edge. If you try to measure to the closest windward edge in a direction perpendicular to the truss, you’ll find that 0 is in fact the shortest distance.
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