Why does purlin spacing matter when doing truss analysis? Even if individual pur

Question

Why does purlin spacing matter when doing truss analysis? Even if individual purlins are spaced differently, wouldn't they send the same amount of loading into the respective joints on the truss? So how does changes in purlin spacing change the results in my truss analysis?

Files Home @C ) Building Load https://prod.ca 0 0709 PMS5555- y Roof truss .x how to do struc * truss structured purlin blocking: trusses and puri e chegg Tutors r.com/Streamsen let/Roof+Truss + Design + Project +Summer+201 7 + Revised pdfin CQ, engineering help forums ® of 3 - Automatic Zoom Objective: The Fun City Picnic Park has put out an RFP (Request for Proposals) on the design of a large open air pavilion. Your portion of the project is to design the wooden trusswork to support the roof, including layout of the truss geometry and selection of individual member sizes. You have to analyze and compare alternate designs by varying the truss geometry before selecting a final design. Geometry: Building dimensions are 36 ft by 75 ft, with the roof trusses to span the 36 ft dimension. Bays are spaced 25 ft apart, so the building will have 3 bays requiring a total of 4 identical roof trusses. Roof pitch (rise: run) is to be a minimum of 1:4.5 (4 ft high) and a maximum of 1:2.25 (8 ft high). To adequately support the selected roofing, roof purlins will be spaced no more than 8 feet apart. See sketch below. Individual member lengths are restricted to no more than 10 feet. Alternate your design by (1) changing the purlin spacing and by (2) changing the roof pitch. This combination of roof pitches and purlin spacings gives gives you 3 alternate designs (pitch 1 with purlin 1, pitch 1 with purlin 2, pitch 2 with purlin 1) to be analyzed. Purlins ft min. 8 ft max 8 ft max 7:30 PM 7/23/2017 Type here to search

Explanation / Answer

Purlin spacing is important because the load coming on each purlin depends on the purlin spacing. The reason being , the load on each purlin = area load* tributary area of each purlin; tributary area of purlin = purlin span * spacing of purlin

Hence, the load on each purlin is a function of the purlin spacing.

Since purlins collect the lod from the roofing material and transfers to the truss, the joints at which the loads are applied is again a function of the positioning of the purlins and hence spacing of spacing comes into picture here as well. i agree, that the total load on the truss will remain the same irrespective of the spacing of the purlins. However, the spacial distribution of the purlins changes the spactial distribution of the loads on the truss which affects the truss and truss member analysis and forces.

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