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enter image description here

I know that the King Post and "Truss Web" or "braces" are there to make the whole truss "stronger", but can someone explain what are they actually supporting if there is no load on the bottom chord/tie beam?

If you do a truss analysis with loads on the "upper" nodes, the kingpost and webbing are 0 members. I'm guessing you can't actually do a truss analysis on this (altho it is called a truss) because the majority of forces are bending forces? the braces are there to keep the rafters from bending and the kingpost is there to keep the bottom chord/tie beam from sagging?

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  • $\begingroup$ Those bottom chords are in tension when the top is loaded. $\endgroup$
    – Solar Mike
    Commented Dec 18, 2019 at 14:12
  • $\begingroup$ @SolarMike I know that the bottom cord is there to hold the ends of the rafters from moving out when the top is loaded. I was wondering what the king post and truss web are doing when only the top is loaded? $\endgroup$ Commented Dec 18, 2019 at 14:22
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    $\begingroup$ You state in your question that there is no load on the bottom chord - which is why I commented. $\endgroup$
    – Solar Mike
    Commented Dec 18, 2019 at 14:53
  • $\begingroup$ The plate is used to join the members together and transmit forces between members although the king post is redundant, mathematically speaking, it does resist other variable forces as a safety issue. $\endgroup$
    – Rhodie
    Commented Oct 30, 2022 at 9:55

3 Answers 3

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These are usually prefabricated trusses and should be able to handle all potential variety loading on a roof.

  • In typical light framing structures, rafters are placed at 16" O.C. which will cause moments and will load the web members.

  • Wind load depending on the angle will stress the king post, the web members and even can cause uplift.

  • The bottom cord usually supports the ceiling wight through ceiling joists.

  • The roof may have unsymmetrical seasonal loads such as snow piling up on wind leeward, or rooftop installation of chimineas, HVAC, vents, and live load of the repair crew.

All these and cases not mentioned above will cause loading on all different members of a typical roof truss.

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  • $\begingroup$ There is also the weight of the roofing materials. $\endgroup$
    – Eric S
    Commented Dec 19, 2019 at 0:02
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If the truss is prefabricated with metal plates at the joints as in your picture, it isn't a truss any more.

Pin jointed trusses are a very useful educational tool to teach people how to resolve forces and moments by hand, but that doesn't mean real-world 21st-century structures are built that way.

Medieval wood framed structures were literally pin-jointed, as in this Wikipedia image, but that was then.

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  • $\begingroup$ Will these modern prefabricated efforts have the same lifespan as the medieval ones? :) $\endgroup$
    – Solar Mike
    Commented Dec 19, 2019 at 9:03
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Your guess is correct. Although it is called a "truss", you can't do "truss analysis" on this for most typical loads. Truss analysis implies that all members only carry axial tension or compression. Therefore, for truss analysis, you need (1) pin-joints and (2) loads applied only at the nodes.

(1) As @alephzero said, the metal plates which act as moment connections mean that this is not suitable for truss analysis. However, truss analysis would not usually be suitable for old wood frames either, even though they were literally pin-jointed, due to the way the structure is loaded.

(2) As @kamran said, different loads will load the members in different ways. Furthermore, a uniform distributed load (e.g., something as simple as dead weight) would cause bending moments and shear forces in the members. If you take a free-body diagram of the connection (aka method of joints) between the truss web and top chord and include shear forces and moments in the top chord, you would find non-zero forces in the truss web.

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