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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<h1>Roofs and pylons</h1>
<figure><img class="frame-1" src="techimages/Tech1_LG_gr8_ch1_fig1_opt.jpeg" alt="Tech1_LG_gr8_ch1_fig1.tif" /> <figcaption> <p class="x--Body-Text para-style-override-1">Figure 1: Internal view of a roof structure</p></figcaption>
</figure>
<figure><img class="frame-2" src="techimages/Tech1_LG_gr8_ch1_fig2_opt.jpeg" alt="Tech1_LG_gr8_ch1_fig2.tif" />
<figcaption> <p class="x--caption"> Figure 2: Different types of electricity pylons</p></figcaption></figure>
<figure><img class="frame-3" src="techimages/Tech1_LG_gr8_ch1_fig3_fmt.jpeg" alt="Tech1_LG_gr8_ch1_fig3.tif" /> <figcaption> <p class="x--caption para-style-override-2"> Figure 3: Different types of roof trusses</p></figcaption></figure>
<h3>1.1 How can you make a roof withstand the forces acting on it?</h3>
<p class="x--Body-Text">People make and build many different kinds of things, such as houses, motor cars, roads and dams. We also make items like bottles, clothes, books and furniture. Some things, like forks and spoons and knives, are <b>solid objects</b> that consist of one part only. Other objects, like bottles, pots and water tanks, are hollow objects that can also be called <b>shells</b>. We also make objects that consist of different parts that are put together, like chairs, tables and bridges. These objects are called <b>frame structures</b> and it is important to try to make frame structures strong. </p>
<p class="headerbar">Learn about roofs</p>
<p class="x--Body-text-1-3">Fold a sheet of cardboard in the middle so that it looks like the roof of the house in Figure 4.</p>
<figure>
<img class="frame-4" src="techimages/Tech1_LG_gr8_ch1_fig4_opt.jpeg" alt="Tech1_LG_gr8_ch1_fig4.tif" /><figcaption>Figure 4.</figcaption></figure>
<p class="x--Body-investigation-hanging">1.	Use your hand to press down on the cardboard roof. What happens?</p>
<hr/>
<p class="x--Body-investigation-hanging">2.	Your house’s roof plates may be very strong and will not bend. But what will happen when a couple of big men sit on the roof to fasten the roof sheets?</p><hr/><hr/>
<p class="x--Body-text-1-3">One way to make a roof stronger is to use more and thicker materials. However, this is not always a good plan since it will cost a lot more money. It can also make the roof so heavy that the walls of the building are not strong enough to carry it.</p>
<figure> <img class="frame-6" src="techimages/Tech1_LG_gr8_ch1_fig6_opt.jpeg" alt="Tech1_LG_gr8_ch1_fig6.tif" />
<figcaption>Figure 5</figcaption></figure>
<p class="x--Body-text-1-3">A few ways to strengthen a cardboard roof model are shown in the sketches below.</p>
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<figure><img class="frame-7" src="techimages/Tech1_LG_gr8_ch1_fig7a_opt.jpeg" alt="Tech1_LG_gr8_ch1_fig7a.tif" style="max-width:80%" />
<figcaption> <p class="x--label para-style-override-3">A</p></figcaption></figure>
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<figure> <img class="frame-8" src="techimages/Tech1_LG_gr8_ch1_fig7b_opt.jpeg" alt="Tech1_LG_gr8_ch1_fig7b.tif" style="max-width:80%" />
<figcaption> <p class="x--label para-style-override-3">B</p></figcaption></figure>
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<figure> <img class="frame-9" src="techimages/Tech1_LG_gr8_ch1_fig7c_opt.jpeg" alt="Tech1_LG_gr8_ch1_fig7c.tif" style="max-width:80%" />
<figcaption> <p class="x--label para-style-override-3">C</p></figcaption></figure>
</div>
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<p class="x--caption" style="text-align:center">Figure 6</p>
<div class="technote">
<p>The word “brace” comes from the French word “bras”, which means “arm”. When you brace a structure, you put in something like an extra arm to make it stronger.</p>
</div>
<p class="x--Body-investigation-hanging--1-3-">3.	Look carefully at each of the sketches. Then write a sentence for each sketch to describe the method to strengthen the roof by <b>bracing</b> it.</p>
<p class="x--Body-investigation-hanging-a-">(a)	Case A: </p><hr/>
<p class="x--Body-investigation-hanging-a-">(b)	Case B: </p><hr/>
<p class="x--Body-investigation-hanging-a-">(c)	Case C: </p><hr/>
<p class="x--Body--above para-style-override-6">Many roofs are supported by frame structures called <b>trusses</b>. <span>Trusses can be made of wood or steel. The different parts of a truss are called </span><b>members</b><span>. Each truss has a vertical member in the middle. This is called a </span><b>king post</b>.</p>
<p class="x--Body-indent-1-3 para-style-override-7">In some truss designs, there are more vertical members. You can see more roof truss designs on the first pages of this chapter.</p>
<figure> <img class="frame-12" src="techimages/Tech1_LG_gr8_ch1_fig8_opt.jpeg" alt="Tech1_LG_gr8_ch1_fig8.tif" />
<figcaption> <p class="x--caption">Figure 7: Incomplete roof trusses</p></figcaption>
</figure>
<p class="x--Body-Text">Roof trusses have to support the weight of the roof materials, such as roof sheets.</p>
<p class="x--Body-investigation-hanging para-style-override-8">4.	What part of the roof trusses shown here will prevent them from tearing apart when the roof sheets are loaded onto them? Mark this part on one of the roof trusses on Figure 8.</p>
<figure> <img class="frame-14" src="techimages/Tech1_LG_gr8_ch1_fig9_opt.jpeg" alt="Tech1_LG_gr8_ch1_fig9.tif" />
<figcaption> <p class="x--caption">Figure 8</p></figcaption></figure>
<p class="x--Body-text-1-3 para-style-override-10">The horizontal member at the bottom of a truss prevents the two sides from ripping apart. Instead of a plank, a rope or a wire can be used to tie the bottom ends of the two sides together. When a plank or piece of steel is used for this purpose, it is called a <b>tie beam</b>.</p>
<p class="x--Body-indent">A tie beam has to be strong enough so that it will not rip apart by the forces acting on it. The weight of the roof plates pressing down on the trusses can cause the ends of the trusses to pull apart. </p>
<figure> <img class="frame-16" src="techimages/Tech1_LG_gr8_ch1_fig11_opt.jpeg" alt="Tech1_LG_gr8_ch1_fig11a.tif" /><br/>
<img class="frame-15" src="techimages/Tech1_LG_gr8_ch1_fig11b_opt.png" alt="Tech1_LG_gr8_ch1_fig11b.tif" />
<figcaption> <p class="x--caption">Figure 9: The yellow tie beam is under tension.</p></figcaption></figure>
<p class="x--Body-indent para-style-override-10">You can say that there is <b>tension</b> in the tie beam, just like there is tension in a rope you pull.</p>
<p class="x--Body-indent para-style-override-10">Forces that cause tension are called <b>tensile forces</b>.</p>
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