The arches in your image are basically half circles, so if you are building on the ground and have space underneath the build where no-one can see, you can always use the round tube prop (a cylinder with a hole in it) and place it so only the top half of it appears and the bottom half is buried in the ground. It will look like your image. All you have to do is re-size the prop, make it very short/thin and flip it sideways 90 degrees using snap rotate. (Magnet icon with bent arrow, make sure this is highlighted to enable, go into settings and enter "45" in the snap rotate field, any more than this is too hard to drag, with snap rotate enabled, select the prop, click the green rotate icon so the circles appear all around it, find the line which goes in the direction you wish to rotate it in and click and drag that line, nothing will happen until you have moved it 45 degrees and it will snap into place, drag it once more and it will be 90 degrees and sitting sideways and then you can sink it into the ground and hide the bottom and rotate it around horizontally so the arch is facing in the direction you want it to.)
To make the lighted part, you would use (in materials section) the first tile in the emission section, this one is on all the time and does not flash or glow on and off. You can change this emission to any colour or intensity you want using the colour palette. To make the underside lit part, all you have to do is clone the original piece, then move it and re-size it so it only shows visible where you want it to, on that surface/side. This does not mean you have to make it completely thin, as long as it is contained within the other piece and nothing of the new piece is sticking out, you will only see the slice where you want it to regardless of how big the object is. Clone the prop and make it any contrasting pattern so you can see the two pieces clearly, at this point it does not matter what the pattern is. With the cloned piece selected, click the left hand green move icon (4 arrows) and using the yellow central box to drag, slide the whole thing in the direction you wish it to show up, whether a couple of pixels only or a thicker slice. When you have the surface of the new cloned object with the contrasting pattern showing where you want it to, then click on the re-size green icon and drag the edges of the cloned object towards the center of itself so that all sides are buried inside the original prop and only the one side you wish to have visible is showing. By now you should have the original object, and along one surface of it, a contrasting pattern. With the cloned object still selected, switch to the emission texture and adjust the colour and intensity as desired. When you are happy with your emission texture, close the colour palette, it will save your colour choice even if you close it. (EDIT: see below) Repeat the above for all the surfaces you wish to create, when changing the texture to the emission, and having selected the correct emission tile first, open your colour palette and making very sure not to move the colour dot in any way, just click onto the dot once and the surface will then match the previous one. This is the way I often do it, but if you wish to change all of them to the new texture in one step at the same time which can be easier, you can leave the temporary texture in place until you have made all your new surfaces, then select all objects at once (hold down CTRL and left click so they are all highlighted green) and work on your texture/colour/light-intesnity on all parts at once, as this is possible in multi-select mode.
*edit* I'm silly, looking at your image, you will only need to create one arch and lit surface then you can clone them, so no need to open and shut the colour palette. Make one arch and clone it and make the contrasting layer, then apply the emission texture and adjust the colour and intensity. When you are happy with it, you will only have two pieces. So select both of them and click the copy icon, then drag the new copy along, when it is where you want it to be, copy them again and repeat until you have all your arches. Each clone will copy the colour settings on the lit part anyway so no need to redo them. If you wish to make it exact and don't want to guess the gaps, enable 'snap move' (magnet icon with straight arrows) and drag the cloned pieces along using this method (you may have to fly way up high in the air and backwards somewhat to have enough room to snap them along if they are large, with snap move you need the 'screen space' to be able to drag them along, and if your cursor reaches the top of the screen, or any edge of the screen before snapping to the new position, it will not move, depending on the setting you have for snap move of course. So you need plenty of open screen space to make it snap along.).
Edited by Rockster, 13 September 2018 - 10:14 PM.