Structures questions

[architect23]

Two more from my set which I couldn't quite grasp.. thanks again for any input

Forgive my ignorance on what are probably basic questions-

1) For the first question, Thaddeus says concrete lift slab designed as flat plate is illegal. Can someone clarify?
Why are the other three choices incorrect in terms of access, cost, headroom, loading, construction time?

2) My answer was compression, since force is being pushed down on this vertical member. Why is this member in tension?

Thank you!

[introversed]

First things first.

These aren't basic questions at all. These are top-level exam level questions. You should give yourself more credit. And kudos to you and everyone else pushing through the SS exam study, because it's probably the most dreaded exam among all architects in the western hemisphere.

Here's a rule of thumb which explains the answer to question 1: Always use steel when you're adding new structure to an older building
There are two reasons for this. One reason is that steel structures are much lighter than concrete structures carrying the same load. Existing buildings have already been engineered to support a certain amount of weight, so if you're going to add more weight, add a little as possible by using steel. Otherwise you'd need to reinforce the hell out of the old building, and the cost and scope of those structural improvements starts to multiply really quick. The Second reason is that it's much easier and quicker to park a truck of prefabricated steel members in front of the building, open a hole at ground level, carry them in, and install them. Pumping concrete requires 28 days of curing, bad for the time constraint.

[architect23]

Thank you!!

Can anyone explain the tension in the member of the truss below? I often have trouble understanding when truss members would be in tension or in compression. Yes, I took Thaddeus, but his live course was very fast paced.

[corbismyhomeboy]

When the load presses down, it causes the legs of the scissor truss spread apart. The space where the dashed line is widens. The dashed line has to stretch out (aka it's in tension) to connect between the two points.

I took the online Thaddeus course, the go at your own pace one, and it was great! Do you still have your workbook from the class? There are a lot of notes in the key that may help answer some of your questions. Lots of good color-coded diagrams too. He actually uses this exact example of a spreading scissor truss to explain why the dashed member is in tension.