More 'Concept' Questions

[architect23]

1) A specific wide flange beam is required for architectural reasons. Calculations show that its strength in shear and bending are satisfactory, but that its deflection is 20% more than allowed. Without changing the section or welding additional steel to it, how can the deflection be brought down to the allowable value?

-By using a steel with 25% higher yield
-By using a steel with 44% higher yield
-By using a steel with 73% higher yield
-Deflection cannot be reduced by changing the grade of steel

-The answer is "Deflection cannot be reduced by changing the grade of steel." Is it because the grade of steel is always a constant?

2) Which of the following construction types has the lowest dead load to live load ratio?
-Precast concrete, prestressed tees
-Open web steel joists, metal deck, and concrete slab
-Cast in place concrete waffle system
-Flat chord wood trusses with plywood sheathing

Can someone explain what the low dead load to live load ratio means? Does this mean dead load and live load must be similar? I am having trouble understanding the language of a "low ratio."

[introversed]

When you're taking about deflection, yield strength has nothing to do with it. What matters is another factor which measures how elastic the steel is, or how much it will stretch/squeeze under a given load. The technical term is MODULUS OF ELASTICITY (also called E), which is always 29,000 ksi for steel.

If you haven't signed up with Thaddeus, please do so. He's got a great workbook on the deflection equation and how t works.

Question 2:

Yes, this is absolutely a section modulus question. For a simple beam like this, the formula is S=bd^2/6. In other words, the strength of the beam is actually looking at the SQUARE of the beam depth, not merely the beam depth. Hence, 4 is the answer..

Question 3:

Sprayed fiber is the answer because of the LABOR COST. All the other answers require at least a day of labor, but spray on fireproofing is done by one guy with a pressured nozzle in minutes

[architect23]

Thank you- I guess I need more clarification on the above two. I think with the first one, it's the way that the question is worded.
My understanding is that (d) is the answer because its modulus of elasticity will never change (29,000 KSI)- therefore deflection is not affected?

I am sorry if this is repeating anything, having trouble understanding the relationships presented between the concepts here.

If anyone can offer input on the following one, I'd appreciate it

2) Which of the following construction types has the lowest dead load to live load ratio?
-Precast concrete, prestressed tees
-Open web steel joists, metal deck, and concrete slab
-Cast in place concrete waffle system
-Flat chord wood trusses with plywood sheathing

[Coach]

2) Which of the following construction types has the lowest dead load to live load ratio?

Wood.

[architect23]

thanks, I should clarify, I have the answer as listed above but am having trouble understanding what the question is asking :

Can someone explain what the low dead load to live load ratio means? Does this mean dead load and live load must be similar? I am having trouble understanding the language of a "low ratio."

[corbismyhomeboy]

If you take the dead load of the material and divide by the live load it can handle, that would be the ratio. So you're looking for what material makes the smallest fraction aka has the lowest dead load with the highest live load. They aren't expecting you to figure this out mathematically (there are no numbers in the problem), but using what you know, which material is lightest and can still handle a lot of load?

In regards to the Modulus of Elasticity question, go look at the equation for deflection. What are the variables you see there and what do they represent? Try to understand their relationships. If you increase the E, what happens to the overall deflection? If you decrease the I, what happens to the deflection? etc., Talk your way through it. The grade of steel is not a variable in the equation for deflection therefore has no impact on the deflection.

[introversed]

D is the answer because it's the only answer that has correct information.