Structures Flashcards
Modulus of Elasticity
Property of material itself that describes it’s stiffness relative to strain
How does beam depth and girder dpeth generally relate to beam span? How does the width relate to the depth?
Beam depth is about 1/20 of the span. Span = 60' Depth = 3' Girder depth is about 1/15 of the span Width is 1/3 of its depth Depth = 3' Width = 1'
Bolt connections
High strength bolts:
- Bearing-type holds it in shear
- Slip critical is super tightened and operates in tension
Shear connection
Resists gravity forces but transfers shear forces. Only bolted on web but not flanges. If there’s bending in the beam, the columns don’t bend (but lateral forces will cause it to sway).
Moment connection
Resists gravity and shear forces. Bolted on web and flange. If there’s bending in the beam, there’s bending in the columns.
Fully restrained = more rigid, negligible rotation between connected members
Partially restrained = transfers moment with some rotation between connected members
Braced frame
Uses shear connections for most and one or more bays with diagonal bracing to resist lateral forces. More cost effective than moment frame.
Eccentrically braced frame used in seismic areas (less likely to buckle)
Shear wall
Resists lateral/shear forces. Need one in X, Y and Z directions.
Total loaded width of column
Tributary area it is supporting x number of floors it is supporting
Staggered truss system
Every other floor is a truss and spans the full width of the building. Allows column-free floor plates. MInimizes floor to floor height.
What provides more resistance to lateral forces? Rigid core or rigid perimeter?
Rigid perimeter is more stiff because it acts across larger area
Composite steel and concrete columns/beams pros and cons
Encase WF in concrete or fill HSS tube with concrete
Pro: reduce amount of steel used by half
Cons: Too heavy for beams
Shear stud
Steel pins that attach poured concrete floor slab to top of steel beam with welded connection to resist shear forces
Girts
Metal channels that run horizontally between columns (in z channel family). Metal wall panels attach to them.
How tall can an unprotected steel structure be built?
5 stories
Which steel members do not have to be fire proofed?
Members that only resist lateral forces like cross bracing
What adds embodied energy to a material?
Higher finish Heaver material More petroleum used More glue used More heat used in production More mining or refining Less recycled content US: 90% steel recycled, 30% aluminum recycled
Which open web steel joist type spans the longest?
DLH - 150’
LH - 96’
K - 60’ (most common)
List metals from most anode to most cathode that contribute to galvanic action (corrosion)
- alum
- zinc
- steel
- iron
- stainless steel - active
- tin
- lead
- copper
- stainless steel - passive
The further materials are from each other, the more current that will flow between them and speed corrosion (electrolysis). Best choice is using the same metal for everything touching but also be careful with other alkalyine materials like mortar and concrete. Separate materials with a nonreactive material such a rubber, neoprene or plastic.
K values for likelihood of column buckling
Different connections have different values (found in table). Multiply length of column x k to determine how it will behave under buckling. Higher value means higher likelihood of buckling.
Ex. 10’ column with k value of 0.5 will behave more like a column 5’ high.
Theoretical k value is used for steel (consistent)
Recommended k value is used for wood (has variation)
Where are rotation fixed and translation free connections used?
In seismic conditions (allowed to pivot side to side)
What controls the beam size calculation the most?
Bending moment
Other: shear, deflection
Relationship between column length and…
a. shear
b. moment
c. deflection
a. shear: L
b. moment: L^2
c. deflection: L^3
Ex. doubling the L means: Vx2, Mx4, Dx8
Section modulus
A geometric property for a given cross-section used in the design of beams. Larger section modulus means less bending stress. Fb = M/S Fb: bending stress M = bending moment S = section modulus
Soft story problem
When buildings are heavier on top than bottom in seismic zones.
Where can you notch or put a hole in a wood beam or stud for mechanical ducts or pipes? What about studs?
- You can notch end up to 1/4 of depth
- You can notch anywhere else 1/6 of depth and width of notch has to be less than 1/3 the depth of the beam
- Cannot notch in middle 1/3 of beam
- A hole can be put anywhere but diameter must be 1/3 of the depth max. and the hole needs to be at least 2” from edge of joist. If there is a second hold, it needs to be at least 2” away from first.
- In studs, max diameter of hole is 40% of stud depth and no closer than 5/8” from edges. Notch cannot exceed 25% of stud depth. If pipe is bigger than 40%, you can get up to 60% if you double stud up.
Direct tension indicator
Washer under bolt with bumps that get flattened out as you tighten the bolt to know if you tightened it enough.
Types of buildings moment resisting frames are appropriate for
Low rise and high rise up to 30 stories. Higher than 30 should use x bracing.
Wood member spans: Joists Solid beams Glulam beams Heavy trusses
Joists: 20’
Solid beams: 30’
Glulam beams: 100’
Heavy trusses: 200’
One way vs two way slab
Slab will behave like a one way slab if aspect ratio is more than 2. (long length / short length > 2)
Steel member spans: Decking Lightweight joists Beams Open web joists Heavy trusses Space frame Suspension
Decking: 20' Lightweight joists: 30' Beams: 75' Open web joists: 150' Heavy trusses: 300' Space frame: 500' Suspension: 500+'
Concrete span ranges One way Two way Waffle slab Beams Precast slabs Precast double tees Precast single tees
One way: 20' Two way: 40' Waffle slab: 55' Beams: 70' Precast slabs: 45' Precast double tees: 100' Precast single tees: 150'
Structural systems recommended for a library
Reasons: heavy live loads of stacks, long spans
Steel beams and girders
Sitecast concrete beams and girders
Sitecast one way or two way slab
Structural systems recommended for a concert hall
Reasons: fire protection, long spans
Steel beams and girders Steel trusses Sitecast concrete beams and girders Concrete one way or two way slabs Waffle slab
Structural systems recommended for tall buildings
Reasons:
Steel beams and girders Steel single story rigid frame Sitecast concrete beams and girders Two way flat plate slab Precast beams and girders Hollow core planks
When to use a column and slab system
No beams means columns can be located more freely
Possibly more economical due to simplicity and reduction in floor depths
Most are sitecast concrete systems but can also be space frame systems
Precast concrete
High early strength mix (type III) and steam curing allow shapes to cure quickly for faster production. Warehoused until they have passed 28 day cylinder test.
Shapes: floor planks, beams, columns, wall panels and domes
Conditions better controlled than site cast concrete.
Slip joint at head of aluminum storefront purpose
Accommodate deflection of the structure above
Types of attachments for securing precast panel to floor assembly
2 Points of Bearing: One rigid and one tie back which allows for both vertical and lateral movement
Allowable stress ratings for lumber are based on what?
The wood species
Depths of open web joists
K - 30”
LH - 4’-0”
DLH - 6’-0”
Depth of concrete beam relative to span
1/16
Semi-rigid connection
Beam or end plate welded all around. Limited moment to compensate for. No tab plates or shear bolts required.
How to calculate snow load on a ridgebeam per lf
Snow load x 1/2 of total beam span
Which beam is the heaviest? W 27 x 178, 20’ long W 24 x 104, 40’ long W 21 x 147, 30’ long W 18 x 119, 35’ long
Multiply weight per ft x length for each: W 27 x 178, 20’ long = 3,560 pounds W 24 x 104, 40’ long = 4,160 pounds W 21 x 147, 30’ long = 4,410 pounds W 18 x 119, 35’ long = 4,165 pounds
