Assemblages Test 2

Why is carbon content important to steel quality?

carbon content is a crucial determinant of the properties of a ferrous (iron-based) metal
too much carbon makes a hard but brittle metal
too little carbon produces a soft, relatively weak material

How are steel structural shapes formed?

in the structural mill
hot steel blank passes through a succession of rollers
the rollers squeeze the metal into progressively more refined approximations of the desired shape and size

What are the most common steel beam shapes?

wide-flange shapes are used for most beams and columns, superseding the older American Standard (I-beam) shapes

How do you find the depth and width of a wide flange shape?

depth is the nominal first number after the W
width must be looked up in the Manual of Steel Construction

How are open-web steel joists formed?

prefabricated steel trusses designed to carry heavy loads, particularly bays of steel joists

What are the four series of open-web steel joists?

K series joists
Longer-span LH and DLH series joists
CJ composite joists

What are the ways of joining steel members?

Rivets
Bolts
Welds

Rivet

a steel fastener consisting of a cylindrical body and a formed head

How are rivets used to join steel members?

it is brought to a white heat in a forge
inserted through holes in the member to be joined
hot worked with a pneumatic hammer to produce a second head opposite the first
as the rivet cools, it shrinks, clamping the joined pieces together and forming a tight joint
riveting was for many decades the predominant fastening technique, but it has been replaced by the less labor intensive techniques of bolting and welding

What are the two categories of bolts commonly used in steel frame connection?

carbon steel bolts
high-strength bolts

Carbon steel bolts

similar to the ordinary machine bolts that can be purchased in hardware stores

High-strength bolts

heat treated during manufacture to develop the necessary strength
inserted into holes slightly larger than the shank diameter of the bolt and then various ways can be used to determine if they bolt is tightened to the necessary tension to transfer the load

How is welding used to join steel members?

welding can join the members of a steel frame as if they were a monolithic whole
welded connections, properly designed and executed, are stronger than the members they join in resisting both shear and moment forces

How do you know high strength bolts are tightened correctly?

pneumatic impact wrench
turn-of-nut method
load indicator washer
tension control bolt
swaged lock pin and collar fastener

What are the edges of steel members beveled for?

a beveled edge allows full access for the welding electrode so that the joint can be filled with weld metal to the same thickness and strength as the member being connected

Moment connection

a joint that allows the transfer of bending moment forces between a column and a beam
connection of flanges strongly across the joint by means of a full-penetration groove welds across the beam flanges

framed connection

a beam is connected to a girder or a stanchion by means of two angles placed on the two sides of the web of a beam
joins only the web of the beam and not the flanges

How do you stabilize a steel frame against lateral loads?

diagonal bracing
eccentric bracing
moment connections
shear panels

How do you attach a steel column to a foundation?

steel baseplates, which distribute the concentrated loads of the steel columns across a larger area of the concrete foundation, are shop welded to all but the largest of columns
the baseplate is then attached to the foundation with anchor bolts cast into the foundation

How do you plumb-up a steel frame building?

plumbing up is straightening and squaring the steel frame in order to make sure that the frame is level overall
the two-story pier of framing is them plumbed-up with diagonal cables and turnbuckles while checking the alignment with plumb bobs, transits, or laser levels

Why are columns spliced at waist level?

column splices are made at waist level above this platform, both as a matter of convenience and as a way of avoiding conflict between the column splices and beam-to-column connections

What materials can be used to fireproof steel members?

brick
concrete
metal lath and plaster
gypsum board
spray-on fireproofing
loose insulating fill inside a sheet metal enclosure
water-filled box column made of a wide-flange shape with added steel plates
rigid slab fireproofing
suspended plaster ceiling
mineral fiber slabs
intumescent mastics and paints

What are some types of improved beams?

castellated beams
plate girders
rigid frames

Castellated beams

produced by flame cutting the web of a wide-flange section along a zigzag path, then reassembling the beam by welding its two halves point to point, thus increasing its depth without increasing its weight

Plate girders

long-span beams tailored to any loading condition custom designed and fabricated

Rigid frames

efficiently produced by welding together steel wide-flange sections or plate girders

How do tensile structures work?

a tensile structure is a membrane that is supported by masts or other rigid structural elements such as frames or arches
the fabric and cables transmit external loads to the rigid supports and ground anchors by means of tensile forces

How are light gauge steel framing members produced?

cold-formed to differentiate from the much heavier hot-rolled shapes that are used in structural steel framing

What shapes does light gauge steel framing come in?

C-shaped sections: studs, joists, rafters
Runner channel sections: top and bottom track

Why is gauge of the steel important?

the strength and stiffness of a member depends on the shape and depth of the section and the gauge (thickness of the steel sheet from which it is made

How is welded wire fabric used?

a reinforcing mesh of welded wire fabric, cut to a size just a bit smaller than the dimensions of the slab, is laid over the moisture barrier or crushed stone
the fabric most commonly used for lightly loaded slabs
the grid of wires and bars helps protect the slab against cracking that might be caused by concrete shrinkage, temperature stresses, concentrated loads, frost heaving, or settlement of the ground beneath

Why are control joints important for a slab on grade?

control joints must be provided at intervals in a slab on grade
the function of a control joint is to provide a place where the forces that cause cracking can be relieved without disfiguring the slab
the reinforcing mesh is discontinued at each control joint as a further inducement for cracking to occur in this location rather than at random

control joint

a straight, intentional crack that is formed before the concrete has hardened fully

How do form ties work?

form ties are inserted through holes provided in the formwork panels and secured to the back of the form by devices supplied with the form ties
the ties will pass straight through the concrete wall from one side to another and remain embedded permanently in the wall after it is poured

Form ties

small-diameter steel rods specially shaped to hold the formwork together under the pressure of the wet concrete

What are the types of formwork used?

wall forms may be custom built of lumber and plywood for each job, but it is more usual to use standard prefabricated formwork panels
insulating concrete forms
lift-slab construction
flying formwork
slip forming
tilt-up construction

Insulating concrete forms

an alternative way of casting a concrete wall, particularly one that will be an exterior wall of a building, is to use insulating concrete forms that serve both to form the concrete and to remain in place permanently as thermal insulation

Lift-slab construction

used chiefly with two-way flat plate structures, virtually eliminates formwork
the floor and roof slabs of a building are cast in a stack on the ground
the hydraulic jacks are used to lift the slabs up the columns to their final elevations, where they are welded in place using special cast-in-place steel slab collars

Flying formwork

for slabs that are cast in place, flying formwork is fabricated in large sections that are supported on deep metal trusses
the sections are moved from one floor to the next crane, eliminating much of the labor usually expended on stripping and re-erecting formwork

Slip forming

useful for tall walled structures such as elevator shafts, stairwells, and storage silos
a ring of formwork is pulled steadily upward by jacks supported on the vertical reinforcing bars, while workers add concrete and horizontal reinforcing in a continuous process

Tilt-up construction

a floor slab is cast on the ground, and reinforced concrete panels are poured over it in a horizontal position
when curing is complete, they are hoisted into position by a crane and grouted together, thereby eliminating most of the usual wall formwork

How are concrete columns formed?

the column form may be a square box of plywood or composite panels, a cylindrical steel or plastic tube bolted together in halves so that it can later be removed, or a waxed cardboard tube that is stripped after curing by unwinding the layers of paper that make up the tube

Why are reinforcing bars offset at the top of columns?

where vertical bars overlap, the tops of the bars from the column below are offset (bent inward) by one bar diameter to avoid interference

What are the various types of one-way floor and roof framing systems?

one-way solid slab system
one-way concrete joist system (ribbed slab)
wide-module concrete joist system

One-way solid slab system

spans across parallel lines of support furnished by walls and/or beams

One-way concrete joist system (ribbed slab)

as one-way solid slab spans increase, a progressively thicker slab is required
beyond a certain span, the slab becomes so thick that the weight of the slab itself is an excessive burden, unless a substantial portion of the nonworking concrete in the lower part of the slab can be eliminated to lighten the load
the bottom steel is concentrated in spaced ribs or joists
the thin slab that spans across the top of the joists is reinforced only by shrinkage-temperature bars
there is little concrete in this system that is not working, with the result that are one-way concrete joist system can span considerably longer distances than a one-way solid slab

wide-module concrete joist system

when fire resistance requirements of the building code dictate a slab thickness of 4.5" or more, the slab is capable of spanning a much greater distance than the normal space between joists in a one-way concrete joist system
joists are spaced 4-6' apart

What is special about two-way floor and roof framing systems?

two-way concrete framing systems are generally more economical than one-way systems in buildings where the columns can be spaced in bays that are square or nearly square in proportion

What are the various types of two-way floor and roof framing systems?

two-way flat slab
two-way flat plate system
two-way waffle slab system

Two-way flat slab

a system suited to heavily loaded buildings such as storage and industrial buildings
this formwork is completely flat except for a thickening of the concrete to resist the high shear forces around the top of each column
traditionally, this thickening was accomplished with both a funnel shaped mushroom capital and a square drop panel, but today the capital is usually eliminated to achieve a greater economy of formwork cost leaving a drop panel to do the work alone

Two-way flat plate system

in more lightly loaded buildings the slab need not be thickened at all over the columns
because there are no beams and girders, only a thin slab, the story heights of the building may be kept to an absolute minimum

Two-way waffle slab system

the waffle slab, or two-way concrete joist system, is the two-way equivalent of the one-way concrete joist system
metal or plastic pans called domes are used as formwork to eliminate nonworking concrete from the slab, allowing considerably longer spans than are feasible n the two-way flat plate system

What are the innovations in sitecast concrete?

lightweight concrete and admixtures
formwork materials and methods
lift-slab
flying formwork
gang forms
slip forms
tilt-up
shotcrete

What are the spanning differences of the different precast floor elements?

solid slabs - short spans and minimum slab depths
hollow-core slabs - intermediate spans
single tees - longest spans
double tees - longest spans

What are topping slabs used for?

after the slab elements have been erected, a concrete topping is poured over them and finished to a smooth surface
the topping, usually 2" in thickness, bonds during curing to the rough top of the precast elements and becomes a working part of their structural action
the slab elements may be topped or untopped

What are the standard precast concrete beam and girder shapes?

rectangular beam
L-shaped beam
inverted tee beam
AASHTO beam

How are stirrups at the top of precast elements used?

mild steel reinforcing bars are used for stirrups
the projecting tops of the stirrups will bond to the sitecast topping

Why are L-shaped and inverted T-shapes important?

the projecting ledgers on L-shaped beams and inverted tees provide direct support for precast slab elements
they conserve headroom in a building by supporting slabs near the bottoms of the beams, as compared to rectangular beams without ledgers, where slab elements must rest on top

What is used to connect precast elements?

bolting
welding
grouting
the simplest joints in precast concrete construction are those that rely on gravity by placing one element atop another, as is done where slab elements rest on a bearing wall or beam, or where a beam rests on the corbel of a column
posttensioning can be used to combine large precast elements into even larger ones on the site

What is posttensioning used for in precast beams?

a posttensioned, structurally continuous beam-column connection may be created by passing a tendon from a pocket in the top of one beam, through the column, to a pocket in the top of the other beam
the tendon is anchored to a plate in one pocket as it is tensioned by a jack in the other pocket

Why is a masonry cavity wall important?

a cavity wall prevents water from reaching the interior of the building by interposing a cavity between the outside and inside wythes of the wall
insulation
reduces dead weight

What is an advantage of using adjustable masonry ties?

adjustable masonry ties are convenient for the masons because they allow for irregularities in course heights

What is a disadvantage of using adjustable masonry ties?

they may be insufficiently rigid for some structural purposes

Where are internal flashings used?

typically at the bottom of a cavity wall
at every location where the cavity is interrupted, such as:
above the heads of windows and doors
below copings and window sills
below lintels and above shelf angles
over spandrel beams

What happens when an internal flashing meets the exterior surface of a masonry wall?

the flashing should be carried at least 3/4" beyond the outside face of the wall
flashing should be turned down at a 45 degree angle so that draining water drips free of the wall
if the internal flashing is made of a flexible material, it should be cemented to a sheet metal flashing just before it exits the wall

What are end dams used for?

to keep water from running off the ends of sill and lintel flashings

Why are high quality flashing materials important in masonry walls?

flashings within a wall are almost impossible to replace if they should fail in service
even the most expensive flashing materials cost only a very small fraction of the total price of a masonry wall, so there is little reason to use cheap ones in a misguided effort to save money

What spanning systems may be supported on masonry bearing walls?

wood balloon framing
heavy timber framing
structural steel framing
sitecast concrete framing
precast concrete framing

Which masonry units shrink after installation and which expand?

new clay masonry units tend to absorb water and expand under moisture conditions
new concrete masonry units usually shrink somewhat as they give off excess water following manufacture

Does brick masonry have any tensile strength?

no, the allowable tensile strength of brick is 0 psi

Does reinforcing need to remain continuous through an expansion joint and control joint?

joint reinforcing must be interrupted at movement joints so that it does not restrain the opening or closing of the joint

What does efflorescence indicate?

water has begun to enter the wall
best controlled by investigating and correcting the source of leakage

What do you need to do to mortar when outdoor temperatures are below freezing?

warming the mixing water (and sometimes the sand as well) to produce mortar at the optimum temperature for working and curing
enclosure
using a Type III (high early strength) cement to accelerate the curing of mortar
mixing the mortar in smaller quantities so it does not cool excessively before it is used
chemical accelerators and so-called "anti-freeze" admixtures are, in general, harmful to mortar and reinforcing steel and should not be used