Draft
3.4Masonry 63
30 Density of normalweight concreteis 145lbs=ft
3
and 100 lbs=ft3
forlightweight concrete.31 Coecient of thermalexpansionis 0: 65 10
5
/degF fornormalweight concrete.32 Whenconcreteis poured(orratherplaced),thefreewaternotneededforthehydration
processevaporatesover a period of timeandtheconcretewillshrink. Thisshrinkageis about
0.05%afteroneyear(strain).Thus if theconcreteis restrained,thencrackingwilloccur
3
.33 Concretewillalsodeformwithtimedueto theappliedload,thisis calledcreep. Thisshould
be takeninto considerationwhencomputingthede
ections(which canbe upto threetimes
theinstantaneouselasticde
ection).
3.4 Masonry
34 Masonryconsistsof eithernaturalmaterials,such as stones,or of manufacturedproducts
such as bricks andconcreteblocks
4
, stackedandbondedtogetherwithmortar.35 Asforconcrete,allmodernstructuralmasonryblocks areessentiallycompressionmembers
withlow tensileresistance.
36 Themortarusedis a mixture of sand,masonry cement, and eitherPortlandcement or
hydratedlime.
3.5 Timber
37 Timber is oneof theearliestconstructionmaterials,andoneof thefewnaturalmaterials
withgood tensileproperties.
38 Thepropertiesof timber varygreatly, andthestrengthis timedependent.
39 Timber is a good shock absorber (many wood structuresin Japanhave resistedrepeated
earthquakes).
40 Themostcommonlyusedspeciesof timber in constructionareDouglas r,southernpine,
hemlock andlarch.
41 Memberscanbe laminatedtogetherundergood quality control,and
exuralstrengthsas
highas 2,500psicanbe achieved.
3.6 Steel SectionProperties
42 Dimensionsandpropertiesof rolledsectionsaretabulatedin thefollowingpages,Fig.3.7.
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3
For thisreasona minimumamount of reinforcement is always necessaryin concrete,anda 2%reinforcement,canreducetheshrinkageby 75%.
4
Mudbricks wereusedby theBabylonians,stonesby theEgyptians,andiceblocks by theEskimos...