Construction Layout Basics

 Purpose - to transfer REFERENCE POINTs (ათვლის წერტილები) & ALIGNMENT (სართულის გეგმა) from drawings to the site.

we start by measuring from known points  (they were set by surveyors who came before design. they established CONTROL POINTS/REFERENCE POINTS from which they mapped the site. they usually set those in protected/out-of-the-way places) shown on the construction plans

to set stakes (პალო) & nails at key geometry points like center lines or offset points, then

we establish elevations and locations of points relative to the proposed construction.

we use survey stakes to hold information. (variety size and shapes)

short square stakes called "hubs" (to mark horizontal and vertical positions)

tall thin stakes called "lath" act as guard stakes for "hubs" - a way to label the survey point მაგ. აწერენ 3D კოორდინატებს N:111111111.02 E:2121212121.212 ElV:123.21

all these stakes must be firmly driven to ensure stability.

also sometimes we can drive hardened nails in asphalt paving, we can also set nails in concrete if we pre-drill a hole.

ყველა შემთხვევაში reference points and lines must be: durable - marked with sufficient permanence 
& located to maximize longevity. understandable by user - adopted to the contractor's specific needs & marked with locally accepted terms. Sufficiently accurate - measured according to the intended use. 

for few weeks project - wooden stake might be ok to locate horizontal reference point, for longer projects - iron rods.

when we build some type of construction from erroneous layout can lead to costly repairs. technicians must anticipate error sources & check all work rigorously. taking time to check field notes and doublecheck all the computations

communicate clearly and proactively - "what do you want me to layout?" "how do you want me to mark it?"

observe construction practices on the site - how contractor will use laid out points

we must read construction plans proficiently and must visualize proposed work from plans.

***

heavy black lane needs to be laid out in this lot (მიწის ნაკვეთი) on a cul-de-sac (მომრგვალებული ჩიხი) in a residential subdivision

fairly typical with a pie shaped lot like this to minimize the front yard and maximize the back yard.

dash lines are the SETBACK lines and those are often shown on the subdivision plans and they are specified by the city where subdivision is being built.

what is critical in this case, as we try to put this building as far forward in the lot as possible -is the side yard setback. many contractors will avoid trying to lay this structure out themselves because of risk getting it wrong. if they get the the front corner, the one closest in the street right place but they get the building rotated just a little bit. they can end up with an encroachment (საზღვრის დარღვევა) in the opposite side yard. this can lead to very expensive modifications or expensive legal actions. therefor complex layouts like this often get contracted out to land survayors.

***

typical building layout scenario - we've laid out the building roughly and then we started to dig a hole.
but where in the hole does the building go.

Layout marks inside the excavation are prone to damage, if we put them down indicating the wall lines they are likely to be damaged by activity on the site. that's why we use OFFSET points.

offset points enable layout for digging and structure work without interfering.

one way of doing this is to use a set of batter boards with string lines crossing the 4 stakes that form the 4 corners of this simple structure:

Batter boards holding points that are set beyond the corner of the structure:

Mark appropriate locations for batter boards on this building:

we have 3 North-South lines and 3 East-West lines we need to mark with offsets beyond the limits of our excavation.

consistent offset distance  can be 3m or 4,5m or 6m (as needed)

excavation limits intended to be between the offset stakes and the building

for inside corner  we extend them beyond the far side of the excavation.

***

Another building

12 corners..too many stakes to be practical so

we will stake what we call the minimum rectangle - the smallest rectangle that completely encompasses (მთლიანად ფარავს) the building footprint.

we can see on the south-east corner 36.65ft side and 108.40ft side form one corner of the rectangle 
& north-west 105.53 and 47.43
after we can extend lines up as parallel lines L 7

when we lay this out , we need to perform independent checks to verify that we have done it properly.

one of the great risks is that our corners aren't square.

we can calculate the diagonals of this minimum rectangle  by
1) summing up (შეჯამება) the lengths that will give us the sides of our rectangle

35.65+41.68+47.43=125.76 eastwest
108.40+48.82-15.79= 141.43 ან 105.53+48.10-12.20= 141.43

2) and then Pythagorean theorem to calculate the distance from northeast corner to southwest corner and from northwest corner to southeast corner. X 

D=√(125.75² + 141.43²)=189.26  (189.24985..... ?) 

both diagonals should measure 189.26 , if they are significantly different more than one hundredth of a foot (1/8 inch) (3,175mm) or two, that we would have to do some investigative work to figure out where we have some kind of blunder (უხეში შეცდომა) or inaccuracy.

if these are building corners , that's one check we can make but really once the hole has been excavated for the foundation, those corners no longer exist and our contractor is reliant upon our offset stakes so therefore we need to check diagonals between our offset stakes. 

was 141.43 between building corners, now we added two 15ft offsets(=171.43) to it across the bottom of the triangle, but our 125.76 distance is still the same because top offset stakes are in line with the east-west lines of this structure. 

კალკულატორზე 

125.76
x2                sqr (125.76) 15,815.5776
+
171.43
x2                sqr (125.76) + sqr (171.43) 29,388.2449
=   45,203.8225
2V`x  212.61190......

D= 212.61

we dont want to do this for just a few stakes, we want to do it for as many as we can, so there are several different diagonals we would check:

instead of 171.43 now we have 156.43 (-15) and instead of 125.76 we have 140.76 (+15)
why? because offsets affect our triangle differently in this configuration. 

so its very important whenever we do construction layout that we do very careful and independent checks, we have to anticipate (გათვალისწინება) that we can make mistakes.

Todd Horton , PE, PLS Parkland Collage