Ársrit Verkfræðingafjelags Íslands - 01.01.1914, Page 29

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IV. Reports. — Referate.
1. l'he Bridge across Ytri-Bangá.
A lecture given in tlie Association of Civil-Engineers in Iceland on Okt. 15‘ll 1912 by Jon Thorlaksson civil engineer,
1. The site of the bridge.
Ytri-Rangá has its source in the western edge of
the lava round Hekla and flows in a southwesterly
direction through Rangárvalla-district, between Holt
and Land on the west side and Rangárvellir on the
east side. It joins Þverá a little distance above the
latter’s mouths. Already in 1899 a road passable for
carriages had been carried forward to the western
banlc of the river at Ægissíða in Holt. The highway
led across the river there, which was passed by a
ford. It was very difficult to cross the ford with
carriages, but the country to the east of the river is
dry and llal, all easily passable for carriages, so that
no roads are needed there. The need for a bridge
across Ytri-Rangá was tlierefore seriously felt, and
after some delay caused by difíerence of opinion
about the most suitable site for the bridge, the Rudget
for 1912 provided 45 thousand ”krónur“ *) for the
building of it.
The sile of tlie bridge is immediately above the
ford, mentioned above. A cross-seclion of the site is
shown on the map, Plate 1 (pag 8). At the western bank
tliere is a rock level with the water, but everywhere else
a lajrer of sand on a cliff of sandstone. The current
progresses at a rate of 0,9 metre per second at the
deepest parts when the river is lowest, and the ave-
rage rate is probably about 0.7 m. per sec. So the
least quantity of water is very likely about 50 cl,!,'/Sec.
The sandlayer above the cliff of sandslone is
very lose and consequently usatisfaclory as found-
ation, and so it was thought necessary to dig it away
and rest the piers immediatly on the cliff.
The length of the bridge is 92 metres, and equal
to the breadth of the channel at the site. Two piers
are built in the river, and the spans measure 31,5
-29—31,5 metres. The rivers is much narrower bolh
a liltle above and some way below the site, at Ár-
bæjarfoss and Ægissíðufoss; but at either of these
places it would have been necessary to build a bridge
with one span only (very likely a suspensionbridge),
and such a bridge, although sliorler, would not have
been correspondingly cheaper. Besides, both these
places are at some distance from the higd-road.
Stone was not to be had above ground at a
shorter dislance from the site than aboul 3 kiloinetres
to the east of the river, and it is very unsatisfaclory,
') ca. 2500 pounds.
small, cracked, very hard and easily broken. Still
some of it was transported to the site the preceding
winter and employed in the eastern pier and abut-
ment. At the western end of the bridge there is a
rock in tlie ground; a part of it was blasted and the
stone employed in the western pier and ahutment.
It is dolerite, very solid and easily dressed, resembling
in quality the best dolerite to be had near Reykjavik.
2. The abutments and piers.
The western abutment is built on a rock, and
made of dolerite on the outside with a backing of
stone concrete. The eastern abutment is founded on
a firm layer of sand, which was reached by digging
to the depth of 1 metre. It was built of the same ma-
terials and in the same way as the western abutment.
The western pier stands near tlie rapidest cur-
rent and greatest depth of the river. It was built in
the following manner. A casing of piles, 4 by 8 metres
wide, was rammed down round the site of the pier
with a 300 kg. monkey. The planks were 3 inches
thick, and 6 by 6 in at the corners. Then the layer
of sand was remowed with a bagdredger. At a depth
of 2,20 metres below the level of the water, solid
bottom was reached, impenetrable with the bag. Then
the bottom was levelled, b}r dragging across it a
channel iron ([ iron) hanging in a wooden frame
which was driven to and fro on the top of the pile-
planking. In this manner tlie boltom was dressed
completely level. Then a caisson of concrete was made
in the following way. A platform was placed below
ffie surface of the water, hanging in 8 screw-bolts of
ll/8 in. iron. The studs under the upper beams were
placed, some on the pile planking and some driven
down into the bottom inside it. The caisson is ex-
hibited on Plate 2 (page 8); its height was 2,30 metres.
It was all reinforced. Eight days after it was finished,
it was screwed down till it floated on the water
wilhin the pile-planking, drawing then about 1 metre.
Then the platform was removed from under it, and
with a íirst layer of concrete filled in. A lillle
before the caisson liad reached the bottom, it was
sunk by filling it with water, in order to try if it
settled on the boltom without leaning. It stood per-
fectly plumb; and then the water was pumped out
of il and it was filled with stone concrete. Then the
pier was built on this support. The total height of
the pier from bottom is 5,2 metres.