Question about asce 7-05

[hello12345hello]

trying to calculate wind load on solid free standing wall using asce7-05. I am using Fig-6-20 to calculate the wind pressure on wall. Wind pressure is 30 psf. the wall is 10ft high and 30ft wide. How much concentrated load do i have to apply. i have to consider full width for that ?

 

[RHTPE]

The pressure is evaluated at height "h" as shown on Figure 6-20. You then use the force calculated from 6.5.14, applied for the various cases as shown in Figure 6-20.

Ralph
Structures Consulting
Northeast USA

 

[hello12345hello]

all right that's fine but Force = 30psf *(10ft) * (1ft) = 300 lb at mid height. but then how about the criteria how about Case B and Case C.

 

[RHTPE]

Case A is simple - F = qh * 0.85 * 1.40 * 10' * 30', applied at the middle and at a height of 5.5'.

Case B uses the same force coefficient as Case A, only applied 0.2B either side of the plan mid-point of the wall, where B = the plan length of wall.

Case C divides the plan view of the wall into various regions. In your situation there will be 3 regions - 0 to s, s to 2s and 2s to 3s, each with their own force coefficent (2.60, 1.70 & 1.15).

Pretty simple (but involved) once you've done a few.


Ralph
Structures Consulting
Northeast USA

 

[hello12345hello]

Ralph,

The force say (for qh 25 psf )= 8925 lb total. But generally we will check 1ft tributary width of wall for A psf wind pressure. i will check the cantilever wall for moment due to wind = (Pressure psf * ht*ht/2). If i have to check for concentrated load then how i am going to check. this is very heavy concentrated load apply at mid height

 

[RHTPE]

A little engineering wisdom must come into play here. The reality of wind force is that it is a varying, but relatively uniform load. ASCE 7-05 is recommending the total force applied to the structure due to wind. How you resolve it to the structure's support(s) is a determination that you must make based on how your structure is to be constructed.

Not meaning to get snippy, but this forum is about getting advice to help formulate your own reasonable and rational premise regarding an issue. You're the one who gets to determine the final answer.

Ralph
Structures Consulting
Northeast USA

 

[ron9876]

I don't like the way that section is presented. They have you calculate a total force but for designing a wall as presented by the op it doesn't make sense. I calculate the pressure instead of the total force and design the components accordingly.

If you were designing the columns for a large sign along the interstate the force procedure would make more sense for the column design but for me it doesn't make any sense for designing the sign itself.

 

[azcats]

IMO, you don't have to apply concentrated forces. 6-20 is showing where they want the resultant force to act.

Ultimately, in the OP's situation, the end condition of case C will govern his design. It will be very stout as his pressure will probably increase to around 60 PSF based on the Cf values.

I don't like this section. How many typical screen wall details (that never had problems) were made obsolete or ridiculously confusing because of Case C? When this code was adopted, Clark County, NV added an amendment which removed case C from screen wall design.

http://dsnet.co.clark.nv.us/ds/building_services/codes/ibc_amendment_1609_1_1.pdf

Meh...done venting for now.

 

[a2mfk]

I have been assuming that the resultant force Case C is in there to produce a possible torsion, depending on your structure. Since some monopole sign systems would be vulnerable to this type of loading (like large highway advertising signs , ie ron's example). Like the rest of you we are using our engineering judgment on how to design screen walls, uniform loading is all you should be using in most cases, IMO.

Codes should not be read like IRS tax code anyway. Well, for that matter, the IRS tax code shouldn't read like the IRS tax code (sorry, wayyyy of topic).

Make your structure stand up and use your judgment, even the best code/spec writer can't cover every example out there and the attempt to do so is probably why we are all complaining about ASCE 7 and codes so much. Too complicated is a bad thing, K.I.S.S., words to live by...

 

[RHTPE]

I believe the intent behind Figure 6-20 is to provide a simplied (ha!) means to allow for the varying wind force against a sign or free-standing wall. The result is either the total force (Cases A & B), or the total force on a specific region (i.e. Case C). Obviously given the nature of wind loading, it is applied to the sign or wall surface as a distributed load, NOT as a point load. I also believe that this total force is intended to included the negative (suction) pressure on the leeward side.

Those who study these things in a laboratory wind tunnel are so wrapped up in the data they acquire and are way too cerebral to write a code in such a way that an average engineer can easily understand the intent. The Code and Commentary should be side-by-side, and the commentary should be proof-read by practicing engineers before publishing. I agree with the K.I.S.S. principle as well, and would rather over-design a tad rather than get it theoretically exact.

Enough soap box stuff.

Ralph
Structures Consulting
Northeast USA

 

[oldrunner]

Are you using ASCE 7-05? I believe the latest ASCE 7 has a bunch more wind chapters - more like Harry Potter continuing sagas. On the other hand, wind does do a lot of damage - especially lately - so even it'd painful to learn, we gotta do it. Agree with the commentary being more accessible - similar the AASHTO's Pole code or the ACI code. Mistakes in the codes are an ongoing annoyance. Once called a guy about some anchorage code language that he and some others had authored. He had caught the errors but who ever published his stuff didn't make the changes.

Not to long ago there was a discussion on the number of erratas for the Steel 13th edition - so check AISC's website for whatever might be there.

 

[steellion]

It is a confusing figure to be sure. I design the sign (or screen wall or RTU) itself for a uniform pressure of qz*G*Cf. Also, for rooftop structures, don't forget the multiplier from 6.5.15.1! I interpret Case B and C to apply only to apply when figuring out torsion in the columns (a pseudo-MWFRS approach) if you're truly dealing with a free-standing sign.